Location

An early decision is whether or not to have a permanent installation; assemble a portable setup in the back yard each time or to travel to a dark site for your photography. Light pollution and the typical atmospheric conditions for a general location set the upper performance limit for astrophotography. Dark sites offer the best conditions but one must be willing, ready and able to set up at a remote dark site each time the weather looks good. On the other hand, the convenience of a back yard beckons, but with the possibility of greater light pollution, an interrupted horizon and the neighbor’s insecurity lights. Early enthusiasm can wane and although we might start off with good intentions, the practicalities and effort of remote site operation may be worthwhile for just a few guaranteed prolonged imaging runs in fantastic conditions or in a social context. In my case, I have a worthy dark site about 20 miles away on the coast, with low light pollution and a convenient grassy car park, set in the marshlands of east Essex, but I have not tried it.

Most of us would love the turn-key convenience of a permanent observatory but cannot justify the cost or eyesore, especially if the weather limits its use. With a little practice it is possible to deploy a portable setup and be imaging within an hour. If a shower takes you by surprise, a large waterproof cover can be thrown over in seconds and at the same time permit a setup to remain for a few days. The installation decision influences the equipment choice, since telescopes, mounts, counterweights and tripods are neither light or small. After all, a portable setup needs to be just that, portable, and it is surprising just how quickly the repeated assembly of a large articulated mass in cold dark damp conditions can reduce the appeal of this hobby! For example, my first acquisition was a used 8-inch Meade LX200 SCT, weighing in total at around 43 kg. The previous owner had bought it for his retirement but quickly changed to a lighter telescope. It is not just a case of lifting a large weight; equipment requires transport, carrying and assembly, often in the dark, without tripping, damage or injury. The box for the LX200 head filled the back of my car on its own. The same story plays out in the many adverts for large, used telescopes. In a permanent setting however, larger and heavier mounts, scopes and installations are a onetime problem. These systems remain fully assembled and the cables routed permanently so that remote operation is safe and feasible. In a portable setup, the needs are different: There will be trade-offs in weight and rigidity and all the mechanical and electronic components must endure repeated assembly without failure or damage. In these situations, refractor telescopes are certainly more compact and robust during transport and do not require alignment before use.

fig.1 This 8-inch Meade LX200 telescope with an equatorial wedge is a handful for a portable setup; the equivalent 10- and 12-inch versions weigh considerably more and are better suited for a permanent installation.

Safety

It goes without saying to “be safe” but I’m compelled to highlight a few things on personal safety. The conditions and the remote locations that astrophotography encourage can create some unique situations. Clearly lighting is essential; a powerful torch and a wind-up spare, in case the batteries fail. You may also have to consider personal security in a remote location; although mobile phones are commonplace, there may be no signal at the site and someone should know where you are. Increasingly, there is also the subject of physical safety and the best practice of lifting and moving large heavy pieces of equipment (in the dark). Those vehicles with a flat load-space (like a van or wagon) are particularly back-friendly since they allow heavy objects to be slid in and out without lifting with a bent back. Capped boots are a sensible precaution too; a dewy counterweight once slipped through my fingers and missed my sandaled feet by a few inches.

Power

Power has its own safety considerations. Most astronomy equipment requires 12–14 volts DC, but some devices, like USB hubs, only require 5, for which I use an encapsulated 12 to 5 volt DC converter. (A few mounts also use 24 or 48 volts.) Lead-acid cells are the most common source for mobile DC power. They conveniently store a high capacity (measured in amp-hours) but this lowers after each discharge/charge cycle, an effect which accelerates with the level of discharge. There are several lead-acid battery designs; those ones for hobby-use are best; they are designed to be more tolerant of deep discharge, whereas car battery versions are optimized to deliver bursts of high current but quickly lose charge capacity after repeated cycling. Gel-filled batteries or AGM designs are maintenance free and do not need to be kept upright. Large capacity lithiumion batteries, up to about 24 Ah are also available, but at premium prices. These are considerably lighter and smaller than the lead acid versions.

Power supply quality is important too and the DC supply to a CCD camera should have as little electrical noise as possible. A simple solution is to use two batteries; one for the imaging and guiding cameras and the other for the “noisy” motor and switching functions such as dew heaters, focus control and the telescope mount. Battery charging is a potentially hazardous activity. Dead or damaged batteries should not be used but properly recycled. Modern batteries require a little care to prolong life and capacity. Some do not like over-charging or being charged too quickly. If in doubt, check the recommendations for charging and use the recommended charger.

For a domestic site, mains power is also an option but only with care: Any mains extension cable run out through the back yard or buried in the back yard should not only be armored to protect from accidental rupture but employ an earth leakage current breaker (ELCB) at the power source. Just as significantly, many regulated DC power supplies, including some that are supplied by mount OEMs, are designed for indoor use only. Dew and general dampness go hand-in-hand with astronomy and there is an obvious risk of electrocution or failure with some models. One alternative is to place the power supply in the house or in a suitable enclosure and use a length of heavy-duty speaker cable to carry DC, with minimal loss, to the telescope. The same safety considerations apply to domestic plug-in-the-wall power adaptors and desktop computers sited in outdoor situations. They need protection from moisture and should be appropriately earthed. Spare mains sockets should be kept away from moisture and fitted with a plastic child safety cover for good measure. If there is any doubt, ask a qualified electrician to check over your installation. The only stars you should be seeing are those through the telescope!

fig.2 The combination of frugal power management settings in the MacBook Pro with an external lithiumion battery is sufficient for a full night’s imaging. The orange plastic armor and keyboard cover add some protection against knocks and dew.

In a portable setup, the laptop is the computer of choice, preferably with a battery life in excess of 5 hours. Aggressive power saving settings are great but remember to turn off the sleep-mode or your imaging run could unexpectedly terminate! Some models have hot-swappable batteries but most are internal. An alternative is to use a reserve high capacity lithium-ion battery. These have a programmable output voltage and are supplied with multiple adaptors to suit the most popular laptops. A third option is to use an inverter. These convert 12 volt DC into AC mains, which then supplies the laptop’s power adaptor. As before, check the models are safe for outdoor operation. Lastly, some power supplies are floating and to avoid potential static discharges, connect all the equipment together before powering up.

Comfort

Astrophotography is physically demanding, especially in cold conditions. The manual exertion during the setup may keep you warm but the body cools down with the extended inactivity during image-capture. It’s the same with hill walking; you need to layer up when you stop exerting yourself. When I’m watching the exposures roll in I add a few layers and put on a ridiculous hat, gloves and warm boots. Over extended imaging times, food, drink and diversion are essential. I use an iPod rather than the vehicle radio or mobile phone to preserve the important batteries. A number of laptops and tablets have touch sensitive controls and will not work with conventional gloves. If you look around, gloves are now available with conductive fingertips. Extreme cold does have one advantage though; it keeps the insects at bay. Insects like astronomers, so mosquito repellent and bite cream are a few more essentials to keep in mind.

fig.3 This SkySafari® application on an iPad® can plan a night’s imaging, from left to right, determining when the object will cross the meridian, checking the field of view for the telescope and sensor combination and essential object information, including magnitude.

Weather and Planning

It happens to all of us: The cloud is unbroken and each weather front merges into an unending overcast month and you have almost come to the point of giving up astronomy. Then, one evening, surprise, it is clear. We suddenly have an opportunity for imaging … of what exactly? Meteorology is not an exact science but weather systems are predictable to some extent (even in the UK). It is possible to recognize cloud sequences from cold and warm fronts and the characteristics associated with areas of high pressure. A little knowledge of clouds certainly comes in handy; for instance, small cumulus clouds in the evening generated by thermals are more likely to disperse than high cirrus clouds signalling a warm front. The on-line weather reports and those with dedicated sky forecasts are a useful resource to anticipate clear skies in the next 48-hour period although the timing may be slightly out. During the day, the local cloud conditions help to realign the forecast, especially if corroborated with information from other sources, say a weather station.

fig.4 This polar scope setting application on an iPad not only shows the position of Polaris as viewed through the eyepiece but a readout of the Hour Angle (HA) which can be quickly set directly on the mount’s RA scale (after a one-time calibration).

Next, we have to decide what to image. Many objects, especially those at lower declinations have a preferred season when they are visible for some of the night. Ideally, they will have an altitude of about 30° in the east (for those of us in the northern hemisphere) at the start of the night, which maximizes imaging time. Those objects nearer the pole never set and those over 70° from the celestial horizon (for UK sites) are safe bets too. At a latitude of +50°, objects with a DEC between −10° to −90° will never have an altitude greater than 30° and will be a challenge to image. Planetarium programs usually have a “tonight’s best” option, usually for short term visual purposes. Luckily every object’s visibility is predictable and it helps to identify those objects that will be optimally positioned each month beforehand. You can make a target list with a planetarium program or an application like AstroPlanner. These identify those objects that are in a good starting position for an imaging run, for each month of the year. For example, the high declination objects or those low declination objects whose altitude is about 30° in the east at dusk. (Charles Bracken’s The Astrophotography Sky Atlas has a comprehensive set of maps for all seasons and latitudes.) Ideally, I set up at dusk when my object’s position is low on the Eastern horizon. By the time I have calibrated the mount and set the focus, it will have risen sufficiently to clear my neighbor’s roof line. I also check the azimuth of the target and note if, and when, it is going to cross between east and west over the meridian (if the image acquisition system requires manual intervention). Before using Sequence Generator Pro I had to stop the exposure, flip, realign the target and restart autoguiding and the imaging sequence with the mount on the opposite side.

Timing

Imaging is not a quick fix (well, other than short planetary videos). It is really frustrating to start a promising imaging sequence only to be obliged to break off before you have enough imaging data to make a quality image. To prevent this, a quick evaluation of the likely overall exposure duration and available time helps. Image quality benefits hugely from multiple exposures (or “subs” as astrophotographers call them), the more the merrier. Dim galaxies and nebula benefit from at least 10 hours of data or more, especially when using narrowband filters. For high quality images it is normally necessary to image over several nights and combine images. Again, programs like Sequence Gen-erator Pro are intelligent and remember what you were imaging and how far through the intended exposure plan you were when you shut down. They can start up and continue on another night with ease.

fig.5 Good planning helps with the setup too. Here, the balance point for the telescope (fully assembled and in focus) is marked on the dovetail plate with a piece of white tape and aligns with a corresponding marker for quick and easy balance setup.

The Internet once more is a useful resource to plan exposures: An Internet search of the deep sky object often links to other’s images that usefully indicate the equipment setup and exposure details, possibly with the same sensor. Some bright objects might be captured in a single night, others require perseverance over several.

Familiarity

Equipment and software setups soon become quite complex and complete familiarity really helps with quick and reliable operation. This reduces “real-time” frustration, which often occur in portable setups or after a software update. My better half calls it “playing” but a few daylight dry-runs to practice assembly, balancing and alignment make night-time operation second nature. Some preparatory work can make this easier still; for instance, the balance points can be marked on the telescope dovetail plates for instant assembly and the polar scope reticle calibrated to the RA scale and centered during daylight. The mass of a dangling cables affect balance and tracking and loose cables may catch during slewing. I found Velcro cable-ties were an easy way to secure cables from snagging and relieve stress on the connectors. In addition, I made simple colored markers with electrical tape around each end of the many USB cables. This is a quick way of identifying which cable is which, handy for when a single device requires a power reset, without disturbing the scope-end of things.

Little things like systematic storage, say using clear and labelled plastic food storage boxes, protects small items from dew and dust and avoids rummaging in the dark. The smaller boxes can be organized into larger ones for imaging, observing and common accessories, which helps when loading up. I save the silica gel bags from various purchases and pop them in for good measure.

Understanding the software operation is essential and often underestimated: My early setup used 8 pieces of software, six of which interact (C2A, Maxim DL, FocusMax, MaxPoint, ASCOM and EQMOD) with many USB devices (camera 1, camera 2, focuser, filter wheel, GPS receiver, EQ6 Mount and a USB over Cat 6 extender). It saves a lot of time to pre-configure each to work with the others and enter the site (time, horizon, location) and equipment data (camera type, mount type, focal length, pixel resolution, filter setup and so on) that most of these programs individually require. Even when you think everything is perfect, software can kick back; in some versions of Windows, if a USB camera plugs into a different port connector, the operating system demands the driver file is loaded again, causing unnecessary aggravation, especially at a remote site, without the CD or Internet.

fig.6 This figure illustrates the information flow between the software applications and the hardware of my initial system. It looks more complicated than it is in practice but it does illustrate the need for preparation and familiarity with the software and hardware settings and connectivity. In this setup, the full planetarium program resides on an iPad and a simple one in Maxim DL.

I am not the first or last to waste imaging time due to poor preparation: Recently, after a SSD drive upgrade to my laptop, I failed to realize that the PC clock had changed to daylight saving, confusing my alignment and plate solving attempts for two precious clear nights. The software startup sequence can be established during a dry run and the hardware drivers checked beforehand for reliable operation on each of the different USB ports, especially after any software upgrade or update.

One simple way is to record the power-up sequence. This increases the chances that the equipment connects flawlessly and follows a reliable alignment and calibration sequence, including polar alignment, focus, star alignment, object location, autoguider calibration and exposure determination. It helps to try out alternatives using the simulator driver versions of camera, focuser and mount interfaces in the comfort of the home. Astrophotography challenges our ability to remain clear headed; few of us are at our best at night and I use a laminated checklist for repeatability. It can also include the essential data for the telescope and imaging configurations that are required by the imaging software. I have included an example checklist in the Resources section.

fig.7 The back end of a telescope is sometimes more important than the front end. For a precise and robust assembly of this William Optics field-flattener, the 2″ adaptor and nosepiece are unscrewed from the focus tube and field-flattener. The clamp system is replaced by a threaded custom adaptor that screws both items together firmly and squarely. There is a small global industry which manufactures and designs adaptors for every conceivable requirement, made known to all via the Internet.

Mechanical Integrity and Stability

A stable imaging platform is vital in any imaging setup and is equally if not more important than the optical quality of the telescope. Every vibration, slippage, flex or movement, degrades the final image. It might seem pedantic, but a little attention to every detail ensures the best possible result. For example, in a previous chapter the calculation for a telescope resolution hovered around one arc second or 1/3600^th of a degree. That is equivalent to a 0.005 mm movement over 1 m! Working from the ground up, the tripod or pier and the telescope mount must be as inert as possible and minimize vibration, slippage or flexure. This is principally a combination of mass and stiffness. A permanent pier set in concrete is very rigid, even more so if it is de-coupled from external sources of vibration. For portable setups, a rigid and secure tripod is essential, with feet that do not slip or slowly sink into the ground under load. Adjustable tripod legs can flex and sometimes the locking design does not secure in all directions. Height is not important in imaging and one way to minimize leg flex is to extend them as little as possible and choose a tripod with locking legs that prevent lateral movement too. Loose cables are another source of error; they can catch on things as the mount swings around or flop about and change the balance of the telescope.

fig.8 Little things count: Tripod spikes provide a secure footing but an oversize nylon washer prevents the spike sinking into soft ground. The underside of a pillar extension, shimmed with electrical tape for a snug fit and with three sandpaper pads. When clamped, these grip to the tripod top plate and prevent accidental rotation.

The telescope mount has the lead role in the imaging system’s performance. Even without tracking, the mount’s many mechanical interfaces affect general stability. Mounts vary enormously in weight, stiffness, payload and price. When planning the equipment budget, the mount takes the top spot. Conventional wisdom suggests that for the best results, the weight of the scope, camera and hardware should not exceed 2/3rds of the mount’s payload capacity. It is also essential that the mount cannot move about on the tripod or pier. This requires tightening all four polar adjustment knobs on the mount and re-tightening any base fixings after polar alignment (which requires the mount to move). In the case of a tripod mounted system, if the mount locates around a raised cylindrical spigot, check for a snug fit. If it is not, insert a few shims or packing to eliminate the possibility of lateral movement.

The need for stability equally applies to the telescope, especially flexure from the back half of the telescope. The focus mechanism, field-flattener and filter/camera system need to be secure and orthogonal to the optical axis. This may require a little ingenuity in some cases: For example, many standard telescopes are equipped for observing use, with 2-inch and 1.25-inch clamping systems for diagonals and eyepieces. The imaging adaptors often make use of these but the weight of heavier cameras cause flex or slip. The less expensive adaptors use a single screw fixing, which not only marks the tube but will almost certainly wobble on the other axis. The up-market versions use a brass ring, clamped at three points but these too do not guarantee an optimum assembly. Any tilt or flex has a direct impact on the shape and focus of stars around the image periphery. It is always better to screw things together to ensure an aligned and stiff assembly and a scour of the Internet will often locate a threaded adaptor to mate items together or a machining company who can make an affordable custom design. In the cold I use a pair of rubber gloves to grip damp accessories that have tight threads.

Mechanical play, sometimes known as backlash or hysteresis, is a consequence of mechanical tolerances. Each mechanical interface has a little gap or flexure that results in backlash between each gear, belt and bearing. Backlash is not an issue if things only move in one direction; it becomes a problem when a motor changes direction (or when the balance point flips over and the engagement forces reverse). It can be measured by noting the amount of reverse movement needed before a system starts to change direction. It is normal to have a small amount of play in any system but it is prudent to check your mount for severe issues, as consistent factory adjustment is not guaranteed, even in premium products.

With the mount fixed on a tripod and the clutches secured, if you can feel play with a gentle twisting motion in either motor axis, some immediate attention is required. For a quick backlash measurement, mount the telescope as normal, disable the mount tracking and attach a reticle eyepiece or an imaging camera. Center a terrestrial object by using the mount’s slew controls on the handset. When a feature is close to the reticle, slow the mount slew-speed down and approach slowly from one direction. Set the slew speed to the sidereal rate (1x) or less and use the opposite slew control button to reverse the direction of the telescope. The approximate value of backlash for a 1x slew rate is the duration of the button press (in time) before the mount starts to move. This time, multiplied by 15, gives the approximate value in arc seconds. Gear engagement is often adjustable, either by the user or manufacturer, though you can never eliminate it entirely. Most guiding and alignment software will also measure it during its calibration routine and account for any residual backlash in its operation. It is important to note that the effect of backlash mostly affects the initial star alignment accuracy of a telescope and the accuracy of subsequent slews to an object, but it can also interfere with autoguiding performance, when the declination motor changes direction or arising from a balance issue in the right ascension axis when an in-balance is not opposing the tracking.

Tracking and Alignment

In theory, an equatorial mount tracks a star’s movement around the sky with a single motor: A star’s declination is fixed and an equatorial mount only has to rotate around the right ascension (RA) axis. (This is true so long as the RA axis points directly to the celestial pole, there are perfect tolerances in the mount system and atmospheric refraction does not exist.) Ours is not a perfect world, however, and two issues, periodic error and drift, compromise performance.

Periodic Error or PE is the consequence of the dimensional tolerances of the drive system in the mount. As the RA motor turns, the imperfections of the mechanics cause the mount to run a little fast or slow at different angles and the image does not track perfectly. The error from each part of a rotating system repeats itself each turn, hence the name. These errors combine to form a complex cycle of tracking errors. During a long exposure, these cause a star’s image to elongate, to an oval or worse. We measure PE in terms of angular error; typical peak-to-peak values range between 4–60 arc seconds. The more advanced mounts use optical encoder systems and self-correct their mechanical to sub arc second values.

fig.9 This used Meade LX200 telescope was an early model with nylon gears. From accidental abuse, the teeth had worn badly and needed replacing. On the left the motor, two gears and worm gear can be clearly seen in relation to the large diameter altitude axis gear. The other two close-ups show before and after the gear drive clean and upgrade.

Price and brand are not always a guarantee of good PE performance; many forums bear testimony to the performance differences between identical models, which is an unavoidable outcome of the statistical nature of manufacturing tolerances. In addition to physical tolerances, grit or debris trapped in the lubricant of the gear mechanism can create an abrupt tracking error. Some of the more confident engineers in the community will thoroughly de-grease, clean and re-lube the gear system of a used or even a new mount. This is not a task for the faint hearted and only considered after the warranty situation, ease of access and a risk assessment is completed! After some tense moments, I successfully replaced, aligned and cleaned the gears in my used Meade LX200 to huge effect (fig.9).

My current equatorial mount has a better than average PE and I use Periodic Error Correction (PEC) to improve it further. It is practically sufficient and I’m not tempted to tune the mechanical components further. Even so, for a typical setup, my PE is often larger than the telescope resolution, remaining CCD resolution and astronomical seeing combined. For that reason, deep sky imaging requires autoguiding to remove the residual error and ensure small tight star images. Many specifications focus on the peak to peak magnitude of the PE but the rate of change is often more important, since it is difficult to correct a rapidly changing error through mount motor adjustments.

If the mount’s RA axis is misaligned with the celestial pole an additional tracking issue, “drift”, occurs. When the rotation of stars and that of the telescope are not perfectly concentric, the stars slowly and steadily drift out of view. The rate of drift is minimized by improving the mount’s polar alignment and its sensitivity is also dependent on the star’s position. Alignment is covered in detail in later chapters but for now we assume it can be improved through the use of a polar scope, computer assisted polar alignment or by repeated drift-rate observations and adjustment. Some techniques obtain a close but not perfect alignment in a few minutes. High precision drift alignment uses more of the potential imaging time and is a chore in freezing conditions but worth the effort in permanent installations. Some modern mounts track automatically in both axes to compensate.

There appears to be an insatiable interest in perfecting polar alignment and reducing periodic error on the Internet forums. These go to the extreme of replacing or upgrading gears, bearings, complete overhauls of new equipment and extensive use of numerous polar alignment routines. It seems to be a goal in itself to achieve perfect alignment and run long unguided exposures without star elongation. I apply the 80–20 rule to polar alignment and mechanical performance and use autoguiding to correct for both drift and periodic error during exposure. In context, if one tripod leg sinks 1 mm, it defeats advanced polar alignment.

Autoguiding

Practically and within reason, autoguiding corrects for periodic error and drift. Autoguiding, as the name implies, automatically guides the mount to track a star. It is achieved by a small software application that takes repeated short exposures of a star, using a still or video camera, either through a slave telescope or via an image splitter attached to the imaging scope. The autoguider software records the initial pixel position of the star and calculates small corrections to the both motors after each exposure to correct for any tracking error. These adjustments are sent back to the mount’s motor control board, either directly to the mount’s guide port connector or via the mount’s control software. The improvement is significant; one mount has a (peak-to-peak) periodic error of 12 arc seconds and with autoguiding this reduces to 1.5 arc seconds. The average error is less than 0.2 arc seconds, considerably better than the diffraction-limited resolution of the optics.

Autoguiding is not a cure-all, however; it assumes a reasonable level of mechanical and alignment integrity in the first place and it is a good idea to first check the mount’s raw performance and alignment accuracy. For instance, to check the polar scope one could, after a one-time extensive drift alignment, set up the polar scope, as if to perform an alignment and note the precise position of Polaris for future reference.

fig.10 Focusing is critical and difficult to do well in all conditions. Here is a “V” curve from SGP of a RCT, where the collimation is a little off. (Centrally obstructed scopes are the most challenging to autofocus and the slopes either side of the minima are not necessarily equal.) The latest versions of SGP’s focus algorithms now work well with the “donuts” from out-of-focus SCT and RCTs. V-curves are also employed by Maxim DL and FocusMax (which was a free utility that had a similar proactive community as PHD2). In practice I found it required some effort to make it work reliably and it has since been made into a commercial product by CCDWare. These all work on the basis of acquiring images at different focus settings and minimizing the half flux density (HFD) of a star (or stars) for a range of focus positions. The optimum position is the lowest point, or the intersection of the V-curve. The slope of the V-curve is consistent for an optic. SGP uniquely samples many stars in its autofocus routine but takes a single reading at each focus position.

Dew Control

The need for dew control varies by region, season and by telescope type too. As the air temperature falls at night its relative humidity increases. At the dew-point temperature, air is fully saturated and cannot hold any more water vapor. Below the dew point, excess water vapor condenses onto cold surfaces. Those surfaces which are exposed are worse affected and optical surfaces will not work with condensation on them. Long dew shields (lens-hoods) certainly help but do not entirely eliminate the problem. In reflecting telescopes the primary optics are generally well protected at the bottom of the telescope tube. In the case of truss design reflector, a cloth shroud will similarly shield the primary mirror but the secondary mirror will require mild heating.

Those designs with glass optics at the front will likely need gentle warming; just enough to prevent condensation forming. (You should avoid aggressive heating as it will create your very own poor seeing conditions within the telescope’s optical path.) This is conveniently achieved by passing current through an electrically resistive band (dew heater tape) which wraps around the telescope near to the exposed optical surface. The dew heater tape plugs conveniently into a 12 volt power source for full power or a pulsed supply for finer regulation. With dew control, prevention is always better than cure; it is best to turn on the dew heaters a few minutes before you remove the lens cap. Removing condensation takes a lot of heat and the heated surface will then produce image-blurring air currents until it cools down.

Focus and Focusers

Several refractor telescopes from competing companies share the same optical cells, sourced from the same Far-Eastern supplier. Their major difference lies in the other aspects of the telescope. Putting aside optical quality for the moment, these include the finish, internal baffling, sundry accessories and most importantly, the focus mechanism. Precise focus is critical and it took me some time to realize just how important it is for high-quality imaging. A poor focuser design is a source of intense frustration. In the more demanding setups the precise focus position may require updating during an imaging session to account for thermal effects on optics and tubes, as well as any differences in focus between each colored filter.

Focus mechanisms must be a dilemma for telescope manufacturers; the needs of the visual user are less demanding and it makes no sense to design in cost for a performance that is superfluous to many customers. In astrophotography, the combined mass of the imaging equipment places considerable load on a focus drawtube and more importantly, the need for maintaining precise focus and stability. A good focus mechanism is strong, does not slip or flex in any direction under load and, when the focus position is changed, the image should not shift significantly.

Even with a substantial mount, touching the focus knob on a telescope will cause the image to jump about, making assessment tricky. For that reason it is really handy to focus a telescope via an electronically driven motor attached to the focus knob. The better designs use a gearbox reducer and a stepper motor, which enable fine control and absolute positioning, usually via a serial or USB interface. Several companies specialize in upgrade focus mechanisms, with and without motor actuation. Some telescope manufacturers even offer third-party focusers as a point-of-sale option for imaging use.

You might be wondering just how essential is accurate focus for high-quality imaging? A star is smaller and brighter when it is in focus, since all the photons fall on the fewest number of pixels (this also maximizes the signal to noise ratio). It was a revelation, however, to realize just how sensitive the focus position is: As an example, a short telescope of 400 mm focal length and a 100 mm aperture has a diffraction-limited image disk diameter of 2.7 arc seconds. A focus error of 0.1 mm blurs this to a circular patch 13 arc seconds wide, about 5x wider and covering 20 pixels of a typical CCD sensor! Correspondingly, the photon intensity per pixel will be, even accounting for average seeing conditions, about 20x less.

A stepper motor driven focuser controlled by an autofocus program makes the most of the mechanical abilities of a quality focus mechanism. These autofocus programs reliably determine a more accurate focus position and in less time than manual methods and accommodate backlash adjustment at the same time. They vary in sophistication, but most work on the principle of minimizing the width (FWHM or HFD) readout for a bright star. Some programs can save precious imaging time and autofocus with just a few exposures and a prior saved profile with a resolution of 0.01 mm or better.

Essential Software

Software is clearly a necessity for digital astrophotography and at the same time is the most difficult subject to give specific guidance on. The basic applications used in astrophotography are a planetarium, mount controller, image-capture and image-processing. These are only a fraction of what is available: It is easy to collect multiple software applications and utilities like confetti. Many are free or reasonably priced, like the thousands of “apps” for mobile phones. The trick is to be selective; in addition to the essentials, an autoguiding program, polar alignment utility and an autofocus program are the most useful for imaging.

One of the dilemmas with software is there is no consistent feature boundary between software genres and many applications overlap functions with others. The variety and permutations are almost endless. One way to look at this problem is to turn it around and consider the necessary functions rather than applications. Where the functions reside is matter of product selection and preference.

The main planetarium function is to display a chart of the sky for planning purposes, for any geographic time and place, with the essential information for thousands of objects. There are many varieties for computers, tablets and smart-phones. They commonly have additional functions to point and synchronies the telescope mount to an object and show the field of view for an eyepiece or camera. These programs, especially those with high-resolution graphics, use a good share of the microprocessor time and memory as they calculate and update thousands of star positions each second on screen. Once a system is imaging, there is little need for the planetarium function and the application merely wastes computer power. There are many planetarium program styles; some are educational and graphics intensive, others more scientific in approach, with simpler graphics, aimed at practical astronomers. Several excellent programs are free but those with movies and pretty graphics can exceed £100. Most use or can access the same star catalogs for reference but the controls, ease of use and interfaces vary widely. I often use an iPad application to plan my imaging sessions; the intuitive screen controls and built in GPS are equally convenient in the daytime or at night.

fig.11 The popular SkyWatcher EQ6 mount has what looks like a RS232 serial connector on the facia. This is a serial port but at 5 volt TTL levels and can only be plugged into the SynScan handset or a special interface that operates at TTL levels. The SynScan handset has a second smaller connector to allow external RS232 control, often via a USB to Serial converter. (RS232 operates with ± 12 volt signal levels and direct connection to the mount would damage the motor control board.) There are many USB to Serial converters but not all are reliable in this function. Those from Keyspan or use the Prolific chip-set have favorable reports.

Mount control is an equally diverse function. A telescope mount houses a motor-control board that connects to an external handset or computer, which has the essential functions of alignment, object selection, slewing and tracking. After physical polar alignment, which, say, points the RA axis within 5 arc minutes of the pole, the mount needs to know exactly where the stars are. It already roughly knows, within the field of view of a finder scope, but it needs precise registration to improve pointing accuracy. The basic registration function has two steps; slewing the mount to where it thinks a star is, followed by manually centering the star with the handset controls and synchronizing the mount. Basic registration or alignment calibrates both motor positions for a single star and in a perfect world every other star falls into place. Sadly, mechanics and physics play their part and to compensate, the more advanced alignment functions repeat the registration process with several bright stars in different parts of the sky. A hand controller typically uses three stars to align itself but the sophisticated computer programs use more to build a more accurate pointing model. It is easy to go over-board; ultimately, we just need to place the object we wish to image onto the sensor and then rotate and fine tune the camera position to give the most pleasing composition.

In the case of a mount with a handset you require an external computer interface too, either directly into the mount or via the handset. This is when things become complicated. For example, the popular Sky-Watcher® EQ mount has two software interface protocols; the one between the supplied SynScan™ handset and the mount and a basic one with an external computer. The physical connection possibilities are numerous and are shown in fig.12. In each case, a computer, tablet or mobile device, controls the mount using a simple command protocol. (Mobile devices can control a mount too but are unable to image or autoguide.) For this particular mount, a free third-party application EQMOD entirely displaces the handset and allows advanced control of the mount, multipoint alignment, tracking, periodic error correction and guiding directly through a PC interface via wired or wireless interfaces. This program in turn accepts commands from the planetarium and autoguiding applications that are linked to it through ASCOM. This example serves as an illustration on just how complicated these systems can become. For success it makes sense to start off simply and check things work before introducing additional hardware and software complexity.

fig.12 There are many ways to physically control a mount; the SkyWatcher EQ series probably has the most permutations and like other mounts has thriving third-party support for software and hardware alternatives. It is a good idea to thoroughly prove your basic connection reliability before attempting wireless connections, which may not be fast enough for autoguiding.

After mount control, a dedicated image-capture program is desirable for camera control, image download and storage of high bit-depth files. (It is also possible to get by, without specific image capture software, by using a standard camera with a simple cable or timer release.) Image-capture programs allow remote operation and have drivers for most dedicated astronomy cameras, webcams, Canon EOS and increasingly other leading brands of DSLR too. Many photographic cameras are enabled for tethered operation through their USB port.

In addition to the key functions to control exposure and download images to disk, capture programs display an image preview and provide essential data to gauge exposure, including maximum pixel values, noise levels, star FWHM and image histograms. They can also set and report the CCD temperature and filter wheel position. Many dedicated image-capture programs have a function to disable guiding and sensor cooling during the image download to reduce power supply interference and in turn, amplifier noise in the sensor. Interruptions during a USB image download may show banding on some sensors.

Image processing requires several sequential functions that are performed by the capture software, dedicated image processing software or a standard photographic product like Photoshop. An imaging session produces multiple exposures either in color or as separate monochrome files. Processing these images is a long journey. The key image processing steps are calibration, alignment, stacking and a group of functions that collectively enhance the image. Calibration, alignment and stacking functions are included in several image-capture programs or available as a separate utility. Image enhancement functions also reside in many astronomy programs as well as Photoshop. Here again, many functions overlap between application types and the user has a choice of how to process an image. Photoshop is a universal tool in photography but image calibration, alignment, stacking and advanced processing techniques require dedicated programs. Image enhancement is a huge subject and worthy of its own book and it is important to realize that there is no one way to process an image: Each application has a range of tools for reducing noise, sharpening, enhancing faint detail, improving color saturation and so on, some of which will be more or less effective for your particular image than a similar function in another application. It is an area for patient experimentation on a cloudy day.

There are several utility functions that are particularly helpful in a portable setup; polar scope alignment, autofocus and plate solving functions. I use a small iPad application to determine my location and time via GPS and display the hour angle of Polaris to set up the polar scope. I can polar align my mount in a few minutes and quickly move onto general star alignment.

Of those systems with an electronic focuser, the remote focus application can be standalone or imbedded in the image-capture software. They obtain consistent and accurate focus without the need to touch the focuser. They can also quickly apply a predetermined focus offset to compensate for a change in temperature or filter selection, or perform an autofocus routine between exposures. Some of the recent developments achieve highly accurate focus by using software to analyze the diffraction pattern of a bright star taken through a focus mask.

Plate-solving is a small luxury that delivers extremely quick and accurate alignment. Depending on its implementation in the application, this function provides benefits all-round; precise mount alignment without the necessity of centering alignment stars, effortless mount re-alignment to a prior image, precise automatic image stacking and supernova detection. A plate solving routine correlates the stars in an image with a star catalog of the general area and calculates the precise image center, rotation and scale in arc seconds per pixel. It is smugly satisfying on a cold night to simply polar-align the scope, retire to the couch and watch the telescope automatically slew, expose and self calibrate its alignment with a dozen stars. The satisfaction grows when a slew to the object places it dead center in the image, ready for focus confirmation, autoguider calibration and a night of imaging. It is quite surprising just how useful plate solving is in general astro-photography and although some might claim it to be an indulgence, for me it was important enough to make the move from OSX to Windows 7, to enable effective remote control with a minimum of time at the scope. (TheSkyX uniquely features plate solving in its OSX version.)

Cleanliness

In the days of film-based photography and darkrooms, with a little care, one could avoid the need to “spot” a darkroom print. Those precautions are still valid today and in digital photography, we need to be equally careful to avoid dirt on the sensor or optics. Astrophotography is more demanding; a dim dust spot on the original image becomes a black hole after image manipulation. The calibration techniques (described later on) that compensate for variations across the sensor can also improve the appearance of dust spots. These techniques work up to a point but are ineffective if the dust pattern changes between imaging and calibration.

A dust speck becomes smaller and more pronounced the closer it is to the sensor and indeed the shadow diameter can infer its position along the optical path. It is easy enough to inspect and clean optics and filters but sensors need more care. Cleaning a sensor is not without risk; they are delicate devices that can be ruined from the wrong cleaning technique. SLR cameras have a mode that expose their sensor for cleaning but a dedicated astrophotography CCD sensor is often inaccessible and requires disassembly to gain access, which may invalidate its warranty. A few dust specs are normal but excessive dust within a sealed camera unit may require a service by the manufacturer. If you do choose to clean your own, proceed with care, check the available resources on how to proceed and observe the normal precautions of a clean environment, natural fiber clothing and static electricity protection. (Some sensors are fitted in a sealed cavity, filled with dry argon gas and cannot be opened without causing potential condensation issues.)

fig.13 CCD sensors used in astrophotography vary considerably in size and will give a different field of view with the same optics. This full scale depiction illustrates some of those commonly used in astrophotography cameras.

A few sensible precautions reduce the chance of dust build-up. In a portable setup, the camera / filter / telescope are frequently disassembled for transport. Any optical surfaces should only be exposed in a dust-free environment and capped as quickly as possible. It also helps to cap the telescope at both ends and store small items in a clean container with a bag of desiccant. Many cameras have threaded adaptors and protective caps. I find push-on plastic caps are lacerated by the end threads. I use threaded dust caps for C and T-mounts to avoid this potential source of contamination. Screw in covers for T-threads are quite common and an Internet search will locate a source for the less common C-mount caps.

Sensor Size

Camera choice is ever increasing. More DSLRs, mirrorless interchangeable lens cameras, astronomical CCDs and video cameras appear each year. As far as the context of this chapter is concerned, whichever camera you choose, it will have pros and cons. Apart from cost, your choice (or choices), should consider a number of things. One of the surprises to many is the need for a wide field of view to image large nebula and galaxies.

The ratio of sensor size to focal length determines the field of view and although full size (35 mm) digital cameras are now common, many telescopes are unable to produce an image circle large enough to cover a full frame sensor, or do so with significant shading (vignetting) at the corners. Most telescopes have an image circle that covers the smaller APS-C sensor size. Many of the popular deep sky objects (for example, the Messier objects) will only occupy a part of the sensor area and an image will require cropping for aesthetic reasons. Conversely, very large objects can be imaged by photographing and assembling a mosaic of several partially overlapping exposures, but this takes a lot more time and effort. If you wish to try out different things and if funds allow, two sensor and telescope sizes will broaden the range of possible deep sky subjects, providing the telescope’s image circle covers the larger sensor. This is where a telephoto lens on an SLR comes in handy or a medium format camera lens adapted to fit a CCD for those few wide-field shots.

Pixel Size

The sensor pixel size (pitch) affects image resolution and can limit the overall optical resolution of the combined optical system and prevailing seeing conditions. If the pitch is too large, the resolution is compromised, too small and fewer photons will land on each pixel for a given exposure, with the outcome that electrical noise is more noticeable. Large megapixel counts sell consumer cameras but in the case of astrophotography, less may be more, as the pitch has to decrease to accommodate more pixels within the same area. In general terms, for a sensor to resolve the light falling on it, with small optics the sensor’s pitch should be about 1/2 that of the telescope’s diffraction limit (FWHM) and for larger telescopes, about 1/3 of the typical seeing conditions. The table in fig.14 suggests the theoretical pixel pitch to match the resolution of some focal length and aperture combinations. In practice, using a sensor with a larger pitch size (or “binning” a sensor with a small pitch) can improve image noise and lower resolution.

Sensitivity

The sensitivity or quantum efficiency of a sensor is a measure of its ability to convert photons into electrons. It changes between models and varies with wavelength. High efficiencies are obviously desirable for visible wavelengths and especially, in the case of deep sky objects, deep red wavelengths associated with Hydrogen Alpha (Hα) emission nebula. Typical peak values for sensitive CCDs are in the range of 50–75%. Many popular SLRs have built-in infrared cut-off filters that reduce their Hα sensitivity to about 20%, although these filters can be replaced or modified by third-party specialist companies to improve Hα sensitivity. The benefit of a high efficiency is twofold; shorter exposures and better image noise from a given imaging session.

Image Noise Reduction

Sensors are not perfect and teasing faint details from an image’s background places extreme demands on the initial file quality. Of the various imperfections, sensor noise is the most detrimental to image quality. If an exposure has a poor signal to noise ratio, it will show up clearly in the final manipulated image. Noise refers to any additional unwanted image-forming signal and is typically made up of constant and random elements. There are several sources of noise in a camera system, each of which has a unique effect and remedy. The two most significant types of sensor noise are thermal (dark) noise and read noise.

I have to digress to explain a little about noise, even though it is explained in a later chapter. Thermal noise adds random electrons to each sensor pixel. The average count increases linearly with time and temperature and its randomness also increases. In many sensors, the electron rate per second doubles for each 6–8°C increase. To combat this many imaging CCDs have cooling systems that lower the temperature by 20–40°C. This significantly reduces the average and random level of non image-forming electrons. (In comparison, consumer cameras do not have cooled sensors and their electronics warm up with extended use.) The average level of thermal noise can be removed from an image by subtracting a dark-frame (an image taken with the lens cap on) using the same exposure duration and sensor temperature. Unfortunately this does not remove the random thermal noise.

Read noise originates in the interface electronics and is present in every exposure, including zero-length exposures. Again, there is random and constant element. The random noise level is a critical specification for astronomy CCD cameras and as discussed in the prior chapter, low levels increase the effective dynamic range. The constant element is called bias noise and like dark noise, can be subtracted. Out of camera, the image forming photons themselves have noise, referred to as shot noise.

The key to remove any random noise is to take multiple exposures and combine them. Combining (averaging) the results from multiple events reduces the randomness. It is standard practice to acquire and combine multiple exposures to reduce random image noise. For each doubling of the exposure count, the noise of the averaged images reduces by 40%.

So, to reduce all forms of image noise requires a series of calibration and averaging processes:

1 average multiple dark-frames to isolate the mean temperature and duration contribution for each pixel

2 average multiple zero-length exposures to isolate the mean electronic circuit difference (bias) between pixels

3 acquire multiple image exposures

4 subtract (calibrate) the mean effects from each image frame, pixel by pixel

5 average multiple, calibrated image frames to reduce the random noise, (think smooth)

The sensors and exposure chapter, as well as the one on image calibration explain things in more detail.

Calibration

Image calibration improves image quality as part of the process to reduce image noise. In addition to reducing thermal and circuit noise, it also addresses a third source of imperfection caused by the small gain variations between pixels. Each pixel signal is amplified and sampled into a digital signal and small variations, including the effect of optical transmission, dust spots and reflections affect the perceived signal strength. Luckily, this is measurable from another set of exposures, this time taken of a uniformly lit target. These image “flats” are used in calibration to normalize each exposure by applying a correction factor, rather than an offset, for each pixel.

Image calibration requires some effort on the part of the astrophotographer and makes good use of a cloudy day. It requires many exposures to measure the average image errors for bias noise, dark current and the system gain for each pixel on the sensor. A complete set of calibration files may take several days to generate but can be reused, with care, for many imaging sessions with the same optical setup. These files are statistically averaged and the calibration process uses these image files to subtract and normalize the pixel anomalies within each of the image files before alignment and combining (stacking).

fig.15 This filter wheel (shown with the cover removed) is inserted between the field-flattener and the sensor. A USB-controlled motor positions each filter over the aperture. An off-axis guider mirror can be seen behind the UV/IR filter. The guider camera is screwed in place of the silver cap on the right.

Camera Filtration

Color images require color filtration in one form or another since all photo sensors are monochromatic. Color cameras achieve this with a fixed RGB color filter mosaic (Bayer array) precisely aligned in front of the sensor elements. An unfiltered sensor requires three separate exposures, taken through colored filters, one at a time, to form a color image. A common solution is to fix the filters in a carousel or filter wheel directly in front of the sensor, a kind of cosmic disco light. Later, combining the three separate images during image processing forms a color image. In the latter case, separate filtration provides additional flexibility when we replace the common red, green and blue filters with specific narrowband filters, tuned to the common nebula emission wavelengths.

Filters can also reduce the effect of light pollution. Light pollution comes from all light sources, the most prevalent of which are from low-pressure sodium and mercury-vapor street lamps. Luckily, these sources emit distinct wavelengths that do not correspond to the nebula emissions but do unfortunately adversely affect images taken with sensors fitted with Bayer arrays. Light pollution filters block the majority of these unwanted wavelengths but transmit the remaining visible spectrum. They are not a perfect solution and require careful color correction during image processing. Separate RGB filters and narrowband filters also minimize the effect of common light pollution wavelengths by using carefully selected transmission characteristics. In particular the red and green filter pass-bands exclude the yellow sodium-lamp wavelength.

Interface and Software Support

Modern astrophotography CCDs use USB 2.0 interfaces and come with Microsoft Windows drivers. In addition, there may be an ASCOM driver for the camera and additionally, dedicated driver support within the imaging software. ASCOM in their own words is “a many-to-many and language-independent architecture, supported by most astronomy devices which connect to Windows computers.” In basic terms, standards help with inter-connectivity and reduce software development costs. There is no equivalent in Mac OSX but there is no reason why it could not be done. As such, hardware support in OSX is highly dependent upon the individual image-capture and planetarium software provider.

Consumer cameras are a mixed bag and some are capable of remote operation or image download through a USB interface. Although low cost T-adaptors exist for most camera bayonet mounts, remote triggers and compatible remote control software do not. In the case of my Fuji X cameras, although tethering software exists, it is awkward to use for long-exposure astrophotography.

Optics

The optical design of a telescope is quite different from the telephoto lenses we fit onto photographic cameras. Al-though they have longer focal lengths, those with simpler optics with a few elements are optimized for visual use but not for imaging onto a large sensor. (Photographic lenses are also optimized for a focusing distance of about 20x their focal length, whereas telescopes are optimized for infinity.) Most telescope designs, either based on glass optics (refractors) or mirrors (reflectors) require additional optical modules in the optical path to produce round, tightly focused stars from corner to corner on a flat sensor. When I started astrophotography, I incorrectly assumed the touted benefits of apochromatic and triplet designs implied excellent imaging quality too.

In the scope of this chapter the telescope choice is the sum of its parts: The principal characteristics, its aperture and focal length need to match the imaging field of view and the focuser design needs to be robust enough for imaging. As far as the optics are concerned, there are some differences between designs. Apochro-matic performance will reduce color fringing on color sensors though it is of lesser significance with narrow band or LRGB imaging. Some optical configurations are naturally better suited for imaging but in the case of many, a good telescope for visual use is not necessarily the last word in imaging.

fig.16 These three T2-adaptors for Fuji X, Micro 4/3 and M42 cameras have different lengths to compensate for the varying camera lens mount to sensor distance. They ensure the T-thread to sensor distance is the same 55 mm distance. Many field flatteners are deliberately optimized for a flange to sensor distance of 55 mm.

fig.17 The top two images were taken from a short (500 mm) focal-length APO refractor, without and with a field-flattener. The top image shows the effect of a curved focus-plane at the corner of an image, the image beneath has been optically corrected. The two 3D plots were generated by CCDInspector® software, which calculates the plane of focus from the shape of the stars in an image. The slight tilt in the plane of focus, which can be seen in the bottom plot, is likely to be attributable to a droop in the clamp and lightweight focus mechanism.

A standard refractor telescope has just two or three glass elements at the front end and irrespective of its optical prowess, focuses light onto a curved plane. When stars are in focus in the middle of a flat sensor, the stars near the edge are elongated and fuzzy (fig.17). A long focal length or small central sensor will be less sensitive to these issues but shorter scopes / larger sensors will show obvious focus issues and distortion at the image periphery. For imaging purposes, you need a special lens module, called a field-flattener, which is fitted just in front of the sensor. Field flatteners typically have two or three glass elements and have a generic optical design compatible with a range of focal lengths and apertures. Some advanced telescopes, (astrographs) incorporate these additional elements within the telescope body and are specifically optimized for astrophotography. Some field flatteners also change the effective focal length. Most are “reducers”, which shorten the effective focal length, give a wider field of view and improve the focal ratio. The most common have a power of about 0.8x. There are two other common sizes 0.63x and 0.33x especially suited for the longer focal lengths found on Schmidt-Cassegrain Telescopes (SCTs).

The distance between the field-flattener and the sensor is critical for optimum performance and it takes some experimentation to find the best position. The reward is sharp round stars across the entire image. The camera-end of a field-flattener is often fitted with a T(2)-thread that assumes the sensor spacing is 55 mm. There are many compatible camera adaptors (fig.16), which automatically maintain this sensor spacing but in the case of filter wheels and astronomical CCDs, some careful measurement and juggling with spacers is required to find the sweet spot.

In the case of most Newtonian reflector telescope designs, the principal imaging aberration is coma. As the name suggests, stars assume the appearance of little comets. The issue is more apparent at the image periphery and with faster focal ratios. This is not a flaw in the optical elements as dispersion is for glass optics; it is physically not possible to design a parabolic mirror that eliminates the problem. Again, larger sensors are most affected and in a similar fashion to the issues with simple refractors, a coma corrector lens reduces the effect when placed in front of the sensor. Some coma correctors are optimized for imaging and have additional lenses which improve the field flatness at the same time.

The Newtonian reflector is the granddaddy and there are many derivative designs, with folded light-paths and additional lenses at the top end or sometimes in the focus tube, to trade-off optical defects (aberrations). Some of these designs replace the parabolic and plane mirrors in a Newtonian with hyperbolic or spherical mirrors. Each is a compromise between economy and performance. As manufacturing methods improve, designs that were previously reserved for professional observatories have become affordable to the enthusiast. At one time it was the APO refractor, then the Schmidt Cassegrain, now it is the turn of the Ritchey Chrétien design, with its pair of hyperbolic mirrors, the most famous of which is that used in the Hubble Space Telescope. Some designs have wonderful double-barrelled names and are worthy astrographs and are described in greater detail later on. Depending on the configuration, a purchase may require an additional corrector lens to ensure pinpoint stars on large format sensors.

Setting Up and Note Taking

The setup process for a portable system is extensive. In my location, imaging opportunities are sometimes weeks apart, if not months and it is easy to forget an essential step that causes a hang-up later on. Using a simple checklist is a simple solution or, for more advanced users, set this all down into a small program (scripting language) that runs like a macro in Windows. Automation is fantastic until something goes wrong, so it is best to pursue that goal after you one is satisfied that the system is reliable during manual operation.

Note taking is a basic and useful activity. Although most imaging software includes essential information in the image file header, a few choice notes help with future setups. Some imaging software, such as Maxim DL and Sequence Generator Pro save setup “configurations”; storing all the hardware and software settings in one click. In the same manner, some autofocus programs do the same; focusing algorithms can re-use saved focus information and “V” curves from a prior session, assuming it has the same optical configuration.

In addition to manual note taking, most programs produce a log-file. These are stored as simple text files and are a useful resource for debugging problems and as a record of activity. Many software providers ask for a copy of the log file before they will actively debug code. In the case of autoguiding, log files document the telescope pointing error after each autoguider exposure. A number of utility programs can interpret this log to produce a periodic error characteristic for your telescope mount or recommend backlash and other settings.