Spoiled for Choice

The U.S., Japan, Russia and to a lesser extent Europe are the main players in optical design. The top-end equipment is often hand-crafted in the county of design but where there is potential for economy of scale, typically at the middle and lower end of the market, component manufacture is moved to the Far East. These components mostly comprise optical cells, castings, turned metal parts and labor-intensive assembly operations. In some cases, several companies will use the same optical cell, or an entire assembly will be re-badged with minor cosmetic changes. To achieve the required price-point, telescopes and mounts are value engineered and it is this rather than the actual source country that sets the quality level. This essential thrift affects component assembly tolerances and component finish. This is not altogether a bad thing, since it provides us with an opportunity to purchase a functionally OK part, at a discount price and spend time and resources tuning and upgrading it as required. As with most things there is a law of diminishing returns and also a level beneath which the product is best avoided altogether.

Dealer Choice

Perhaps the most important part of a key purchase decision is who to buy it from. Dealer choice is an important consideration. The specialist dealers are often practicing astronomers with hands-on knowledge. These are more than box-shifters and give pragmatic advice depending on your needs. I’m a strong believer that if you value their service, they should have your business. In the UK at least, I have found the dealers to be refreshingly collaborative and an enquiry with one will lead to another that has stock or deals with a different brand that better suits your needs. On several occasions this advice has made me rethink my requirements and avoided costly equipment overkill. This is a very different experience from those special deals one finds in a newspaper advert or ordering from a catalog, which sells everything from furnishings to electronics. Small telescopes are increasingly available in photographic stores too but these retailers are less likely to offer meaningful advice or after-sales help. Astrophotog-raphy is still testing the boundaries of feasible economy and customer equipment-tuning and adjustment is part of the game. It is generally not a plug’n’play experience, as it is with computer equipment and digital cameras. A bargain is not a bargain if you make the wrong choice or you are unable to use a product to its full potential. As a result, the experience and backing of your dealer easily outweigh their premium over a discount warehouse or grey import deal.

Specifications and Misleading Claims

As the customer base becomes more educated they are, at the same time, more susceptible to the allure of “badges”, especially at the lower price points. The potential buyer should be aware that the execution (by which I mean design and implementation) of a feature is as important as the feature itself. Telescopes with fancy glass names or designated as “APO” may not perform as well as a well-executed simpler design. The same applies to mount design. For the astrophotographer, there is little merit if a mount with an integrated handset and an amazing deep sky database is not capable of slewing and tracking a star accurately under load.">well-executed simpler design. The same applies to mount design. For the astrophotographer, there is little merit if a mount with an integrated handset and an amazing deep sky database is not capable of slewing and tracking a star accurately under load.

fig. 1This slightly tongue-in-cheek table of suggested systems classifies the level of sophistication and cost from aspiring to committed astrophotographers. The pricing is a conservative guide only and assumes a mix of new and used equipment and is not dissimilar to enthusiast and professional DSLR outfits. This can be a starting point to help plan the overall budget for your setup.

In essence, the would-be buyer has to do some background research; the on-line forums are a good place to discover if there any obvious equipment short-comings and at the same time, the bargains. Since everything has to come together to make a successful photograph, an Internet image search for say a globular cluster and the model name will show what is possible. For instance, some otherwise optically excellent telescopes are let down by their focuser. It is not uncommon to find an upgraded used model in which the improvements have already been implemented by the first owner. Purchasing used models and accessories in general can be very useful but keep in mind that refractor models age better than mirror based designs. On-line astronomy classifieds do provide some measure of seller feedback, as does eBay, but rogue sellers do exist. Face-to-face selling is preferable, especially if the equipment is delicate and prone to damage from shipping, usually as a result of the original packaging having been discarded.

German Equatorial Mount

This is perhaps the most common all-purpose mount for amateurs. In this case, “amateurs” refers to small to medium mounts; the very largest professional observatories use fork mounts and camera rotators for economic and practical reasons. The GEM is compatible with both refractor and reflector telescope designs and has the useful capability of simple polar alignment through a polar scope, aligned with its motor axis. An advantage of this simple polar alignment is that the tripod does not have to be precisely levelled and orientated towards the celestial pole. The telescope attaches using a dovetail adaptor plate that come in two standard sizes; Vixen (31 mm wide) and Losmandy (75 mm wide). Since the mount, tripod and telescope are separate, the smaller, lighter components also facilitate transport and storage. Price, size and mass vary enormously; the lower-end models may use low-pressure aluminum castings, plastic and mild steel fixings whereas the high-end models often are machined from solid alloy, use ceramic bearings, hardened stainless fixings and precision gearing. Belt, friction and gear drives are used by different companies and advanced features such as optical encoders fitted to the motor axis can improve tracking and pointing accuracy as a result of immediate closed-loop position feedback. There are two physical drawbacks of the GEM design: imaging through the meridian and leg clashes.

fig.2 This figure shows the main features of a German Equatorial Mount or GEM, with a refractor set up for visual astronomy in which the control and power cables have been disconnected for clarity. It is sitting on a pier extension (though not actually required with this short refractor) to avoid the back end of the telescope touching the tripod legs when the telescope is aimed close to the meridian. In this design, the polar scope aligns to Polaris through the hollow RA motor axis.

Meridian Flips and Leg Clashes

During normal tracking, the main body of the GEM rotates around the RA axis. At the point at which the counterbalance shaft is horizontal and pointing east, the telescope will be pointing due south or north (at the meridian) depending on its declination setting. (In the Southern Hemisphere, the counterbalance shaft points west.) As the mount continues to track, the camera-end of the telescope continues to lower and may collide with the tripod legs at high altitude. To avoid this, the mount controller detects when the telescope crosses the meridian and flips the telescope 180° in both the DEC and RA axes, so that it is pointing to the same spot before the flip but from the opposite side of the mount.

For the user, this poses a few issues that need intervention; “the same spot” may be a little way off, the camera image is rotated 180 degrees and an auto guider system requires its sense of direction corrected. In practice, most GEM mounts will continue to work without issue past the meridian for a few degrees and in some systems the user can set a precise limit, before any touch condition can occur. The resumed image sequence may require a small adjustment to re-center the object and the guider software told to flip the guider polarities on one or more axis, depending on the guider configuration. Realignment may take several minutes and require repeated slews and exposures, or more swiftly using plate solving to establish a pointing correction.

With bulky telescopes, leg clashes may occur just before the telescope reaches the meridian, especially on mounts that are fitted directly to a tripod. A pier extension can lift the scope away from a leg obstruction and a permanent or mobile pier support allows an even greater freedom of movement without fear of obstruction.

Other GEM Considerations

Selecting a GEM model can be quite tricky and may be an expensive mistake. Appearances are skin-deep and unlike a telescope, how do you evaluate and rate the claims of the internal mechanical performance? Budget is only a guide; every model, no matter how expensive, will have different user experiences. In the last few years there have been a growing number of competent mounts in the £700–£1500 range that compete with the old favorite, the SkyWatcher NEQ6. At the other end of the range, above £4000, there is less consensus between users. At this level users have very specific needs and there are distinct genres; the established models (typically from the U.S. and Japan) and the more radical offerings (Europe). Whatever the price, the mechanism sets the performance; comparisons of weight, maximum load and mounting options are a good place to start. That being said, the thorny subject of software reliability (firmware and application) is also a major consideration for imagers and more difficult to quantify. Maximum load specifications should be treated with caution: The maximum load for imaging purposes will be less than that for visual use and the supplier’s specification may include the counterweights as well as the optical system. Some designs are more sensitive to accurate telescope balancing than others, which pop up later as a tracking or guiding problem. The lightweight designs may be sensitive to cable drag or external disturbances, for the same reason.

fig.3 This pier extension on a Berlebach ash tripod is a reasonable compromise so long as it can be secured without flexure. The tripod legs lock with excellent stability and the metal extension lifts the mount , reducing the interference between the filter wheel on my long refractor and the legs.

The obvious purchasing consideration is the precision of the mount; periodic error, backlash and positioning accuracy. Any particular model’s specification is a starting point. This is usually a range of values according to the manufacturing tolerances, with tighter control and less variation in the high-end mounts. There are several drive mechanisms to choose from; friction, belt and gear, each with their supporters. Some companies use stepper motors and others use DC brush-less motors with positional feedback. The implementation, however, is as important as the design architecture (just as there are awful carbon-fiber bicycle frames and fantastic steel frames). For instance, gear systems have less flexure but the mechanical interfaces have potential for backlash and periodic error, the opposite of toothed belt-drives. A few companies use friction drives; an interesting solution, similar to a rim-driven record turntable. These work on the principle of large diameter metal disk driven by a small plain-metal wheel connected to a DC motor. Since there is no actual coupling, as with a gear or belt system, it requires precision optical encoders on each axis to give position feedback. As a result, the motor control is more complicated and expensive but corrects for its own issues, including changes over time. (This simple metal to metal friction interface is not unique, it is the basis of the popular Crayford focuser mechanism too.) The software implementation for these advanced mechanisms is a key part of the system performance. The mount’s own control loop has to accurately track the telescope on one or both axes and at the same time react to guiding commands and dither and track accurately without excessive delay or overshoot. This is not an easy task and from my own experience, to get the best from these advanced systems requires a rigid, permanent installation and to operate without guiding.

Mounts with low periodic error are sought after and highly regarded. Even when autoguiding, it is equally important that the mount does not exhibit sudden changes in periodic error, since these errors are difficult to measure and correct within a few exposure cycles. Sudden large changes to PE are often attributable to grit trapped in a gear system’s lubricant and more confident users will strip, clean and carefully lubricate their gear system to improve its performance; indeed some mount designs have easy gear access for user-servicing and upgrading.

Weight is not the only consideration for portable use. Some mounts do not have a provision for a polar scope, to facilitate quick alignment, others have no handset and require a PC to run (although this is less of an issue for the astrophotographer, it is convenient to leave the computer at home for observing only). At the other extreme, some models are capable of remote operation, via an Internet connection and maintain their alignment by using precision shaft encoders which determine the mounts absolute position. These can maintain sub-arc second tracking and alignment without manual intervention on a permanent remote site.

It is an interesting time for the buyer right now. In the higher price bracket, novel designs (Gemini, Mesu, 10Micron and Avalon) are challenging the established standard precision belt or gear-driven worm-drive (As-troPhysics, Losmandy, Takahashi and Paramount). It is very much a buyer’s market and the Internet forums are buzzing with opinion and discussion, perfect to while away those cloudy evenings. In the £1,000 bracket, SkyWatcher iOptron models dominate the market.

fig.4 This Meade LX200 is a typical example of an integrated telescope and fork mount (sometimes called an Alt-Az mount). Both motor axes are employed to track stars and although precise guiding can be accomplished (through say the top-mounted refractor) there is a risk of field rotation and elongated stars in the image.

Fork and Wedge

Unlike the GEM, a fork mount is often a permanently connected part of a telescope system, typically a Schmidt Cassegrain derivative, an example of which is shown in fig.4. In its standard configuration, the fork mount moves in altitude and azimuth and is not orientated towards the celestial pole. Unlike a GEM, this type of mount requires accurate levelling and alignment with true north to track stars accurately. Even if it does, it moves along horizontal and vertical axis. The stars in the meantime endlessly wheel around the pole celestial pole and during a long exposure, stars will show some field rotation. This elongation increases with distance from the tracked star (typically the one used for guiding) and proximity to the celestial pole. There are two ways to correct for field rotation; precisely rotate the camera during the exposure or align the fork mount axis with the celestial pole using a wedge. Most opt for a wedge (shown in fig.5) which, assuming the base is level, inclines the mount by your current latitude to align the azimuth axis with the celestial pole. Wedges are under considerable load in the mount assembly. It goes without saying that the more substantial versions are more stable and several third-party vendors offer heavyweight upgrades. I found the lightweight one in fig.5 to be a little springy under load.

Unlike a GEM system, a fork mount can track through the meridian but this architecture is not without its own unique drawbacks; the combined weight of the telescope and motorized fork mount make portability an issue and a fork mount may not image near the pole, since at high declinations a collision is likely between the camera system and the fork arms and base. In a similar manner to setting meridian limits, upper DEC limits are set in the system software to avoid system damage, typically at about 75 to 80° in DEC. In another twist, a Hyperstar® system attaches a color camera at the front of a folded reflector telescope, in place of the secondary reflector and blanks off the rear optical port. This allows full freedom of movement, albeit with a shorter effective focal length and an equally faster aperture.

In terms of product choice, the purchase decision has to consider the combined merits of optics and mount, typically between models in Meade’s and Celestron’s range. In each case, the mount is designed and scaled to suit the size and specification of the optics. (Schmidt Cassegrain telescopes are also available as an optical tube assembly or OTA, for mounting on an equatorial mount.)

Tripods and Piers

Cost-conscious designs often target the unexciting tripod for compromise. Light-weight models packaged with a mount benefit from upgrading: there are several companies that make piers and tripods (in aluminum, carbon fiber, steel and ash) supplied with adaptor plates to suit the popular mounts. The better models have firm clamps on the adjustable legs, leg bracing and tight leg-to-mounting plate interfaces. The heavier models may have screw-adjusted feet for accurate levelling. Several companies choose ash hardwood for the legs, as this has a natural ability to absorb vibration. The simplest test is to firmly grip and man-handle the tripod’s mounting plate; the tripod should remain firm and while viewing, the effect of a small tap on the telescope should dissipate within a few seconds. Some tripods appear strangely short: For imaging purposes, the telescope height is of little concern and a tripod is more stable when its leg extension is at a minimum, providing of course that it does not topple over during the process of attaching the telescope and counterweights.

fig.5 The scope in fig.4 has been mounted on an equatorial wedge, which inclines the mount by the observer’s latitude, to alignmen with the celestial pole. At high declination settings a bulky camera or diagonal and eyepiece may collide with the fork arms or base but, unlike the setup in fig.4, images do not suffer from field rotation.

The base makes a difference too. In a portable setup soft ground can yield over time and balconies or decking may suffer from vibrations, say from passing traffic. In the case of soft ground, a concrete paving slab placed under each foot spreads the load and keeps the system stable. Similarly the concrete pier foundation in an observatory installation should be isolated from the floor of the observatory. Observatories not only provide convenience and a permanent installation but offer some shelter from the wind too. A mobile windbreak or strategically placed car may equally provide some shelter from a prevailing breeze in a temporary setup.

Most mounts are designed with damp outdoor use in mind and use stainless steel fixings and aluminum throughout. Hardware is one of those things that succumb to economy; for instance, the popularity of after-market hardened stainless steel adjustment Alt/Az bolts for the SkyWatcher EQ6 mount tells its own story. There may be other occasions too when a little “finishing off” can improve a value-engineered product, to improve its performance and protect your investment; one that comes to mind is to upgrade the dovetail clamp to a machined rather than cast design that offers better support.

Glass Versus Mirror

Glass and mirror systems have unique pros and cons. When light reflects off a mirrored surface, all colors reflect along the same path and the resultant image has no colored fringes (chromatic aberration) around the stars. Not all the light is reflected and there is some diffraction from the mirror edge. A simple glass optic bends (refracts) light, with some transmission losses, diffraction and additionally suffers from an optical property, dispersion, that causes different colors of light to bend by different amounts, producing chromatic aberration. (A simple lens will bend blue light more than red light.) This may appear as one-nil to the reflector but the image formed by a parabolic mirror, commonly found in a Newtonian reflector, has its own unique optical defect, coma, which is increasingly obvious the further way you get from the image center.

The image plane of a telescope is an imaginary surface upon which the stars come to focus. Ideally, this should be flat and coincident with the imaging sensor. Unfortunately, both systems have curved image planes and both require optical correction to make them suitable for high-quality imaging onto a large sensor. These issues can be minimized by introducing an additional lens or mirror elements or a combination of both into the optical system.

fig.7 These telescopes essentially are as long as the focal length of the design. At the top is a simple refractor doublet design, in this case set up for visual use. Beneath that is a triplet design, using positive and negative lens elements with different dispersion characteristics to minimize chromatic aberration. A field-flattener lens is inserted before the camera to ensure a flat focus plane across the whole sensor area. The three Newtonian designs show increasing sophistication: The classic simply uses a parabolic mirror but the bottom two are optimized for astrophotography and make use of a more economical spherical mirror and a glass corrector. These are designed for imaging and their focus point extends further outside the telescope body, which gives room to insert the motorized focuser, filter wheels and the camera system.

One of the interesting things about astrophotography is the demand placed upon the optics: Everyday photographic subjects are surprisingly forgiving of optical defects whereas small intense points of light show up every flaw. These demands multiply with aperture; as the glass or mirror-area doubles, the quality control becomes increasingly difficult. Telescopes are also used at full aperture, unlike photographic lenses, which are often stopped down to improve image quality. (Interestingly, an image of a bright star using a stopped-down camera lens will show diffraction effects caused by the non-circular aperture.) Diffraction is also the culprit for the characteristic 4-point spike around bright stars on those reflector telescopes that use a secondary mirror support, or “spider” in the optical path. The physics of diffraction cause, in general, the simplest optical designs to have the highest image contrast. Since each optical boundary introduces a little more diffraction, a refractor has a higher image contrast than a Newtonian, which in turn is higher than the more complex folded reflector designs. Similarly, each optical surface fails to reflect or transmit 100% of the incident light, with the consequence of slightly dimmer and lower contrast images.

Refractor Designs

The first telescopes were refractors, made from a positive primary lens and a negative eyepiece lens (often using available spectacle lenses) and produced an upright image. These are attributed to Galileo, who improved on the implementation and used them to observe the moons of Jupiter. Today, we use the later Keplerian designs that use a positive eyepiece lens and produce an inverted virtual image. These designs produce a wider field of view, higher magnifications and longer eye relief (the distance from the eyepiece to the eye).

Today, multi-coated, multiple cemented or air-spaced elements replace the simple single-element primary and eyepiece lenses of the 1600s. Combinations of positive and negative elements with different dispersion characteristics reduce chromatic aberration in the doublet design and better still in the triplet designs, often tagged with the term APO. Aspherical elements, now common in photographic lenses, are appearing in some astronomy products too.

When used for imaging, refractors require further compound elements just in front of the sensor, to flatten the plane of focus, or modify the effective focal length. These “field flatteners” are normally a matched add-on lens, but in the case of astrographs, they are built into the telescope. Most refractor designs can image over an APSC sized sensor with minimum darkening of the corners (vignetting), the better ones cover full frame 35 mm or larger. This specification is referred to as the image circle diameter. The image circle is altered by the addition of a field-flattener or reducer.

Newtonian Designs

The layout of a Newtonian is optimized for visual use; the focus position is quite close to the telescope body and may not give enough room to insert coma correctors, filters and cameras. The open body of a Newtonian reflector cools down quickly but unprotected mirrors may also tarnish more quickly than in an enclosed design. Some Newtonians are optimized for imaging with a slightly shorter tube, to extend the focus position.

The Schmidt Newtonian design addresses some of these limitations. It has a sealed tube with a glass corrector element at the front upon which the secondary mirror is mounted. The primary mirror is now spherical, which is more economical and the deletion of the mirror spider removes the diffraction spikes on bright stars. The corrector plate reduces aberrations and produces a flatter field. These designs are ideally suited for a narrow field of view and as with the Newtonian it is necessary to check that in a particular model there is enough back-focus for imaging purposes (short-tube version).

The last of the sealed tube Newtonian designs is the Maksutov Newtonian, which further improves upon the Schmidt Newtonian, with lower aberrations and is arguably the best of the Newtonian designs. The meniscus corrector plate at the front of the telescope is thicker than the Schmidt Newtonian glass element with the possibility that if the telescope has been stored in a warm place it will take longer to cool down to ambient temperature. During the cooling period, the possibility of focus shift and air turbulence arise. In all of these designs, the primary mirror is fixed (in so much that it is not moved to achieve focus) and has screw adjustments to tilt the plane and precisely align (collimate) the optics to produce round stars. These same adjustments may need periodic correction or after transporting between sites. Some adjustments can be achieved visually using a defocused star, others may require a laser collimator to align the optical surfaces.

Folded Designs

Advanced folded telescope designs were once very expensive but since Meade and Celestron led the way with affordable designs, they have become increasingly popular. The optical tube assembly is very compact. A 2,000 mm, 200-mm aperture telescope is only 450 mm long and 6.5 kg, compared to a Newtonian, with the same aperture and half the focal length is 920 mm long and about 9 kg. Whereas all the Newtonian designs use a flat secondary mirror, the folded designs use a convex secondary to extend the focus point beyond the primary mirror. The primary mirror has a hole in its center, through which the image passes and has a long black tube, or baffle, to prevent light straying to where it is not wanted. These incur additional diffraction which lowers the image contrast slightly over simpler optical designs. These designs, with their large aperture and long focal lengths, excel at planetary imaging onto small-chipped video cameras.

fig.8 The schematics above and the table on the next page compare the folded telescope design concepts. The top two both use an economical spherical main mirror and are similar to their Newtonian namesakes, with Schmidt and meniscus glass corrector plates at the front and a secondary mirror attached to its rear face. In these designs, the principal difference is the secondary mirror is convex, which allows the folded geometry to work and the focus position to be behind the main mirror. These designs are commonly referred to as “CATs”, “SCTs” or catadioptric (simply meaning a combination of lens and mirror) and require a field-flattener lens for quality imaging onto a large sensor. The Ritchey Chrétien design at the bottom uses two hyperbolic mirrors. These are more expensive to make but the large image circle has significantly less coma than the others, although there is still some field curvature. There is no front corrector plate and the secondary mirror is supported on a spider, which generates diffraction spikes on stars. The best known implementation of an “RCT” is the Hubble Space Telescope. The Ricardi Honders and Harmer Wynne mentioned in the table opposite (not shown) are two less common variations that use slightly different configurations of glass and mirror elements, trading off aberrations for field of view and speed.

The Schmidt and Maksutov designs, as with their Newtonian cousins, offer increasing levels of refinement and lower aberrations. For high-quality imaging over a large sensor, a field-flattener is required. In these designs, many combine image reduction and field flattening, with the outcome that a 2,000 mm f/10 design transforms into a 1,260 mm f/6.3 or 660 mm f/3.3. The shorter focal lengths are more useful for imaging nebulas and the larger galaxies, with the added advantage of plenty of light gathering power. Some scopes with longer focal lengths achieve focus using two systems; a small external standard focus mechanism with limited travel and by moving the primary mirror back and forth along the optical path. This moving mirror can be an Achilles’ heel since it is very difficult to engineer a sliding mirror mechanism without introducing lateral play.

In practice, the coarse focusing is achieved with the primary mirror and then the mechanism is locked. Even locked, tiny amounts of mirror movement can cause the image to shift during long exposures. If the guiding system uses independent optics, an image shift in the main mirror can cause image quality problems. One solution is to use an off-axis guider, which monitors and corrects image shift through the imaging system from flexure and focusing.

The Ritchey Chrétien, (RCT or simply RC) is a specialized version of the folded reflector design that uses two relatively expensive hyperbolic mirrors to eliminate coma rather than use a glass compensator. Since, there is no front glass element, large aperture designs are feasible, suitable for professional observatories. The optical design is not free from field curvature and requires a field-flattener for imaging onto a large sensor. The Hubble Space Telescope is a RC design with a 2,400 mm mirror with a focal length of 57.6 m! A typical amateur 200 mm aperture telescope, with a focal length of 1,625 mm is only 450 mm long and 7.5 kg. These systems have a fixed primary mirror and use extension tubes or an adjustable secondary mirror to achieve focus.

Aperture Fever

It is a wonderful time to be an amateur astronomer. The choice and quality is fantastic and it is easy to overindulge. The thing to remember with all these remarkable instruments is that although larger apertures do enable visual observation of dimmer objects and shorter exposures, they do not necessarily provide better resolution in typical seeing conditions. Seeing conditions are a phenomenon occurring between air cells a few centimeters across and larger apertures have greater susceptibility. (The effect of turbulence within a telescope tube increases too with the number of times the light beam passes through it. Newtonian designs have two passes and the SCT family three.)

fig.9a A standard red dot finder, or RDF, which projects a variable intensity red led dot onto a window, a system that is tolerant of the viewer’s eye position.

Long focal lengths and high magnification amplify any issues, and at the same time, it is pertinent to remember that a larger aperture does nothing to overcome the effect of light pollution. Some texts suggest, in the UK at least, that imaging quality tails off above a 250 mm aperture. (A 250 mm aperture has a diffraction-limited resolution of about 0.5 arc seconds, equivalent to outstanding seeing conditions.) I don’t want to spoil your moment as you unpack your 300 mm aperture lens and if my words of “wisdom” may feel a little sober, I will finish on a positive note: A big aperture enables shorter and hence more sub-exposures in any given time and combined, they improve the signal to noise ratio. Subsequent image processing gives a cleaner result. There you go now, do you feel better?

Magnification

The magnification for a system is calculated by the ratio of the focal lengths:

There are useful limits to achievable magnification, based on the diffraction limit imposed by the telescope aperture and seeing conditions. The upper limit is approximately 2x the aperture (mm) but can be lower due to seeing conditions. A focal length may be too short for your telescope’s ability.

fig.10 This William Optics® 2″ diagonal, with 1.25″ eyepiece adaptor, fits into a 2″ eyepiece holder. The coupling can be unscrewed and replaced with a screw fitting for direct attachment to an SCT telescope.

Field of View (FOV)

Every eyepiece has an apparent field of view. The size and cost of an eyepiece increases with angle. The actual angle of view through a telescope is calculated with the simple formula:

The right combination of telescope and a wide angle eyepiece can give breathtaking views. Wide angle eyepieces have up to 100° apparent field of view.

fig.11 A selection of eyepieces from 6.5 to 32 mm focal length.

Exit Pupil

Exit pupil size is less obvious: We all know our eye’s pupil diameter is at its maximum in dark conditions but it also shrinks as we get older. If your system has a considerably larger exit pupil size than your own eye, much of its light output is wasted. A typical human adult pupil size is 5–6 mm and the exit pupil size of a telescope system is derived by the following equation, where D is the telescope aperture:

Although low magnification views are brighter, a lower limit is set by our pupil diameter. This is reached when the magnification is D(mm) / 6. In practice, with several telescopes and eyepieces, some eyepieces will be compatible with all and some of the shortest and longest focal lengths will not.

fig.12a The DewBuster™ controller has several temperature-regulated outputs and fixed power levels for multiple dew heater tapes.

Hobbyist Dew Heater Controllers

Dew controllers are a magnet for hobbyists. Various circuits exist using oscillators and switching transistors. It is relatively straightforward to make a variable pulse width oscillator and switch current with a power transistor or field effect transistor. An Internet search finds many “555” timer-based designs in a few seconds. The design issue that may not be apparent to non-electronic engineers is how to switch a few amps on and off without creating interference; both transmitted along the power lines and as radio frequency interference. Interference can create havoc with USB communications and CCD cameras (and some mounts, as I discovered). I required a simple low-cost controller to embed in an interface box. Rather than develop a printed circuit board from scratch I choose to modify an 80W PWM motor control module available from Amazon. This switches up to 3 amps with a switching frequency in the kilohertz. There are many poor designs that generate considerable electrical interference. I chose one whose photograph showed lots of surface mount components, including some output power resistors and diodes, to reduce interference.

Reducing interference occurs in two stages; avoidance and screening. The PWM module was a well designed CE-rated unit and a simple test, using an AM radio close by, did not detect radio frequency interference. I placed 10nF capacitors across the output for good measure and an inductive choke on the power feed to the unit to filter any high-frequency transients in the power line. One might think the next step is to filter the switching circuit, with say a capacitor on the gate of the transistor. This slows the switching slew rate down but at the same time this linear operation will increase the power dissipation in the transistor, for which it is not designed. A better solution is to place 1 watt resistor and series capacitor across the output help to dissipate any back voltage generated by the switching of an inductive load. My unit already had this on its circuit board.

The operating frequency of most commercial dew heaters is approximately 1 hertz or less and is determined by two main timing components, a resistor and capacitor. If you can identify them and the PWM circuit board allows, it may be possible to replace these with much larger values to lower its switching frequency. If you suspect the PWM unit is generating interference, placing the assembly in a die cast aluminum box and using shielded cables for both power and dew heater output reduces radiated radio frequency emissions. I initially routed standard dew heater cables away from sensitive USB or power lines but subsequently swapped over to microphone cable, which employs an independent grounded shield. This cable, with filtering at the connectors, is routed through my telescope mount with no apparent issue.

My system works well and there is no apparent difference in the noise level of images taken with it switched on or off. I cannot replicate the clever feature of the DewBuster, which accelerates warming to a threshold, so I switch my dew heater system on 10 minutes before imaging, with the lens cap in place. I keep an eye on the humidity and dew point and when the humidity tops 85%, I increase the temperature offset to about 8°C. Over 95%, atmospheric transparency reduces and impairs image contrast, and it is time to go to bed.