Internet Resources

Useful Formulae

Many relevant formulae are shown throughout the book in their respective chapters. This selection may come in useful too:

Autoguider Rate

This calculates the autoguider rate, in pixels per second, as required by many capture programs. The guide rate is the fraction of the sidereal rate:

Multiplying this by the minimum and maximum moves (seconds) in the guider settings provides the range of correction values from an autoguider cycle.

Polar Drift Rate

This calculates the drift rate in arc seconds for a known polar misalignment:

Polar Error

Conversely, this indicates the polar alignment error from a measured drift rate:

Sensor Read Noise Density

Sensor read noise is often quoted in electrons but ignores the pixel size. A more effective comparison between sensors is by normalizing the read noise to the pixel area (in microns2). This equation does the conversion:

Periodic Error

This calculates the periodic error in arc seconds for a given declination and pixel drift:

Critical Focus Zone

This alternative equation calculates the zone of acceptable focus for a given set of seeing conditions and uses a quality factor Q. Q is a measure of de-focus contribution in relation to the overall seeing conditions, expressed as a percentage (with a working value of 15%). F is the focal ratio.:

Coordinate Conversion from Ra/Dec to Alt/Az

(LAT=latitude, HA = hour angle, expressed in degrees)

When computing the Azimuth, a correction has to be made around the full circle:

If sin(HA) is negative, then AZ = A, otherwise AZ = 360 – A

Messier Objects – Suitable for Imaging from London Latitudes (~50°)

The first edition included a table of Messier objects above the imaging horizon (30° altitude or more at a London latitude) along with the month in which they first attain this altitude at dusk and the likely imaging time-span before each sets below 30°. Charles Bracken has since published The Astrophotography Sky Atlas, created specifically with imaging in mind. In this book he has taken this concept, born from the same initial thoughts of an aide memoir, and compiled extensive catalogs and sky charts, in a similar chronological arrangement. Many worthwhile objects are excluded from normal sky atlases, as they are too dim to observe visually. With extended exposures, however, these jewels become apparent. This book not only includes the common Messier objects but extensively maps out the Sharpless, RCW, van den Bergh, Abel and Hickson objects of imaging merit. It also provides seasonal imaging targets for other latitudes. As such, it is an excellent planning companion, especially to seek out less well-known objects or those with indistinct boundaries and I serve the reader better by making them aware of this book, rather than to reproduce a latitude-specific Messier table, confined to a few pages.

Imaging Record and Equipment Templates

Rather than break the back of this book by laying it flat to photocopy printed templates, the book’s support website has downloadable spreadsheets. These include templates to record imaging sessions and documenting essential data for a particular equipment configuration (which is often required by the imaging software for a quick and repeatable setup).

I keep an A5 logbook to record the settings for each imaging event. The record sheet is a digital alternative, that can also work as an A5 printout for reference. It is not the last word by any means and serves as a starting point for further adds and deletes. In both cases, the values in green cells are automatically calculated from other data and the sheet is customized by changing the data in the yellow cells. They were generated with Apple’s Numbers application for more universal appeal on PC, Mac and portable devices. (Anyone with an iCloud account can also view and edit these files using a browser.)