Astrophotography for the Amateur How to Use a Computerized Telescope Celestial Objects for Modern Telescopes Digital SLR Astrophotography


Corrections, Updates, and Notes to the Second Edition

Actual corrections are in red.

(p. 16) Transferring files from one computer to another: One method I use is to copy them to a 32-GB SD card. However, the default file system (FAT32) will not accommodate files larger than 4 GB, so you must format the card as exFAT or NTFS instead (which also makes it faster). With exFAT there is a further problem that some Linux systems do not timestamp the files correctly when Daylight Saving Time is in effect. NTFS works reliably, but then the card cannot be used in digital cameras, only computers.

(p. 47) Nikon Manual Movie Settings: For "1/60" read "1/60 or 1/30."

This bug has not been fixed in D5300 firmware 1.01, 1.02, or 1.03. Manual Movie Settings still needs to be turned off when you are doing still photography with Live View.

(p. 82) Dark frames: When taking dark frames indoors, particularly for sensor testing, note that your lens cap may not be perfectly opaque to infrared light; also, a tiny amount of light can enter through the eyepiece. It is best to take dark frames in the dark.

(p. 158) Noises from mounts: One aspect of mount performance that I didn't mention is noise. Computerized mounts with servo or stepper motors can make strange noises and move in irregular jerks when slewing. Noises mean nothing unless there is actually a problem with tracking performance and the noise is correlated with it. Some mounts squeal, hum, buzz, creak, and alternately fall completely silent at unpredictable times while tracking. As for irregular jerks, when the mount slews from one place to another, an algorithm manages the motor acceleration, and sometimes the criterion for using a higher speed is fulfilled for only a fraction of a second. As long as the telescope ends up pointed correctly, the noises it made getting there are of no significance.

(p. 161) Backlash: Celestron tells me there can easily be 60 arc-seconds of backlash in the gearbox (not the worm gear); this is normal, and no adjustment of the worm gear will affect it. They also say that when the mount is deliberately unbalanced for better tracking, it is acceptable for the unbalance to be so large that the telescope actually moves when brakes are released.

(p. 166) When autoguiding with a color camera, bin the pixels 2×2 if possible, to eliminate the Bayer matrix.

(pp. 185-6) Alternative to lamp cord: The ideal power cable would be the same size (of wire and insulation) as lamp cord, but more flexible thanks to silicone insulation (rather than PVC) and wires made of many very thin strands. I've found a supplier of such wire, BNTECHGO, in China, and their product is available on Amazon at this link. Do not confuse it with other BNTECHGO products, nor with red and black "zip cord" from other sources.

(p. 189) Linux is better supported with every passing day: For greater long-term stability, many amateurs (including me) are migrating to Linux for telescope and camera control (a process I have nicknamed "defenestration"), and the available software is getting better rapidly. Open PHD Guiding, FireCapture (for video), and KSTARS (for star maps and camera control through INDI) have free Linux versions. PixInsight supports Linux and macOS as well as Windows.

(p. 189) Telescope controller need not be a laptop: If you're going to have a dedicated computer for your telescope, autoguider, and camera, it need not be a laptop. The latest trend is to use a very small computer whose virtual screen and keyboard are accessed by Wi-Fi from another computer, such as your regular laptop.

The small computer may be a Raspberry Pi or an Intel Compute Stick, but ready-to-use solutions are becoming commercially available. One is the Stellar Mate, much cheaper than a laptop. The computer is small enough to ride on the telescope, runs on 5 volts, comes pre-configured with suitable software, and totally frees you from worrying about whether your main computer will remain compatible with your equipment-control software.

(p. 202) PixInsight dark frame optimization should not be used if the sensor has appreciable amp glow. It assumes the dark frames are genuinely dark.

(p. 212) DeepSkyStacker calibration: For "On subsequent runs, it uses the master frame to save time" read "On subsequent runs, you can select the master frames rather than the whole set of calibration frames, to save time."

(p. 295-297, 302) Dynamic range as reported by PhotonsToPhotos is not the same as reported by DxOmark (even when derived from DxOmark data). PhotonsToPhotos uses a different SNR limit and produces values that are about 2.5 stops lower.

(p. 302) Formulae for dynamic range: For "Bias = the average (or preferably median) of the dark frame" read "Bias = the average (or preferably median) of the flat dark." There is no dark frame other that the flat dark already mentioned.

(p. 304) Here is Table 16.1 with a third-generation Canon added, the 200D.


I don't plan to keep adding to the table indefinitely, but this step in technological progress is noteworthy. The Canon 200D is ISOless but has more read noise than the Nikon D5300 — and we don't know if this reflects a real difference in sensor technology or if the Nikon performs some noise reduction in the camera before writing the raw file. In general, past experience is that Canon raw images are "rawer."

(p. 308) Filter modification can bring out chromatic aberration in lenses. This point needs to be emphasized. With my Nikon 180/2.8 ED IF (non-AF) lens on a modified camera, the stars have bright red haloes. With some other lenses, it is necessary to "focus out" the red haloes, and the star images are not quite as compact as with an unmodified camera.

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Last revised 2018 October 18