Why are stars near the edges trailing in my tracked astrophotos at 200mm with a 2x teleconverter?
Asked 4/10/2015
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I’m starting astrophotography with a Nikon D7100, Sigma 70-200mm f/2.8 APO, and a 2x teleconverter on an AstroTrac (unguided). I’ve reduced coma by stopping down, but stars away from the center still look slightly streaked instead of round, and this starts fairly close to the center.
The mount appears to be polar aligned well because stars in the center remain round even at about 3 minutes. I’ve also tried to rule out vibration and wind by using a remote release and mirror lock-up.
Is this likely field distortion, or is it more likely tracking error or something else? If so, what can I do to reduce it?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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Field distortion would be an overall blur nearer the edges rather than anything you could conceivably call a trail; the stars would simply be fuzzier round blobs.
You are using the equivalent of a 600mm lens (200mm times 1.5 (crop factor) times 2 (doubler)). That means that an arc-second is about half a pixel on your 24MP sensor. (The vertical field of view of a 600mm lens is 2°20', and you have 4000 vertical pixels.) The tracking unit you are using claims a typical peak-to-peak tracking error of 5 arc-seconds over a five minute period. That, in turn, means that you should expect about a 2-3 pixel tangential smearing with exposures in the range you're working at. Given that stars are still awfully small at 600mm (35mm equivalent), 2-3 pixels represents considerable distortion. To get better than that, you'd need to either shorten your exposures (by raising your ISO) or move up to a more sophisticated tracking system, perhaps one that uses real-time imaging of the pole star for self-correction (which will not be cheap).
Originally by user38275. Source · Licensed CC BY-SA 4.0
user38275
11y ago
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This is more likely tracking error than field distortion.
If it were field distortion, stars near the edges would usually look more like soft or misshapen blobs, not short directional trails. With your setup, the effective angle of view is very tight: 200mm on APS-C with a 2x teleconverter behaves roughly like a 600mm full-frame equivalent. At that focal length, even small tracking inaccuracies become very visible.
A typical AstroTrac tracking error can be enough to smear stars by a few pixels over multi-minute exposures, which is plenty to turn tiny star points into short streaks, especially away from the center.
To reduce it:
- use shorter sub-exposures
- improve tracking accuracy as much as possible
- avoid the 2x teleconverter if you can
- use a shorter focal length for easier unguided tracking
- continue stacking in DSS after capturing more, shorter frames
Stopping down can help coma, but it won’t fix mount tracking limitations.
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