Why do stars in my tracked astrophotos have a small 'tail' or comet shape?
Asked 4/9/2026
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2 answers
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I shot a series of tracked astrophotos of the Flaming Star Nebula with a Canon EOS 70D, UHC filter, and Canon EF 100-400mm at 285mm, 8 seconds, f/5.6, ISO 2500. In nearly every frame, the stars are slightly distorted with a small tail pointing in the same direction, and some frames show a longer tail than others. I used a newer iOptron tracker, but I had seen similar effects with an older tracker too.
What usually causes this kind of star shape in astrophotography: tracking/alignment error, camera vibration, or a lens aberration such as coma?
Originally by Neppomuk. Source · Licensed CC BY-SA 4.0
Neppomuk
2mo ago
2 Answers
4
Using the "Rule of 600" at 285mm on a 1.6X crop camera, the maximum exposure without a star tracker that will not show star trailing when the full image is viewed at 8x10 inches is only 1.3 seconds.
You're viewing higher magnifications of a portion of the image while exposing for 8 seconds but are using a star tracker. It's quite possible your star tracker was not properly aligned with the celestial sphere, leading to drift during such a long exposure.
Given that the trails are much dimmer than the main part of each star, it's also entirely possible that vibration of some kind right at the beginning or end of exposure is the culprit. Are you using mirror lockup? Live View? If not, the movement of the mirror and/or shutter curtains may be causing enough of a vibration to produce the unwanted artifacts. Or maybe the gearing in your tracker has a dry spot that needs lubrication?
Originally by Michael C. Source · Licensed CC BY-SA 4.0
Michael C
2mo ago
0
Generated from our catalog & community — verify before relying on it.
The most likely causes are either slight tracking drift/vibration or lens coma, and the pattern can help separate them.
If the “tails” point the same way across the frame and vary from shot to shot, that suggests movement during exposure: slight polar misalignment, tracker drift, or vibration at the start/end of the exposure. Mirror or shutter shock can do this too, so using Live View and mirror lockup helps.
If the distortion gets worse farther from the center and looks like tiny comet-shaped stars off-axis, that is typical coma, a lens aberration. Coma is optical and won’t be fixed by changing trackers.
Since you’re at 285mm on a crop body, 8 seconds is demanding even with tracking, so small alignment errors can show up clearly when you inspect stars closely. In practice, check whether the shape is uniform across the whole frame (more likely motion) or increases away from the center (more likely coma). It may also be a combination of both.
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UniqueBot
AI2mo ago
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