Issue with star elongation on my entire field Requests for constructive critique · Valentin JUNGBLUTH · ... · 4 · 239 · 3

Valentin,
The number one most important skill you need to become a successful imager is the skill of problem solving.   It’s good to come to these forums with questions and you’ll get a lot of help but in the long term, figuring out how to address a problem also helps to better define the question, which will ultimately help to produce a more rapid, helpful answer.  

Problem solving is hard but it’s a learned skill and it’s something that we all have to figure out.  You’ve done the first step, which is to spot the problem.  The second step is assess your setup.  How did you take the data, where was the data taken, when was it taken, what equipment did you use, etc. etc?   The third step is to come up with a theory for what might be causing the problem—in your case elongated stars.  Here are some that I can think of right off hand in the form of questions:

1) Is there is a tracking error?
2). Is there a large polar alignment error?
3)  Is there vibration in the system?
4)  Is there an optical problem in the way of aberration or stray light that could cause this error?
5) Is there mechanical flexure in the imaging train causing image shift?

The trick at this point is to come up with a way to test each one of these possibilities to either confirm or dismiss them as the cause.  Here you’ve said that the PA is good but without tracking turned on, you see immediate drift.  You should stop right there and realize that that means that PA is NOT good!  So…first thing:  Go fix it.  When the PA alignment is correct, you will not get immediate drift.

Next, check your tracking.  If you can’t go unguided for at least 30s - 60s without elongated star images, check your tracking rate.  Is tracking set to the correct sidereal rate?  What does your star tracking system say?  Turn off corrections going to the mount and watch the signal.  Is it oscillating or is is trending smoothly?  You may have a large periodic error and this is one way to detect it.

Is your mount in a stable location or do you have fans or other equipment on your OTA that could be producing vibration.  If so, can you turn things off and on to see what might be causing the problem?  Imaging on a balcony (for example) can easily transfer building vibrations into your results.

The bottom line is that problems like elongated stars are common.  It’s a problem that everyone runs into either as a beginner or along the way even with a brand new, high dollar observatory telescope (been there done that!)  It’s fine to come to these forums for help, but I’d rather teach you how to fish than serve you a meal.  So…sit down and think of all of the things that could cause this problem and then think of a way to test each one so that you can find the problem and fix it.   Come back here when you run into a road block, carefully describe your gear and what you’ve done and you’ll get plenty of help.


John

How many sub frames did you take and how many of them show the error. That will tell you a lot right there. It's normal to have a few bad frames which you just to eliminate from your stack. Many things can cause that, guiding error, wind, RA tracking weirdness, unknowingly bumping the mount and so on…