You're making a mistake in thinking that polar aligning a mount to within 30 seconds is going to correct tracking errors. You need to be guiding with that rig. Polar alignment mostly corrects for DEC drift and not for tracking errors. Check to see if your elongation is in the RA or the DEC axis. My bet is on the RA axis. You can try PEC on the mount if it supports it. That corrects for some tracking errors.
So, I think that the apparent variance is almost certainly caused by environmental issues (wind, altitude, etc) and not by the TPPA. I use TPPA for all of my systems that range from 268 to 2540mm in focal length. Even on my systems a 30 second exposure can show elongated stars. I'm not seeing actual trailing in the images, just some egg shaped stars.
You can't determine anything, though, by photographing a bright star like Hamal for 30 seconds. Pick someplace in the Milky way where you have a large star field. Download a copy of ASTAP and use it to diagnose your issue. I would be shocked if you could get round stars at 30 seconds with that rig consistently.
Finally, consider that when you do TPPA it must be completed within 5 minutes or the numbers will be off. So, if you took longer than that without starting over the number you reached is probably inaccurate. Note also that alignment within a few arc minutes is all that you need to correct DEC drift if you guide the system.
Thank you very much for your analysis. I’m still a beginner in astrophotography and I am trying to learn as much as possible. The fact that you advise me to use guiding with this kind of setup is "comforting". Then you mention that even with your equipment you might end up with elongated stars when using exposures around 30 seconds. I’m waiting for the guiding system to arrive so I can try it out.
I would like to understand how to check whether the problem is on the DEC axis rather than the RA axis. I received this advice from other people as well, but I’m not sure how to verify it, I imagine by analyzing the images obtained…
The location from which I imaginig has many limitations. It’s a terrace of a building on a high floor in an urban zone, with a light-polluted sky and a limited view. Besides not being able to see Polaris, I can’t even see the south with the Milky Way. I can only get an unobstructed view from southwest to northwest (or from southeast to northeast from another terrace), and I’m also limited in altitude due to the building’s structure. This is why the other night I ended up in the area of Hamal, not because I wanted to photograph that very bright star, but for the star field around it, other than that I also had a 40% crescent Moon. Next time, I’ll try to find a more suitable star field, given my view.
I already use ASTAP as a plate solver within NINA, but I’m not sure how to use it to diagnose my problem, as you suggested…
I’m having a lot of trouble with NINA’s TPPA because of the two knobs on the CEM60 mount that have to be used simultaneously. Even when moving the knobs slowly and very carefully, always waiting for NINA to analyze the next image, I keep encountering very large error jumps. For example, starting from an instruction like “30″ error, move west,” I immediately end up with “1° error, move east,” making it difficult to achieve an acceptable result. I know that after about 5 minutes the plugin warns you to restart the alignment procedure and I always do. However, the last time I had set in the plugin’s parameters, an error tolerance of 0°, I noticed that the restart message no longer appeared, however I had restarted the procedure several times before finally obtaining that total error of 30″.
If you’re saying that once guiding is implemented it would be sufficient to have a total error of around one minute, that sounds good (if I understood correctly).
If you please could give me some further guidance on how to diagnose whether my problem is related to the DEC axis rather than the RA, and how to verify it with ASTAP, I would be very grateful.
Thanks again for your support,
Ciao, Biagio
