Warning: This is a long and detailed post!
Hi all. This is my first post here on Stargazers lounge. I have been on CloudyNights for some time and love the forum. I created a thread regarding this issue on CN but was never able to get the help I needed to fix the problem, and the thread has been dead for some time now. So I am turning to you guys to see if I can finally get this figured out. CN thread is here: https://www.cloudynights.com/topic/878303-image-calibration-issues-or-problems-with-my-data/
Basically, I got a new telescope - the AstroTech AT115EDT - and am having an issue with a strange artifact/gradient when using the scope. I only experience the issue when imaging in LRGB, not in narrowband. Here is an image of what I see after performing a dynamic background extraction in PixInsight:
Things I have tried so far:
- Eliminating light leaks. I did initially have a light leak from my ZWO filter wheel that I have since eliminated. I am pretty confident that I have no more light leaks, as I can take long exposure darks at high gain in a fully illuminated room without any noticeable difference in the ADU values.
- Ensuring that it is not related to calibration issues. While it is a faint, I can see it even in my individual sub frames - especially after performing a DBE. Even without any sort of processing though, during acquisition even, I can see a faint outline of the gradient from the ASIAir preview on my iPad so I am certain that the artifact is not generated from incorrect calibration. I can also see this ring in my flat frames.
- New reducer. I contacted AstroTech and they agreed to send me a new reducer since I noticed that the problem didn't happen when I imaged without the reducer. This did not fix the problem - however, I think the fact that the problem is eliminated without the reducer may be a potential hint as to what is happening. I just don't quite know exactly what that hint would be
- New telescope. After lots of back and forth with AstroTech they agreed to send me a new telescope - very good customer service from them. Unfortunately this also did not solve the problem, so the issue _must_ be related to my equipment. What is also strange is I can not replicate this problem on my other telescope, the Redcat 71, with the same exact equipment. Potentially another hint...
- Removed the filter wheel and confirmed that the artifact still shows up without any filters - using only extensions to reach 55mm backfocus.
I am suspicious that it may be a reflection somewhere in the optical train, I just don't know where. I bought some adhesive flocking material and flocked the inside of the 16.5mm ZWO extension I am using. That also did not solve the problem. The dew shield on the scope is not reflective at all. It _does_ look a bit like there is a reflection when I look inside of the OTA of the telescope, however I can't imagine that AT would be selling telescopes that have reflective surfaces in the OTA, or that I somehow managed to get 2 defective telescopes.
As you can imagine I am pretty frustrated. I really don't know what else to do or try here. Maybe it is a byproduct of the environment I am in, high bortle zone (7/8 ?) and some stray light coming into my backyard from other sources. Maybe these are causing the problem I am seeing. I guess I could take all the equipment to a dark site and see if the problem persists... /shrug
Any help is greatly appreciated. It is also worth reading through the CN thread as there is also some information there. For what it's worth, I really like my AT115EDT - I just want to get everything working properly. I was still able to capture some nice data of M51 with the scope, and crop it heavily enough that the artifact is not in the image:
The relevant equipment is as follows:
- AT115EDT with dedicated 0.8x reducer/flattener
- 16.5mm ZWO extension tube
- ZWO 36mm 7x filter wheel
- Antlia VPRO LRGB filters, Antlia 3nm SHO filters
- ZWO ASI2600mm
Thanks to everyone who has made it this far and to those who are willing to help!
Edited to include a single light frame, flat, dark, and flat-dark.
Light_M51_60.0s_Bin1_L_gain0_20230620-213554_0.2C_0001.fit
Flat_808.3ms_Bin1_L_gain0_20230620-235855_0.2C_0001.fit
Dark_808.3ms_Bin1_S_gain0_20230621-000446_0.0C_0001.fit
Dark_60.0s_Bin1_gain0_20230614-113619_0.0C_0018.fit