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Bad Flat Calibration?
I've been battling what I believe is a flats calibration issue for weeks now and I am hoping some of the geniuses here can help me sort it. The issue is, on both of my 8" RC with IMX571 cameras, whenever I calibrate and stack Luminance data I end up with this ring around the center of the frame. Here is a calibrated stack that I did not perform any editing on. You can see a circle pattern in the middle of the frame. Generally the LPS tool does a decent job at getting rid but of it but I am trying to figure out the root cause.
Here is the pattern in question circled in blue as well as the master flat.
You can see the shadow of my secondary and spider vanes in the flat which makes me wonder if the pattern in the lights is due to over or under correction by the flats. I take my flats at 30k adu though I have tried going higher and lower (25-35k) and it made no difference. The lights are calibrated with bias and darks and all calibration settings are left to default. Fresh flats are taken at the end of every session. I use a dimmable USB drawing pad as my light source. I have been using this pad for over a year with 4 other telescopes and this problem has only started recently after I got new cameras (ASI2600mm and Hypercam 26m).
2 Rounds of LPS tool minimizes it but its still visble.
@regpratt Hi Reginald. What Luminance filter do you use with the ASI2600MM? Are you sure it blocks IR well enough for this camera?
@wvreeven I am using Astronomiks L2 on the 2600 and Baader UV-IR on the 26m. They both worked wonderfully on my refractors and all my previous cameras. It never occurred to me that they wouldn't work with these cameras. I have not see any information anyone having trouble blocking IR but I also haven't specifically looked for that information.
@regpratt It was just a thought. To my best of knowledge Astronimik filters are quite good though I did notice poor IR blockage myself with one filter in the past. I pointed a remote control to the telescope and took a picture with and without pressing a button on the remote and the difference was striking. This was with an ASI1600MM camera.
Apart from that, it is possible that the USB drawing pad doesn't emit the proper spectrum for the camera too. I usually take flats by pointing the telescope with my new ASI6200MM at a white wall that is illuminated by natural light coming in via windows and open doors. The light level can easily be managed by opening and closing curtains and doors. This may be difficult if you have a fixed set up but a white wall or a large white sheet of paper can help out as well. If that's not possible then taking sky flats during twilight may be a solution.
EDIT: what cameras did you use before? Were they mono cameras as well or OSC cameras?
@wvreeven Interesting. I can try taking sky flats next session, if the resulting calibrated image looks better that would definitely point to the USB pad. I have had great success with this pad until now so maybe it just played more nicely with my previous cameras (183mm, 1600mm, 294mm).
@regpratt Hmm ok so it may not be the USB pad or the filter after all. Like I said, it was just a thought. I did notice that my ASI6200MM flats generally work better with ADU levels as high as 60k. In other words, I make sure that the peak of the flat histogram is as far to the right as possible without clipping any data. If I don't do that, I see artifacts as well though the ASI6200MM has a much bigger sensor than the ASI2600MM.
@wvreeven Ok, I will try taking some sky flats next session as well as taking some flats at higher ADU values. From what I understand the IMX571 and 455 sensors are basically the same chip just in difference size so it is likely they behave the same.
So I imaged on my second RC and Hypercam 26m last night and the same issue as on the first RC and 2600. The shadow of the secondary seems to be under correcting compared to the rest of the field. I've tired flats of different ADU and sky flats and the result is the same. The LPS tool corrects the pattern slightly but not much. Not sure where to go from here.
Looks like a problem a friend of me has with a ASI294MC, we suspect his problem is dew on the camera or coma corrector because the camera doesn't have an integrated heater for the front window, he is waiting for the ZWO front window heater that hopefully will fix the problem.
Could this be your problem too if the heater isn't turned on?
The heaters are always on when imaging so definitely not dew. All the scopes optics are heated so no dew on the primary or secondary either. Its funny you mention the 294MC because I've had 3 of those and never had this problem. The 1600mm and 294mm also did not have this problem on the same 2 scopes so I the issue is the sensor window or the flats.
@regpratt How quickly do you cool the camera? When cooling too fast there may still be dew on the glass cover of the sensor.
Try to calibrate without darks, i don't use darks on ASI6200 and since the ASI2600 sensor is pretty much the same in smaller format it might work for you too.
Something else to check, were all lights and calibration frames taken with the same software, same driver and same settings? I've had bugs where the ZWO Ascom driver has used gain values not even available in the Ascom driver after i used Sharpcap :S
Because of these problems i got some bad lights that did not have 100% proper calibration.
After updating Ascom, Ascom driver and SGP everything seems to be working fine, i now control gain/offsset from SGP instead of letting the driver handle it alone.
@xplode85 Yes same software all captured on the same night. Calibrating without darks didn't make a difference. Didn't really expect it to though lol.
Here is the result with a strong APP stretch so that the ring is more visible.
At this point given that both scopes and cameras are experiencing the same issue, and said issue didnt exist with my previous 2 models of camera, I have to believe that this weird reflection is some how being caused by the sensor window itself. Which is a bummer because that means there is nothing I can do to fix this.
These issues can be hard to find out, it almost looks like internal reflection maybe. Especially because you didn't have that issue with other equipment.
Problem solved! The problem was internal reflection of the secondary mirror due to these new spacers I got that (for reasons beyond my understanding) are SHINY. I spray painted the inside of the spacers flat black and the reflection is gone on both image trains.