StarTools

Hi all,
I imaged the Andromeda Galaxy last night under extremely humid conditions (91-93% relative humidity), and temperatures in the 70's. I got some strange results- the galaxy appears way overexposed in the center. I have an asi 294 color camera and a Skywatcher esprit 100 ED scope. The flat frames showed a dark circle in the center. I attached the DSS autosave file, my attempt at ST processing, and the flat frame stack below. In addition to flats I also calibrated with darks and dark flats (the autosave file contains all calibration frames).

The light frames were taken at a gain of 119 and an exposure of about 4 min (in Sharpcap). Not sure if the over-exposed center of the galaxy was due to the extremely long exposure time for a galaxy, or if there was another cause. I increased the exposure time in order to get more detail of the dust lanes, but maybe that wasn't a good idea! And I'm also not sure if the dark spot in the middle of the flats was caused by high humidity/dew or another reason.

Any ideas on what is going on here? I am assuming it is a capture question, but I thought I'd post this in the event there was a way to fix it in Star Tools.

Thanks,
John

Autosave file:
https://drive.google.com/file/d/1bipent ... sp=sharing

Flats file:
https://drive.google.com/file/d/1EqqQDV ... sp=sharing

Processed with ST:
https://drive.google.com/file/d/1TwWho- ... sp=sharing

Hi John,

I looked at the master flat, finished image, and downloaded the stack, but didn't take it past Wipe. I think you are on all the right tracks with your suspicions.

Have you ever had flats with a big dark circle like that before? Be a good idea to chase that down, maybe take new ones if you can. Even an estimated rotation (if moved since then) and then just go set your infinity focus, ought to work better than these flats here. Check to make sure no dark circle.

Any idea what might have caused that? Take a look at your camera and the scope I guess. Also maybe blink through your flats or otherwise inspect the list, and make sure you didn't include some other file in there with your actual flats.

Dew might be a possibility as well. And if that's the case, might be a good idea to blink through your lights also. IMO PI is the best for that kind of inspection, but if you have ASI Studio downloaded, the ASI viewer will do the trick as well.

So, what I think may have happened during calibration is that after the flat scaling (dubious) and then division, there would have been overcorrection. I believe I see a wee bit in the corners, but of course the massive overcorrected blow-out right over the core of M31.

Heck, you might even be able to re-stack without flats at all and use Wipe in uncalibrated mode, if you can't retake some new flats or re-stack a new master flat. That's a maybe, but perhaps would work.

Anyway with that giant white light bulb in the center and the calibration not working, I'm not sure evaluation of your subexposure lengths is really possible. Personally I would (could) never go near 4 minute broadband subs. Maybe in Bortle 1? Even then there's a potential problem of star core saturation.

I say pick your subexposure times sufficient to "swamp the read noise," as the saying goes, and mind your total number of saturated pixels. That statistic is available in NINA. Sharpcap I'm not sure.

But revealing the dust lanes against the brighter galactic background can be done by just getting more subs for better SNR, permitting (if you so choose) a better and stronger stretch (OptiDev), and then using the various ST modules (as you like, Contrast, HDR, Sharp, SVD) to make them stand out.

Thanks Mike. I think I'm going to call this one a total loss and capture Andromeda again as soon as the high humidity lets up. Sounds like fixing this just isn't worth the time. And I'll take your advice on using shorter subs. Next time I'll try using either 30-sec or a minute (or both, and compare them). But I at least want to blink through the flat subs as you suggested to see if there is something in there that shouldn't be... that's very easy to do.

One of these days I'll try using NINA. Just haven't had the time yet to learn it!

Regards,
John

I have seen similar flat issues, and to me that's humidity.
Starting from the fact that your camera specs states :

Working Relative Humidity: 20%—80%
Storage Relative Humidity: 20%—95%

imaging over 90% humidity as you mentioned is already set for disaster. Besides, as far as i'm aware, your camera doesn't have dew heater on optical window, so dew could have formed there. cameras without dew heater window have tipically dissicant pands inside the chamber, or a disscant tube screwed on the body when humidity is high (as for my QHY8L camera. )

NiNa has this great plug-in for "session history review" that is good for this. You can check individual frames and see if this appears to all of perhaps after few hours later on in the night. In case you decide to try NiNa make sure to get that plug-in, is very useful.

I would stack WITHOUT FLATS to check if that issue is on the lights too, because i don't see many dust specks on your flats, only that dark circle.
If lights have it too..then not worth keeping them. I would suggest you check your camera window for possible dew formation. you can test this during daytime. if the silica pads are saturated, you should check your camera instructions on how to change or recicle them (usually silica heated in the microwave gets dry again and usuable )

Regards