Background pattern cause?

[pfile]

specifically i was responding to your claim that dark skies would be a bad thing if what i said was true. meaning that your reason for disagreeing with what i said makes no sense. just because thing X is good in general does not mean there cannot also be drawbacks to thing X.

i will say it again, low or no signal in the background of an image can lead to problems with calibration. why do i know this? because it has happened to me with a camera similar to OP's camera. this may or may not be the OP's problem, but it is a real thing that has happened. because it has not happened to you is not proof that it can not happen. furthermore how do you know the calibration flow for the atacama images you reference? maybe those people had to add a pedestal to their calibrated lights?!

what's crazy here is that you are agreeing with me that calibration may be the problem, but can't see that calibration might be the problem because of the characteristics of the lights.

your assertion that low/no signal in the background can not be the problem is incorrect, and your assertion that dark skies would be "a bad thing" if this were the case is also not correct.

try calibrating a dark sub with a master dark sometime and see what happens. depending on your camera you will find that there will be pixels clamped at 0 in the result. a short exposure with a narrowband filter of an area of sky devoid of light that passes thru that filter is very similar to a dark. again, not stating that this is definitively the OP's problem but i felt it was worth his checking to see if there are clamped pixels in his calibrated result.

rob

[rdryfoos]

specifically i was responding to your claim that dark skies would be a bad thing if what i said was true. meaning that your reason for disagreeing with what i said makes no sense. just because thing X is good in general does not mean there cannot also be drawbacks to thing X.

i will say it again, low or no signal in the background of an image can lead to problems with calibration. why do i know this? because it has happened to me with a camera similar to OP's camera. this may or may not be the OP's problem, but it is a real thing that has happened. because it has not happened to you is not proof that it can not happen. furthermore how do you know the calibration flow for the atacama images you reference? maybe those people had to add a pedestal to their calibrated lights?!

what's crazy here is that you are agreeing with me that calibration may be the problem, but can't see that calibration might be the problem because of the characteristics of the lights.

your assertion that low/no signal in the background can not be the problem is incorrect, and your assertion that dark skies would be "a bad thing" if this were the case is also not correct.

try calibrating a dark sub with a master dark sometime and see what happens. depending on your camera you will find that there will be pixels clamped at 0 in the result. a short exposure with a narrowband filter of an area of sky devoid of light that passes thru that filter is very similar to a dark. again, not stating that this is definitively the OP's problem but i felt it was worth his checking to see if there are clamped pixels in his calibrated result.

rob

  That's better....Arguing your point with no disrespect.   I have been wrong before...in fact I am quite used to it.  When brainstorming, or thinking outside the box, being wrong is the norm.  They say you learn the most when you make mistakes (or are wrong).  By that, I am a scholar. Thank you for the information.  However, and now I am being the devils advocate, if the issue to which you refer is related to the camera, OTA....all the things you said, it seems to me that the poster would have run into this issue before....unless this is the first time he used the camera (which I don't think is true).  I assume he has imaged from the same location using the same equipment, and may even have imaged the same target.....without experiencing this problem.  So, if you are correct, the real question is...why now?     

Rodd

[pfile]

i don't know; but the system does seem to be relatively new, so maybe he/they have not imaged in this particular way. also as rick pointed out, bias drift could be in play.

rob

[macnmotion]

Hi again guys,

Just to fill in some blanks: The telescope is not new, we've had it for a couple of years. However the first year (and well into the second) we got almost no use out of it due to mount issues. When we were able to shoot, we were using an FLI PL16803. That changed on Feb 21, 2018 when we switched to the ML 16200. As a system we don't have much experience with this specific setup.

We have recently been shooting some other targets with this setup, and I can review those image sets to compare to what I posted of M1. My guess is that I'll find similar results across targets, but I will double check. M1 was a narrowband image (the only one we've shot with this setup), more recently we have been shooting LRGB galaxy images. Our light polluted skies are changeable each day, and even at various hours overnight. We are at the mercy of many nearby locations, including some type of resort, a police academy, local markets with search lights, etc., and of course the lights of Bangkok to the East. We've had pretty good success removing these light gradients with DBE but it certainly is a challenge on all of our systems here.

The camera itself was sent back to FLI earlier this season for an issue, there was a problem with a seal and the sensor was freezing. That has been fixed.

I think that all of the comments and suggestions in this thread are valuable, and provide me with a checklist of items to rule in or out, and I appreciate the time everyone has taken to help (no matter through practical experience or theory). Once again thanks to everyone for these suggestions. As I mentioned, I will run through them when back at my computer and see if I can narrow down the source of this issue. I will update this thread with any results or additional questions.

Andy

[macnmotion]

Just a quick update: We sent sample images to FLI (calibrated/aligned stack, uncalibrated aligned stack, single light) and their initial assessment is that something seems wrong. We're waiting to hear back but this will likely require the camera be sent in for a checkup.

Andy