M81 & M82 - Strange flat frame issue.

[AlastairW]

Hi

I wonder if one of you talented and knowledgeable people could kindly assist with the following issue or provide any pointers, please ? I've being having great difficulties with flats, having tried all of the prescribed methods (bar building a light box). I shot M81& M82 a couple of nights ago and in post stacking when upping the levels in GIMP (yep, on a budget) noticed some strange streaking at the bottom of the image, which is very ugly. I'm not sure if this is a problem with my flats or light pollution as the subject matter was rather low at the time of the light frame exposures. 

The flats I took were ISO1600, 1/60 on a Canon 350d in AV mode using the stretched t-shirt and laptop screen method. The flats were taken approx. 10 mins after the run of 50x50sec lights with the camera at the same focus and orientation.

The first picture is the final stack with the levels upped and the second is the histogram for one of my flats (they'll all almost identical - 30 in total).

Many thanks

Al

[davew]

The general gradient from bottom right up to top left looks like a sky gradient. Bodes will have been down closer to the horizon if you took the image early on.

The diagonal banding seems to be the bane of many DSLR imagers and I understand could be a bias issue.

If it's the later you're talking about then I can't help. You'll have to wait for a DSLR expert ! If it's the former issue then flats won't cure it.

Dave.

[Chris-A]

_{Looks a bit like shutter artefacts to me. If the exposure is too short then the flat field will be unevenly illuminated due to the movement of the shutter. It's the best part of a year since I moved from DSLR to CCD, but from memory my flat exposures needed to be longer than 1/60s to avoid this. I used to downgrade to ISO100 for my flats for this very reason - there isn't really any need to use the same ISO for flats as you use for your Light subs as you are only trying to capture variations in the field due to vignetting / dust bunnies etc.}

[AlastairW]

Thanks Dave, appreciate the help.

[AlastairW]

Thanks Chris, very interesting. I'll have a bit of a play round and drop the ISO as you suggest and I do notice that there is a clear uneven illumination on the final stacked image.