Problems with M31

[terry0905]

Hi,

I have just started LRGB imaging using and ATIK 383L+ with 2" Baader filters in an EFW2. I captured the RG&B subs binned 2x2 from a moderately light-polluted site. I created flats through L, R, G and B filters in case there was any off variation in sensitivity at different wavelengths. So my process was:

Create master bias, master dark and master flats (following the PI tutorial)
Capture L, R, G and B subs
Calibrated using my masters
Registered all R, G and B against one of the red frames
Registered L separately
Combined the R, G and B.
Applied Auto using STF with the channels unlinked and copied an instance into HistogramTranform, then applied that.

The result is as attached.

I cannot remove the red cast in the middle of the picture. Any suggestions as to how this is happening and what I can do to eliminate it?

Thanks

Terry

[Nocturnal]

Your flattening isn't working right, most likely. First step would be to check which of your channels have this problem, perhaps all of them.

[sleshin]

Hi Terry and welcome.

Looks like you have some sort of gradient, perhaps as Sander suggests, related to poor flat fielding. What ever the cause you might try to see if it can be removed with Dynamic Background Extraction. You want to use this tool on the linear RGB image. So, after you create the RGB master but before the non linear Histogram stretch, try using DBE. Also, before using DBE, crop any bad edges that might be in the RGB image related to prior registration and the RGB combine process. If you need to, watch the tutorial videos on DBE by Harry and the recent LRGB video by Vicent.

Steve

[terry0905]

Thanks, Sander, I will go back and have another look at that. I originally just had a master flat created through the L filter, but then retried with separate flats created through R, G and B filters (I couldn't see why it should make a difference but it was a simple thing to try) and that didn't make any difference. I did wonder about the linearity of the chip in the 383L (or any chip for that matter) - is there a completely linear response over the whole of the illumination range or is it non-linear at the bottom end in particular? I ask this because maybe my exposure was too short to get onto the linear portion which, I assume, could give this odd effect.

Your flattening isn't working right, most likely. First step would be to check which of your channels have this problem, perhaps all of them.

Thanks again

Terry

[terry0905]

Thanks for the welcome Steve and for your comments. I have watched many of Harry's excellent videos, but this is a steep learning curve and I pick something else up each time I watch them. I did have a go at removing the background, but with the galaxy occupying so much of the foreground I was a bit unsure how to go about it for the best. Experimentation I guess is the only way forward. And I should have remembered about cropping the edges - so much to remember!

Hi Terry and welcome.

Looks like you have some sort of gradient, perhaps as Sander suggests, related to poor flat fielding. What ever the cause you might try to see if it can be removed with Dynamic Background Extraction. You want to use this tool on the linear RGB image. So, after you create the RGB master but before the non linear Histogram stretch, try using DBE. Also, before using DBE, crop any bad edges that might be in the RGB image related to prior registration and the RGB combine process. If you need to, watch the tutorial videos on DBE by Harry and the recent LRGB video by Vicent.

Steve

Regards

Terry

[chris.bailey]

That is almost certainly an issue with flat fielding. How are you obtaining your flats? I have the same setup as you and find I need different exposures for the flats through each coloured filter so as to get similar exposures. You are aiming with the 383 to get flats with peak exposure pixel values of around 0.3 float or 20,000 integer i.e. a little lower than the 50% normally recommended.

Try a stack of uncalibrated files and see if the result is the same.

Elsewhere the advice seems to be against binning the colour frames for the 383.

[terry0905]

Chris,

A white T-shirt pulled tight over the end of the tube (an 80ED) done when the light level is low enough to get a flat of around 10 secs or more (to avoid shutter effects). I'd need to check my flats, but I think I was in the 30 to 50% range - clearly I need to take a bit more care over creating them. Where can I find info about binning colour frames or otherwise? I thought that was a fairly standard technique with the L being done unbinned.

Thanks for the suggestions, I'll give them a try tonight.

Regards

Terry

That is almost certainly an issue with flat fielding. How are you obtaining your flats? I have the same setup as you and find I need different exposures for the flats through each coloured filter so as to get similar exposures. You are aiming with the 383 to get flats with peak exposure pixel values of around 0.3 float or 20,000 integer i.e. a little lower than the 50% normally recommended.

Try a stack of uncalibrated files and see if the result is the same.

Elsewhere the advice seems to be against binning the colour frames for the 383.