M31 help

[astrodam]

Hi all,

I recently did some imaging of M31 over two nights.  The first night I took around 50x30s subs at ISO400 (plus darks, flats and bias) and was quite pleased with my result.  The following night I decided to try and add to this and took another 50 subs but this time at ISO3200 (plus calibration frames again.  The result is below and I was happy with the details but not with the gradient with the top of the image being brighter than the bottom.  Could this be to do with my flats being taken by holding a white laptop screen over the aperture because I noticed that the shutter speed was a lot faster than before?

I stacked in DSS and used GIMP2.8 to tweak.

Any advice would be most appreciated.  Hopefully I'll get up to the standard of SGL vets some day for the mean time its great to be able to capture my own views of the universe from the comfort of my own back yard!  Amazing

[Daniel-K]

could be your flats as you say, but your shutter speed is suppose to be faster when taking flats. have you tried stacking it without the flats to see if its amp glow? also did you use a light pollution filter when taking the subs?

[ultranova]

Hi Damien, you will get bad gradients with the moon up as is it is at the moment

but flats should take care of these fairly well.

Typically whilst taking flats with a DSLR your speed range can be any thing from 1 second to

250 of a second depending on how bright the computer screen is, as long as you do not touch

your camera / set up after you have taken your subs and do the flats straight away remembering to

put your camera on to AV mode so its sets the correct exposure values automatically it should be fine.

As Daniel said there could be a problem with your flats and has offered some other good advice as well,

Paul

[astrodam]

could be your flats as you say, but your shutter speed is suppose to be faster when taking flats. have you tried stacking it without the flats to see if its amp glow? also did you use a light pollution filter when taking the subs?

Hi Daniel, yes I did run DSS without the flats but I got a lot of vignetting gradient instead of the top to bottom gradient so I do think the problem is my flats. I don't use a light pollution filter  which I think is why it may be a bit orange - I'm still to learn how to turn that down in GIMP

The speed of the shutter is maybe faster than the frequency of my laptop screen? Am I correct in assuming that a display is usually either 50 or 60Hz? If so, a shutter speed faster than this may pick up the "flicker" of the screen? How do you take flats?

Hi Damien, you will get bad gradients with the moon up as is it is at the moment

but flats should take care of these fairly well.

Typically whilst taking flats with a DSLR your speed range can be any thing from 1 second to

250 of a second depending on how bright the computer screen is, as long as you do not touch

your camera / set up after you have taken your subs and do the flats straight away remembering to

put your camera on to AV mode so its sets the correct exposure values automatically it should be fine.

As Daniel said there could be a problem with your flats and has offered some other good advice as well,

Paul

Hi Paul,

Would turning the screen brightness right down help things?

Thanks