Quick question about taking flats

[MylesGibson]

I've just finished my session for tonight and am now heading to bed so I'll respond to replies in the morning!

I've just finished a second nights imaging on a target and I've only recently started using darks/flats etc (yes, I know it should have been done when I started but oh well!) And since doing that, this is the first time I've imaged the same target in multiple nights. 

I read when doing flats, the camera needs to be at the same orientation and focus as the lights. That's fine when taking the flats that night, but after setting up for the second night, the camera wont be at the exact same orientation (it's as close as I could do it). 

When stacking in DSS, it creates a 'master flat' from all the flats you upload. So, if I have some from multiple sessions, with the camera at slightly different orientations, will this affect the quality or the usefulness of the flats? I'm hoping not, but thought I'd ask first before spending tomorrow evening running into problems!

[Ibbo!]

If you move the camera you will need new flats.

Darks will be OK though, I use my darks for months.

[MylesGibson]

6 hours ago, Ibbo! said:

If you move the camera you will need new flats.

Darks will be OK though, I use my darks for months.

I have taken new flats, but I am also using the lights from another session. So do I use both sets of flats and they will be fine? And if I have another night on the same target, and take another round of flats, these are all OK to use together?

As for darks, I know I'm fine with those, it was the flats I was worried about.

Edited May 14, 2020 by MylesGibson

[MarkAR]

It's difficult to say if you should combine them or not. If any dust has moved then combining could cause artefacts.

I would say carefully inspect each set to see if they look the same. Look for any rings or blotches that have moved.

[MylesGibson]

4 hours ago, MarkAR said:

It's difficult to say if you should combine them or not. If any dust has moved then combining could cause artefacts.

I would say carefully inspect each set to see if they look the same. Look for any rings or blotches that have moved.

Fortunately I don't tend to suffer with much dust artefacts, I have a lens cleaning kit that I use opeten and before each session now I use the little air blower just to puff away any excess dust. I mainly use flats for light pollution issues, as I heard they can help with uneven background or something like that? I suppose I could always try it, and I can always re-stack without if needed. I assumed it would probably be fine, but always better to ask I think

[tomato]

Flats are used to correct any illumination artefacts/variations in the optical train, so if anything changes you should take a new set of flats. Most of the dust that has an effect on the image is on your camera sensor or filter, and to be fair, probably doesn't move around too much, but you cannot guarantee that your focus position will be in the same exact place for each session. When I used to set up and take down every session I always took flats each time, I cheated a bit by noting the position of the focuser for each filter and using these positions for each set of flats at the end of the session. It is a pain to do these at the end when you are probably cold and tired, but they make a difference to the quality of your final image, no question.

Flats will not correct for light pollution, but they will correct for uneven illumination of the sensor such as vignetting. There are lots of wonderful software processing tools to help with gradients caused by light pollution. 

Edited May 14, 2020 by tomato

[MylesGibson]

7 hours ago, tomato said:

Flats are used to correct any illumination artefacts/variations in the optical train, so if anything changes you should take a new set of flats. Most of the dust that has an effect on the image is on your camera sensor or filter, and to be fair, probably doesn't move around too much, but you cannot guarantee that your focus position will be in the same exact place for each session. When I used to set up and take down every session I always took flats each time, I cheated a bit by noting the position of the focuser for each filter and using these positions for each set of flats at the end of the session. It is a pain to do these at the end when you are probably cold and tired, but they make a difference to the quality of your final image, no question.

Flats will not correct for light pollution, but they will correct for uneven illumination of the sensor such as vignetting. There are lots of wonderful software processing tools to help with gradients caused by light pollution. 

Thanks for the info. So can I use different sets of flats taken over multiple nights and combine them like I do with lights?

[MarkAR]

If you feel they are dust free and both sets very close to the same focus then go ahead. Keep the originals in separate files just in case there are issues and you have to do them separately.

[michael8554]

The images and Flats for each night should be loaded in different tabs in DSS.

Michael