Multiple Flats per session when AF changed

[Notty]

Hello. This has had me curious for a while. My workflow has me normally automating my light captures then going to bed, then doing flats next day using focus position written to the log. The ASIAir performs a refocus if the temp changes by more than 2degs by default. Do most people take as many different sets of flats for each batch for a given focus position? Or take the average focus and do it that way. I've been doing the former (pain)...but wondered what was best practice. I've no idea if a focus change from one arbitrary number eg 13120 to 13099 represents a big change or not.

Thanks

[scitmon]

I can't imagine focus changing enough to warrant separate flats.  I would be happy to be proved wrong though.

[Adreneline]

17 minutes ago, scitmon said:

I can't imagine focus changing enough to warrant separate flats.

+1

 

[tomato]

3 hours ago, scitmon said:

I can't imagine focus changing enough to warrant separate flats.  I would be happy to be proved wrong though.

Me also. I keep the same flats until I disturb the optical train which is usually a few weeks unless we have a particularly lean spell of clear nights.

[Notty]

Wow ok I think I’ve been making life a bit harder than necessary then. My setup is semi-permanent so perhaps I’ll try reusing my flats per channel and see if I notice. Thanks all.

[ollypenrice]

The refocus distance is trivial in the scheme of things. You'll never tell the difference. I keep flats for as long as possible, which often means months.

Olly

[Lee_P]

I take one set of flats per object that I'm imaging, and can spend more than a month's worth of clear nights on that object. Seems to work ok and is nice and simple! (I am OSC though, so don't have lots of filters).

[ollypenrice]

10 hours ago, Lee_P said:

I take one set of flats per object that I'm imaging, and can spend more than a month's worth of clear nights on that object. Seems to work ok and is nice and simple! (I am OSC though, so don't have lots of filters).

'Flats per filter' is a debatable point. Nineteen times out of twenty I use a luminance flat for every filter and have no problems whatever. If the R, G and B are not perfectly flattened individually, the combined RGB will be when it is illuminated by the luminance-flattened luminance channel. You just need to think through the process logically to see why this will usually be the case. When, exceptionally, it doesn't work it is because you have a truly remarkable contaminant on one filter. It happens, but very rarely. At least that has been my experience.

Olly

Edited July 23, 2021 by ollypenrice
Typo

[tomato]

My experience agrees with Olly. The dust mote pattern (and alas I don't keep and assemble the kit in clean room conditions, so there are a few bunnies on there) is the same on all 4 filters, telling me the dust that counts is on the camera sensor window.

[ollypenrice]

2 minutes ago, tomato said:

My experience agrees with Olly. The dust mote pattern (and alas I don't keep and assemble the kit in clean room conditions, so there are a few bunnies on there) is the same on all 4 filters, telling me the dust that counts is on the camera sensor window.

My conclusion as well.

Olly

[kirkster501]

Slight changes in focus make no difference at all, I can 100% confirm that from experience.  You do not need separate flats for minute focus changes over imaging runs.  There *can* be difference between the filters though and so you should take flats with each filter.  However, you can reuse those flats over many sessions if you have a permanent set up and you  can be 100% confident that you do not move the camera. I reuse flats up to a month or so.

[DaveS]

Although I do take separate flats for each filter I would only take a new set if I had dismantled the imaging train for whatever reason.