Narrowband imaging and taking darks \ flats \ bias

[AlistairW]

Hello,

If you image using Ha, then switch filter to OIII and finally SII, when do you take darks \ flats and bias ? - With each new filter I would assume there could be different artifacts in the optical train. Do you take darks \ flats \ bias for each filter ?

Thanks

Alistair

[MattJenko]

Darks and Bias are independent from the filters, so you don't need them for each one. In theory, you would want flats for each filter.

[johnrt]

Indeed darks and bias have no light hitting the chip at all so can be made at your leisure, flats can also be made at any time so long as you can achieve the exact same focal point for each filter.

[AlistairW]

My filters are apparently parfocal - but i was wondering if things like dust bunnies would be different with each filter - and whether that would have an effect. This is more for Flats i guess.

[MattJenko]

Filters being parfocal and filters being parfocal in a given telescope are 2 very different things. I have a doublet refreactor, and there is a small but very apparent focus difference between my Baader filters.

Dust on filters affects the image in a scale dependant on the distance between filter and chip. The closer the filter, the more dust on the filter impacts the image as the dust image is smaller and more pronounced.

That being said, I have created flats for each of my filters, but I made them once, inside, using a white screen, and have successfully used them for months and months now, so you don't need to spend a long time creating flats every session...

If you have camera + scope together at all times, you shouldn't be creating new dust bunnies too often. I will only redo my flats once they stop working!

[ollypenrice]

Darks and bias you just do at a set temp. point at any time and keep them for quite a while. While the 'correct' way to do flats is to do them per filter I must be honest and say that I find no significant difference between flats per filter and sometimes take the shortcut of using just one flat for all. However, I have highly corrected optics, filters some way from the chip and sealed electric wheels which keep the filters clean.

When you make your flats don't forget to use a master bias as a dark for flats. This is very important. For short exposures a master bias will be the same as a dedicated dark for flats so don't waste time on those.

If you don't dismantle your kit each night flats are pretty long lasting.

Olly