Standard set of Bias/Flats/Darks?

[simondodd]

Heya,

Just wondering if you can use a kind of standard set of Flats or Bias frames? I'm pretty sure you can't do darks as they are more dependent on conditions but can you take a load of Flats at multiple ISO's to then use on all your images with that camera/scope setup, just selecting the flats at the ISO you have shot your lights at?

Thanks in advance

[Demonperformer]

Bias - yes. Minimum exposure time, as many as possible.

Darks - yes, if you have a temp controlled camera.

Flats - no. They need to be taken through the precise optical set up/alignment as the lights.

[simondodd]

Great! Thanks for that. Will save me a bit of time at least!

Shame my Nikon isn't temp controlled.

So the best process would be to Take the lights, then the flats, then the darks at the end of the session (until I buy a temp controlled camera).

Looks like I'll be sorting though all my bias files and organising them into ISO folders then, should have quite a few kicking around now!

Thanks again

[Anthonyexmouth]

take the darks first so temp is as near to lights as possible. flats can be taken later as long as image train hasnt been shifted. 

[Demonperformer]

Ah, DSLR.

ISO should not affect bias - you are trying to measure your read noise.

I used to take half my darks before the lights and half of them after the lights on the assumption the temp of the chip would vary during them. But then, that might be a little OCD ...

[ollypenrice]

Badly matched darks are worse than useless. I don't even bother with them on our three set point cooled CCD cameras. There's a video by Tony Hallas out there in netland in which he runs through the DSLR routine he uses to make his great DSLR pics. In a nutshell he uses a master bias instead of a dark and a large scale dither (12 pixels from memory) to disperse what he calls 'colour mottle' in the background sky and to let sigma clipping deal with hot pixels.

Since I stopped using darks I have cut my 'final clean' of images from half an hour to half a minute.

I use a master bias as a dark-for-flats as well.

Olly

[sharkmelley]

You mentioned Nikon.  For DSLR cameras, as long as the camera doesn't suffer from obvious amp-glow then with dark scaling (a.k.a. dark optimisation) you can use a single master dark across a wide temperature range as long as the ISO matches.  It's the only reliable way to deal with sensor self-heating during the imaging session i.e. the fact that each sub-exposure has a different sensor temperature and hence different thermal noise.  The scaling is calculated statistically from the data itself and not from the (unreliable) temperature that is recorded in the EXIF files of some DSLR cameras. I shot my master dark at 20C ambient temperature a couple of years ago and it still works well for both my summer and winter imaging - even during the recent (but now forgotten) UK heatwave my night time ambient was never above 20C.

Both DeepSkyStacker and PixInsight support dark scaling/optimisation.  I'm sure many other processing packages do so as well.

I use the same bias and darks for a very long period of time but flats need updating relatively frequently because dust has a habit of moving around.  In practice I shoot a new set of flats every time I notice a dust artefact in my processed image.

For completeness I should also add that for set-point cooled astro-cameras there is no problem with darks: the darks will always match the lights without scaling.

Mark