Formula

[carpman]

Guys or Gals

Is there a formula for the amount of flats / darks / bias taken to the amount of subs being used ?????

Also is it correct that u need to take the flats in exactly the same orientation as your subs , Ie need to b taken at the same time as the subs you will be using them for ?????

The reason i ask is as a noob i'm unsure

The other day i took 170 x 10s subs , 40 dark , 30 flats and 30 bias is this over kill or not enough , i know exposures are short but i only have an ra drive that will take max about 25s subs ( until i upggrade and learn how to process the finished stack )

sorry for all the questions but hey ho this is a very steep learning curve

Danny

[Demonperformer]

Sounds like reasonable proportions to me. I've seen somewhere that 20 darks/flats/bias is optimal. Any more than that and you enter the law of diminishing returns, any less and you start leaving in significantly more noise.

Any yes, flats must be taken with exactly the same optical train as your lights.

HTH

[Ben Ritchie]

You don't need bias if you're taking darks with the same exposure time as the lights. The dark already contains the bias.

Also is it correct that u need to take the flats in exactly the same orientation as your subs , Ie need to b taken at the same time as the subs you will be using them for ?????

Same orientation, yes. Doesn't need to be at the same time though, as long as you've not moved the imaging kit.

[carpman]

You don't need bias if you're taking darks with the same exposure time as the lights. The dark already contains the bias.

It looks like answers lead to more questions:)

@ Ben which is the better option darks or bias ?

Obviously bias are faster so save on time....

So if ive got this right it does not matter how many subs you take . you need to take 20 darks or bias / flats. The last thing i want to be doing is losing any detail i might have gotten ...

I have not tried using flats yet does it make a big difference to finished result ?

When taking flats does the histogram need to be 3/4s to the right

any tips on using als virtual lightbox i think i need all the advice and tips peeps can give

Cheers Dan

[FraserClarke]

@ Ben which is the better option darks or bias ?

Obviously bias are faster so save on time....

If Ben doesn't mind me jumping in...

Darks contain the Bias information. Biases DO NOT contain the Dark information.

Whether you need that Dark information to correct your images depends on the camera. For most cameras, any exposures longer than ~20-30 seconds will benefit from proper darks vs just biases. Quite a lot of cameras will see the difference in even shorter exposures.

What camera are you using?

[carpman]

I'm using a nikon d5000 (Prime focus)

[martin_h]

There is no set proportion 20 - 30 dark frames is enough no matter how many light frames you have. Flats will help the image - ie get rid of dust spots on the lens,sensor etc, Again, 20-30 is a good average

[Ben Ritchie]

If Ben doesn't mind me jumping in...

Not at all

As has been said, provided that the darks have the same exposure time as the lights you don't need bias frames, because the darks already contain the bias signal.

However, because dark current is effectively linear with time you can use bias frames to scale darks to match lights of different exposure time. This is most often done by people with temperature-regulated CCDs to avoid taking new darks each night - generally called something like 'master darks' or 'library darks' - and temperature regulation is important, because even if the exposure times differ, the temperature has to be the same for the darks and lights. Dark current is strongly temperature-dependent, which is why cooling is so important in controlling noise, so if your library dark was at one temperature and the light at another, you'll over (or under) subtract the dark current.

There are a few (amateur) cooled CCDs with such low dark signal that you can effectively just use bias and forget darks (the ICX-285, as used in the SXV-H9 and Atik 314L is one), but for most CCDs and all DSLRs you really need darks. I'd generally use fewer than 20-30, just because the time taken to acquire them adds up fast once you're using integrations of minutes. As has been pointed out, you're into the realms of diminishing returns and IMO all you really need is enough for a decent median-combine unless you're planning some extreme stretching.