Darks at different temperatures and exposures

[SteveBz]

Hi All,

When taking darks with my Nikon D5000 can I average over different temperatures?  Eg For a specific ISO and duration, if I have 50x darks at 10 Deg C and 50x darks at 20 Deg C can I use them together to average out to a night of 15 Deg C? 

Ditto duration.  Eg For a specific ISO and temperature, Can I have 10 x darks at 120 secs plus 10 x darks at, say 240 secs to average out to exposures of 180 secs?

I'm trying to build a library of calibration frames and I want to be as flexible as poss.

What about averaging across ISOs?  I'm guessing not.

What do you think?

Thanks

Steve

[Demonperformer]

Not sure but my thoughts are it would depend on whether the dark current on the nikon chip is linear between the points you choose. 

[SteveBz]

9 minutes ago, Demonperformer said:

Not sure but my thoughts are it would depend on whether the dark current on the nikon chip is linear between the points you choose. 

Yes, I guess you're right.  I recall a site where that info was recorded for each chip maker.  I'll see if I can find it.

[ollypenrice]

Before going to all this trouble I'd be inclined to see if darks actually provide any benefit. Even with precisely matched ones available from my set-point cooled CCDs I have stopped using them. I subtract a master bias as a dark and apply a bad pixel map and hot pixel filtration, a process which out-performs dark subtraction for me and saves ages. The very expert DSLR imager Tony Hallas advocates using dither guiding (at a minimum of 12 pixels), bias-as-dark subtraction (I think, but maybe check that...) and a sigma stacking routine.

Darks can be scaled to correct for temperature mis-match by some stacking software if you're determined to try them. Pixinsight does this, as do quite a few other good packages. The master bias is the reference point for the scaling, as I understand it.

Olly

[SteveBz]

2 minutes ago, ollypenrice said:

I have stopped using them. I subtract a master bias as a dark

Nice, I could easily do this.  I have hundreds of standard biases for every ISO.  I'll try it.  Do you think DSS does this if you don't give it any darks, but only biases and flats?

4 minutes ago, ollypenrice said:

Darks can be scaled to correct for temperature mis-match by some stacking software if you're determined to try them. Pixinsight does this, as do quite a few other good packages. The master bias is the reference point for the scaling, as I understand it.

I was going to keep my darks sorted by temperature, among other things, and then take the nearest, say, 100, darks to my ambient temperature, even if they were out by a few degrees, as long as the mean is correct.  Then extend the same idea to other attributes meaning that I need fewer darks over all.

Steve

[ollypenrice]

1 minute ago, SteveBz said:

Nice, I could easily do this.  I have hundreds of standard biases for every ISO.  I'll try it.  Do you think DSS does this if you don't give it any darks, but only biases and flats?

I was going to keep my darks sorted by temperature, among other things, and then take the nearest, say, 100, darks to my ambient temperature, even if they were out by a few degrees, as long as the mean is correct.  Then extend the same idea to other attributes meaning that I need fewer darks over all.

Steve

Stacking programmes don't know what you're feeding them, only what you tell them you're feeding them! So if you put a master bias into DSS's box for darks, it will treat them as darks and subtract them. To save your own sanity you could rename a master bias as something like Dark_universal  and, while you're about it, save another master bias as Flat Dark  because a master bias is all you need as a dark for your flats. Then you wouldn't use a 'bias-as-bias' anywhere in DSS so far as I can see. I don't use DSS but I lie to AstroArt in this kind of way without shame or remorse!

Olly

[SteveBz]

36 minutes ago, ollypenrice said:

Stacking programmes don't know what you're feeding them, only what you tell them you're feeding them! So if you put a master bias into DSS's box for darks, it will treat them as darks and subtract them. To save your own sanity you could rename a master bias as something like Dark_universal  and, while you're about it, save another master bias as Flat Dark  because a master bias is all you need as a dark for your flats. Then you wouldn't use a 'bias-as-bias' anywhere in DSS so far as I can see. I don't use DSS but I lie to AstroArt in this kind of way without shame or remorse!

Olly

Well, I'm not sure that it subtracts darks from flats.  I think the formula may be something like this:

DSS.Image = (Raw.Image - Average.of.Darks[saved]) / (Average.of.Flats[saved] - Average.of.biases[saved])

Because there is no dark current in flats, only electronic noise (bias).  Your flat is, say, 1/80th sec, but your flat is, say, 300 secs - lots of dark current.  If you subtracted a dark from a flat it would provide a large unintended effect.  If you had no bias-as-bias, then your flat would not be corrected at all. 

The bias-as-dark is a great idea, but I think you probably need bias-as-bias too.

I stand ready to be corrected, what do you think?

Regards,

Steve.

[andrew s]

I have never used DSS but some capture software adds header keywords to indicate the type of frame and some analysis software reads this to use in the processing.

For a cooled sxvr h694 I measured 1 adu as the average difference between a bias frame and a dark matching the exposure of a flat and 10 adu for a dark matching a 600s exposure.

Regards Andrew 

PS ccd at - 20c