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How important is darks temperature?


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My astro-modifed Canon 1100D is still relatively new to me and I haven't got a large library of darks.  A couple of nights ago the temperature dropped quite markedly towards the end of the imaging session (19oC).  It was getting light by the time I started the darks off and the temperature rose quite quickly (26oC).  (these are the chip temperatures from the EXIF data).  How important is this 6oC difference between the lights and darks going to be to the final image? 

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Easy answer: 100000% critical

Dark current doubles for every 6~8C temperature rise..  so if you're 1C out, that means you will still be 10% out.

Best solution:  Use IRIS to do your dark subtraction..   it has an optimal dark subtraction routine that does a correlation to confirm it's removed just the right amount of dark from your lights..   end result, your darks can be at substantially different temperatures and lengths (but still same binning of course) and you get near perfect dark subtraction.

I find it makes a visible difference when subtracting darks from my setpoint cooled CCD, so for a DSLR at who knows what temperature it really is worth considering.

Other software may also do this.. though I don't know of any that does... 

Derek

IRIS Tutorial on this, the bit you're looking for is about half way down:  http://www.astrosurf.com/buil/iris/tutorial2/doc9_us.htm

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Back when I was imaging with a DSLR I took darks every time I went out.  But it's very difficult to get exact temperature matching because the sensor temperature changes.  

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Leaving a DSLR

My astro-modifed Canon 1100D is still relatively new to me and I haven't got a large library of darks.  A couple of nights ago the temperature dropped quite markedly towards the end of the imaging session (19oC).  It was getting light by the time I started the darks off and the temperature rose quite quickly (26oC).  (these are the chip temperatures from the EXIF data).  How important is this 6oC difference between the lights and darks going to be to the final image? 

Darks are very sensitive to the sensor temp as are the lights. Ideally they should be at the same temp. Leaving the DSLR in cool box will not help matters . Your best bet is to leave the DSLR in the garage or a shed away from any heat source at the end of imaging session and take the darks, then activate dark optimisation in the stacking software. If the  the temp difference between the lights and the darks are within a few degrees the software can adjust to some extent. It is also imperative to image with "extreme dither " while imaging  with a DSLR. A very large number of Bias frames ( about 100+ ) in conjunction with dithering during capture and a bad pixel map will certainly give you better results than mismatched darks.

A.G

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I have often wondered if the canons inbuilt dark routine has benifits i know it reduces the number of lights during a session but the match must be nigh on perfect has anyone tried it?

Alan

Hi Alan,

All DSLRs have this dark subtraction routine built in, however even if they perform as expected the routine will certainly reduce your imaging time by at least 1/2. It is of much more benfit to use the time for subframe capture with dither and then let the software get rid the thermal noise, in this way assuming that a large number of subs can be captured a very real improvement in S/N ratio will be available something that is not possible with incamera dark subtraction.

A.G

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Thanks. How do I get and use a bad pixel map?

The procedure depends on the software you using. I have both Nebulasity and Pixinsight. In Nebulasity a bad pixel map can be made from a single dark frame and then adjusted. I have not tried it with Pixinsight, perhaps if Olly is reading he could come up with a few suggestions.

A.G

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You can make a hot pixel map in IRIS quite easily, the command is Find_Hot.  Apply that to your master dark frame and it will find and record all the duff pixels  (I have a 383L+, so it's like a snowstorm, and it fixes that)

You can then fix a raw frame with the command COSME  (also in a tick box on menu driven pre-processing)

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