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Old 15-07-2015, 09:32 AM
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Robert9 (Robert)
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More on Darks

Hi,
I'm in the process of setting up a Darks library. Looking for advice on what temperature ranges are reasonable; ie, if I take sets at 5 intervals, (0,5,10,15, 20C etc.) would this be sufficient, or should I opt. for a closer range? I do feel that having a set of darks for each degree would be a bit over the top. Any thoughts please?
Robert
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Old 15-07-2015, 10:24 AM
glend (Glen)
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For which camera? If you have a temperature regulated camera (CCD or DSLR) you can just shoot darks at your TEC setpoint and then only have to match the time intervals that you normally use for your subs.
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Old 15-07-2015, 11:43 AM
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I'm using a stock standard Nikon D90 DSLR, so my darks are necessary. I want a library so that I don't waste exposure time taking darks.
Robert
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Old 21-07-2015, 09:24 PM
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5C increments is acceptable for DSLRs without cooling. You may encounter some black pixel artifacts in your stacked image, if the temperature difference is ~5C, but this can be virtually eradicated by dithering your shots aggressively.
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Old 22-07-2015, 06:25 PM
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Thanks Rowland. Would you say perhaps that 3C intervals would be better?
Robert
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Old 22-07-2015, 07:20 PM
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In practise a maximum difference of 1.5C between dark libraries, as opposed to 2.5C.
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Old 22-07-2015, 09:13 PM
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That will make for one big darks library. What number of exposure groupings would you suggest? 5 to 50 in steps of 5?
Robert
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Old 23-07-2015, 01:24 AM
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That is, 1.5C difference in dark noise where the temperature of the light frames falls midway between two dark libraries. Or 2.5 if the spacing is 5C.

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That will make for one big darks library. What number of exposure groupings would you suggest? 5 to 50 in steps of 5?
Convention is to take sufficient darks at a temperature, iso and exposure length to provide a reasonable average of dark noise.

If you know before hand what the iso and exposure times will be for the majority of your work the variable is temperature, and unless your camera has regulated cooling, you will find that the libraries will be acquired at other than nicely ordered 3 or 5C increments. You can limit your requirements within typical exposure and iso values rather than creating a vast number of predetermined libraries that you may never use.

As dark noise is a function of iso and temperature, a short exposure time at high temperature, may come close to a long exposure at a lower temperature. This can get messy.

Personally, I take darks at each session, except when using my cooled DSLR. These can be stored for future use.
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Old 24-07-2015, 12:08 PM
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Thanks Rowland. Interesting to get another viewpoint.
Robert
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