AstroArt PreProcessing

[AlistairW]

Been playing around with AstroArt. Under PreProcessing there are the following lists:

> Images, Dark Frames, Flat Fields, (F.Dark Frames)

What are F.Dark Frames ?

Then I think you can use a master bias as dark. I presume this is a simple as adding the master bias to the Dark Frame option ?

Thanks

Alistair

[The Admiral]

F.Darks, or flat-darks, are darks taken with the same exposure as the flats. As this is normally a short exposure my understanding is that there'd be little difference between a flat dark and a dark, so can be replaced by a dark.

There does seem to be a practice of replacing a dark with a bias, which is particularly useful when using a DSLR as the temperature is not controlled and for which a dark is not particularly representative. This is what I do, that is, use a master bias in place of both f.dark and dark. Doing this strictly requires the the use of dither and incorporation of a hot-pixel map as well I believe, though I don't. Olly Penrice is the guru on using AA like this.

Ian

Edit: have a look here

[ollypenrice]

4 hours ago, The Admiral said:

F.Darks, or flat-darks, are darks taken with the same exposure as the flats. As this is normally a short exposure my understanding is that there'd be little difference between a flat dark and a dark, so can be replaced by a dark.

There does seem to be a practice of replacing a dark with a bias, which is particularly useful when using a DSLR as the temperature is not controlled and for which a dark is not particularly representative. This is what I do, that is, use a master bias in place of both f.dark and dark. Doing this strictly requires the the use of dither and incorporation of a hot-pixel map as well I believe, though I don't. Olly Penrice is the guru on using AA like this.

Ian

Edit: have a look here

No, there will be no significant difference between a master bias and a flat-dark. Don't use a long exposure dark to calibrate a short exposure flat. It will be harmful. Just give Astro Art a master bias as a flat dark and it will work sweetly.

Olly

[The Admiral]

Olly, I have long wondered though whether the presence of, say, amp glow, or other similar large scale artefacts besides hot pixels would invalidate the use of a master bias in lieu of a proper dark (though not to replace a flat dark as I wouldn't expect such defects to be prevalent with short exposures). What is your experience in this?

Ian

[ollypenrice]

1 hour ago, The Admiral said:

Olly, I have long wondered though whether the presence of, say, amp glow, or other similar large scale artefacts besides hot pixels would invalidate the use of a master bias in lieu of a proper dark (though not to replace a flat dark as I wouldn't expect such defects to be prevalent with short exposures). What is your experience in this?

Ian

You're right to pose the question and you've anticipated my own findings. I do use a master bias instead of a master dark on my Sony and Kodak chipped cameras. On the Kodaks I also give AstroArt a bad pixel map and a heavy dose of hot pixel filtration. (The deep wells of the Kodak mean that stellar cores don't seem to be affected by the hot pixel filter.) I find this gives a generally cleaner stack on the Kodaks than I get from using normal darks. However, there is a bit of amp glow on the bottom right of the Kodaks which the bias-as-dark method doesn't clean up. It is so easy to fix in Ps, using the burn tool, that it isn't an issue. On the Sony I can find no difference between darks as dark and bias as dark. Literally none, so I use bias since one size fits all!

BTW, the snowstorm noise effect on the Kodaks is obvious pre-calibration but vanishes in the stack. There is no snowstorm on the Sony, leading us to assume that it has far less noise. But when I come to process the Sony data I find that I have lots of overly dark pixels in the background sky. A kind of soot-storm instead of a snowstorm! My own conclusion is that the Kodaks are better than first impressions would suggest. I very much enjoy processing the Kodak data. It stands up well to hard stretching.

Olly

[The Admiral]

44 minutes ago, ollypenrice said:

There is no snowstorm on the Sony, leading us to assume that it has far less noise. But when I come to process the Sony data I find that I have lots of overly dark pixels in the background sky. A kind of soot-storm instead of a snowstorm!

Hmm, mindful of the Sony star-eater problem, I wonder if the signal processing to reduce noise is intrinsic to the sensor chip rather than external to it, resulting in artificially 'quietened' pixels. I would be surprised if Sony made a different chip solely for astro imaging use. All a wild guess, of course!

Sorry, I am getting a bit off-thread here.

Ian