Scalable darks

[Forum_2016]

Hi All,

I am interested in using scalable darks when performing image calibration. I don't seem to find much guidance in the AA6 manual or Help files. (perhaps I am missing something). AA6 appears to be able to handle scalable darks, judging by the inclusion of bias frame input provision in preprocessing window.

Any advice on the use of scalable darks would be appreciated. Also, how long can I expect darks (scalable or otherwise) to remain valid, given that I have a temperature controlled SBIG ST8xme CCD camera?

Regards, Grahamh

[Forum_2016]

Hi, in my opinion (for CCD cameras) scaled darks should be used only in these cases:

1) You took a 2 hours exposure and you have no time or no energy to take some 2 hours dark frames, so you will use a shorter dark frame.

2) You took an image with an an uknown exposure time or temperature so you cannot take a matching dark frame (nowadays it's unlikely, since all the data is inside the FITS header).

3) You took some images which are not so important, so a quick calibration with scaled darks is acceptable.

For all the other cases a calibration with the correct dark is recommended.
About validity, the scaled dark is calculated with an iterative algorithm which minimizes noise.

Off topic:

Bias frames are used only with scaled darks. Don't use bias frames for normal preprocessing (this is asked a lot of time). See the book "Lightcurve Photometry and Analysis" page 36.

[Forum_2016]

Thanks Favio,

I thought to use scalable darks so as not to require dark sets for differing exposures. (eg. using one set set of 600sec darks for all exposures less than 600 seconds.) Your comment seems to indicate that for quality results it is better to shoot a full set of darks at each session, for each intended exposure. (which of course takes more time)

My comment on validity of darks was really a question on the wisdom of using a library of darks taken days or weeks separated from the imaging session. I would only use bias subtraction with scalable darks by the way.

Please let me know if I am drawing the wrong conclusions.

Regards, Graham

[Forum_2016]

Hello Graham,
a library of darks is usually valid for several months or more, usually it's an optimization based on the temperature: on winter your camera may be able to work at -40 celsius while on summer only at -20. I still think that a group of darks taken three months ago is better than some scaled dark taken last week.

[Forum_2016]

Hi Favio,

OK, i'll keep that in mind, many thanks,

Graham