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Re: [OM] Digital top of the heap...

Subject: Re: [OM] Digital top of the heap...
From: "R. Lee Hawkins" <lhawkins@xxxxxxxxxxxxxxxxxxx>
Date: Sat, 28 Nov 1998 17:18:57 -0500
Cc: lhawkins@xxxxxxxxxxxxxxxxxxx
In your message dated: Sat, 28 Nov 1998 16:42:06 PST you write:
>The CCD I am most familiar with is an ST-4 by Santa Barbara Instrument Group
>(SBIG).  With the ST-4, you establish your operating parameters, take a dark
>frame, then do the image.  The imaging software automatically subtracts the
>dark frame from the image.  As long as none of the operating parameters are
>changed, then the one dark frame will work for all subsequent images.

Not true, at least with the ST-4.  The ST-4 cools the CCD some delta
below ambient temperature.  If ambient temp changes, so does the CCD
temp.  This can be a really important factor, since on most evenings the
temp will change by at least 6 degrees, which is a 50hange in dark
current.

>However, I've read that one should always take a new dark frame immediately
>prior to taking the image for critical work.  This tells me that the noise
>characteristics are not identical every time.  I suspect this is the case.

Yes, that is correct.  See above.  You can only make "master" dark
frames if your CCD is truely kept at a regulated temperature.

>Variables like the cooling temperature, voltage fluctuations and stuff I
>just don't understand can change.

Voltage fluctuations are minor effects.  The biggest effect is the
change in dark current.  Now image what happens in early evening as the
tempertaure is changing quickly... There is no *way* you can take a
meaningful dark/image combo in such conditions with an unregulated CCD.

Cheers,
--Lee


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