Re: Mark IV dark images

[hjohnson@pluto.njcc.com (Herbert R Johnson)]

On Tue, 20 Jun 2000 19:20:22 -0500, Tom Droege <droege@wwa.com> wrote:
*>Herb writes:
*>
*>>.... But there is no explicit information on the exposure time of
*>>those darks, the 8 I've sampled all look to be of identical but unknown
*>>exposure length. So I will present my data to Tom privately and ask him
*>>to be explicit on the details of these darks, which was my question.
*>
*>This is just not true.  I have answered this question several times 
*>before.  I cannot imagine that anyone cannot understand what I am doing 
*>after the great detail with which I have answered this.
*>
*>Here it is one more time:
*>
*>There repeat is five sequences of short, long, short, and long.  One of the 
*>five is made with the shutter closed.  In every respect the dark sequence 
*>made with the shutter closed is exactly the same as the sequences made with 
*>the shutter open.  It is in fact made with the same code.  When a dark 
*>sequence is to be made, I simply skip the open shutter code line.  The 
*>shutter is still closed (even though it is already closed.)  This preserves 
*>as much timing as possible.  Note that there is a detailed explanation of 
*>this in TN-62 along with a code fragment so that you can read it and 
*>understand exactly what is being done.
*>
*>Tom Droege

The problem with my "not understanding" was that I was not looking at
the right notes. I looked at the "notes" on Disk 15. When Tom
named *Tech Note 62* as the note HE was referring
to, I looked there and found the "notes" he was previously talking about.

One design feature that makes the darks so apparently uniform is the fact
that it takes a long time to clear the CCD relative to exposure times.
The dark current accumulation in each pixel is the sum of the
CCD clearing time plus the exposure time (approximately).
The resulting timings for clearing plus exposure, quoting TN 62, are:

A The Very first short     69 Seconds
B All first longs         183 Seconds
C The second short        128 Seconds
D The second long         183 Seconds
A All other first short   169 Seconds

...with the possiblity of occasional ten second delays. Also, Tom explains
in TN 62 that the dark current accumulation amounts to a few ADU's per second,
which he suggests may be more than he expected, not less.

So, the approximate
timings for dark sequences I saw in Disk 15 were probably 169 seconds,
183, 128, and 183; with small changes in average ADU counts between them.
What I found was consistent with the above information. In Tech Note 62,
Michael Richmond found similar results for an earlier camera run, asked the
same questions, and got the above-quoted answers and
a detailed explanation from Tom which is contained in that Tech Note.

So I'm sorry for my confusion. But I simply did not know where the information
was that I was asking for - we have a lot of Tech Notes, among other
reports and Web references - and the issues I asked about have confused
others before.

Herb Johnson

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