Re: My problem or yours?/test

[John McKendry <mckendry@mediaone.net>]

Tom Droege wrote:
> 
> Chris and all,
> 
> DS-0 gives me a 2064 x 2037 image.  That is what I expect.
> 
> There are 2 or 3 "covered" lines at the top (W) of the image, and 4 or 5 at
> the bottom (E).
> 
> The overscan pixels are where they should be.  I count 15 of them and 6
> covered at the far edge.  It is quite passible that I am one off in
> figuring out the overscann pixels as I am not sure exactly which pixel I am
> digitizing.  This depends on knowing how the pixels match the clock
> phases.  I do not understand the process well enough to know which pixel is
> actually being digitized.
> 
> Does anyone want me to change what I am presently writing in the fits header?
> 
> Tom
> 
Tom and all,

 I'm no expert, and I've been hesitant to jump into this discussion on that account,
but I think maybe some beginner-level questions might be in order. At the very least,
I have the feeling I'm not the only person trying to figure these things out.
So here goes:

 Am I correct in thinking you're using the Fairchild CCD442A CCD? According to
the data sheet, this is a 2048x2048 array with the top 8 and bottom 8 rows,
left 8 and right 8 columns permanently masked, leaving the center 2032x2032
pixels as the live imaging area.

 When your QBasic program calls "downfits" you tell it to read 2037 rows, each
row containing 2064 columns. I presume the 2064 represents 2048 actual pixels and
16 overscan pseudo-pixels. (Is this right?) I don't get the 2037 rows. If the CCD
has 2048 rows, and you only read 2037 of them, doesn't that leave the unread
pixel charges in the bottom rows of the CCD for the next exposure? When I look
at one of the later dark files (hira2157570.fits) I see that the bottom two or
three rows are very light, and the pixels in the lower right corner of the image
area (from x=2029 to x=2049, y=1) are completely saturated at 32767. On the
other hand, I see a very distinct 5-pixel dark strip at the top of these images,
suggesting that you're reading (some of) the masked rows, too. Hmmm.

 Going back to the 2064 columns for a minute (I've looked at your FLAT.BAS and at
the downfits source and I don't see the answer to this), how does the board know
to read the overscan pixels before it clocks in the next row of real pixels? 
Is that just something wired in to the PROM code? Or is it contained in one of those 
mystery variables that get passed to the Stamp?

 The rest of these are questions for people who know about FITS headers. (Where I haven't
posed an explicit question, the question is "does this make any sense?") To answer
Tom's question, I think it's going to be necessary to change the headers somehow,
because even in my limited experience the IRAF CCD reduction tasks are not happy
with the current values. I think the fields involved are DATASEC (and its equivalent
IMAGESEC) and BIASSEC. (Possibly CCDSEC and TRIMSEC, but I haven't seen a need for
them yet.) I have already run afoul of DATASEC and BIASSEC in trying to run "darkcombine", 
and I've found a way to fix them such that darkcombine no longer complains, but I'm not 
sure I've fixed them the right way. In fact I'm rather sure I haven't.

 DATASEC in the DS19 files is [17:2064, 1:2048]. The 2048 is clearly wrong for
an image 2064x2037. But my reading is that this field should describe the central
2032x2032 live-image area. If the file were 2064x2048, then I would guess that
DATASEC should be [9:2040, 9:2040]. This would eliminate the 8-pixel dark-masked
rows and columns at the edges of the CCD, as well as the 16-pixel overscan region.
Because I don't understand the 2037 rows, though, I don't know what's the right
value here. Maybe [9:2040, 1:2032] - that would eliminate the dark strip at the top,
anyway.

 BIASSEC is currently [10:15, 1:2048]. Again the 2048 should be 2037. But this is
supposed to be the overscan region, isn't it? In which case, don't we want it to
be [2049:2064, 1:2037] ? 

 And one last FITS-header question: what's the right thing to do with the dark-masked
regions of the CCD? Surely they contain information that could be processed somehow,
but I don't see any FITS keywords that seem appropriate.

 I thought I had more questions than that, but no. I guess the thing that has me most
stuck is the 2037 rows.

John McKendry