Re: Flatfield images for disk 6

[Tom Droege <droege@wwa.com>]

Ted Woodhouse sent me some flats that he has derived from the images on
disk 6.  Sigh!  I keep making improvements in the data.  Later images may
not have all these problems.  Still, I thought it would be nice to comment
to all on the defects seen in these flats.  I find 5 major problems:

Problems:

1)  Horizontal banding.

2) Circular pattern on the I image.

3)  Skuz in image center.

4)  Finger Prints

5)  Ice Crystals

Non Problem.

First the problems. 

1)  Horizontal banding.

If one looks at the pattern of a small area one sees a dark band every 10
or so horizontal lines.  (I did not count them so I may be off - a lot.)
This is probably due to feed through from the system clock.  It should
repeat from image to image, so it should be able to be subtracted out.  I
have tried, and it is not perfect, but it can be reduced by subtraction.
In any case, it has at most a 6 count rms effect, which is much smaller
than our expected sky noise under most conditions.

2) Circular pattern on the I image.

This one is new to me.  Looks like some sort of optical interference.  I
have no idea of where it comes from.  It is quite off center.  There
appears to be about a 5 count difference in the mean between where it looks
dark and where it looks light.  This effect is probably in the noise, but
something that can be calibrated. 

3)  Skuz in image center.

This is frost.  Frost seems to come in two varieties.  A gentle mist that
just shows up as a whitish area in the center of the CCD, and obvious ice
crystals.  A section through the skuz looks like grass growing in the
center of the image.  Eventually I will work on keep ing the camera dry.
It is connected to a desiccant, but when I am working so much on the
cameras they never have time to dry out.

4)  Finger Prints

One can see a few dark spots in the I image.  These are clearly lower
sensitivity areas in the CCD.  We just have to learn to live with them.
Most of them are will within what is allowed by the specification.  Well, I
just tried to read it.  There really isn't a spec.  The best I can find
allows 10% non-uniformity before they call the pixel defective.  I think
all the "finger prints" are much smaller than this.

5)  Ice Crystals

Ice crystals are obvious.  The V flat contains obvious ice crystals.

Non Problem.

I comment here that the flats are pretty flat.  A typical camera lens would
be 50% down at the edges.  A very expensive ($5000 I recall) camera lens is
down 40% per the ROTSE paper.  I did not try to make a good measurement of
this on Ted's image (I don't know where zero is) but it looks like of order
10%.  So while we have coma, we don't have much optical variation over the
CCD.  

Thanks to Ted for his efforts. 

Tom Droege





At 07:17 PM 1/26/00 -0800, you wrote:
>Hi again, Tom,
>	I'm going to send you a CD containing two median darks
>I made from the images on disk 6.  There is one for each camera.
>I think you will find them interesting to view, and maybe even
>significant.  There are wave patterns going in various directions,
>and some really slimy looking things that might have crawled out
>of an Antarctic meteorite.
>	These are median flats, made up from the first 24 images
>from each camera.  This doesn't include the focus sequences, as
>of course you can't make a median flat from images that are
>almost identical.
>	I'll burn it tonight and get it in the mail tomorrow or
>Friday.
>
>				Ted
>
>