I Camera Flatness

[Tom Droege <tdroege@veriomail.com>]

Michael has replied to my comments about the flatness of the lenses.  This 
has prompted a little work.

First, I want you all to recall how the lenses were designed.  In 
discussions with Elliot, we chose to optimize the lenses in a circle 
tangent to the sides of the 2048x2048 square.  That is, the corners were 
left out of the optimization.  We could have used a circle that included 
the corners.  That would have proved to be somewhat more demanding.  We 
chose to leave out the corners in hopes of a better over all result.  I 
believe that Elliot's program sought to optimize the RMS deviation from 
flatness over the circle above.  I also recall (mail is on a 
dead  computer) that we set 1% as the goal for the flatness.  Below, I 
attempt to look at the flatness in the same way as Elliot's optimization 
program did.

To look at the flatness, I made a sky flat from 20 of the I images on Disk 
17c.  I made a dark from 3 images on 17c.  I subtracted the dark from the 
flat.  I then examined the sky at various points on the image to get a 
rough idea of the flatness.

The mean of the entire image, less a few pixels along the edges to avoid 
putting the covered pixels in the computation was 3677.

I took the image and subtracted 3677 from each pixel.  This left me with an 
image with +/- deviations from the mean for each pixel.  I then computed 
statistics for the image.  The result:

sigma = 30.4

30.4/3677  =  0.8 %

Note that this number includes readout and sky noise, albeit reduced by the 
dark and flat fielding process.  It also includes the corners.

It would thus appear that Elliot did what he set out to do.  Further, I 
think we were all in on this decision.  So if this is not good enough we 
are all to blame.  ;^)  I also note that it is much better than the camera 
lens that I tested which was down by 50% at the circle edges, worse at the 
corners.  I believe ROTSE had similar results (somewhat less deviation) 
with very expensive camera lenses.

I think things are really a little better than this since on TOM the 
optical axis may not be perpindicular to the CCD.  The images certainly 
give this impression.  I also think Andrew's work supports an out of square 
axis.  This will be corrected when I have a chance to change out the 
mounting plates.  Also, looking at the corners, they are not so bad.  The 
worst one had a mean of 3350 compared to 3677 or down by 8.9%

OK, I think this is a lot different way of looking at things than Michael's 
approach.  I think what is important is how it works for measuring real 
stars.  Michael's approach does this.  The above tries to separate out the 
optics.

Comments anyone?

BTW, the server was so slow today that I never succeeded in loading the 
plots from TN-76.  I assume Michael was referring to the plots with various 
sized circles.  I had to go on how I remembered them.

Tom Droege


At 09:37 AM 3/29/01 -0500, you wrote:

>   If you look at Tech Note 76, you can see examples of flatfields in
>both V and I taken with Tom's Mark IV last fall.  The variation from
>center to edge is very roughly  10% in V-band, and very roughly
>20% in I-band.  Tom has warned us that some of the V-band images
>included a tree branch, so my guess is these are worse than the
>performance of the optics alone.
>
>
>                                             Michael Richmond