Re: puzzle in I-band photometry on Data Disks 18

[Tom Droege <tdroege@veriomail.com>]

Michael and all,

This is the prototype camera.  The plates that hold the lenses have about 
1.8" clearance all around.  (This is a big difference from your camera 
Michael, where the plates have been cut with a few mills clearance.

It is thus probable that the optical axis is not lined up with the 
CCD.  The alignment could be as bad as 10 milliradians.  Might this be the 
problem?

This has been improved on the production cameras.  Probably to the 1 mr level.

Tom Droege

At 04:54 PM 5/30/01 -0400, you wrote:

>   This has to be brief, since the power to our building is going
>down in 14 minutes.  I've been looking at photometry of stars
>on Data Disks 18g, 18h, 18i.  Repeated measurements of stars
>in V-band (on the same night) show very low scatter, as one would
>expect.  Good.  However, repeated measurements of stars in I-band
>(on the same night) show a floor of about 0.03-0.05 mag.
>
>   At first, I thought it might be a shutter problem, but the
>variations in stellar brightness from frame-to-frame are
>not correlated with variations in sky brightness.  Kill that idea.
>
>   I've now discovered that the variations depend upon the position
>of a star within the frame.  Specifically, stars on the East side
>of the chip tend to be brighter by about 0.10 mag than the same
>stars on the West side of the chip.  I'm using aperture photometry
>through a circle of radius 4 pixels.  The FWHM is around 3.5 pixels
>on these I-band images.  There isn't much variation in PSF across
>the frame.
>
>   The I-band flatfield does not show a pattern similar to this
>pattern of residuals.  Hmmm.
>
>   This sort of effect isn't uncommon with wide-field cameras.
>I can take it out, once I know how the effect varies with
>position.  For an example of the gory details, see a paper
>by J. Manfroid, "Stellar calibration of CCD flat fielding,"
>published in A&A, vol 113 (1995).
>
>       http://adsabs.harvard.edu/cgi-bin/nph-bib_query?
>           bibcode=1995A%26AS..113..587M&db_key=AST&high=3b152badf716502
>
>   Sigh.  More work for me after I return from the AAS meeting.
>
>   I wonder what exactly is the cause of the effect?  Could other
>people please check this out and verify that I'm not making some
>stupid mistake?
>
>                                           Michael
>