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Re: systematic gradients

aah@nofs.navy.mil wrote:
>   With the Tycho catalog and the transformations previously posted, we now
> have enough information to check how well the sky-based flatfielding works.
> If we use the mean of the 100+ Tycho stars in a frame to set the zero
> point, then we can derive a table of differences between a Tycho star's
> V magnitude from the posted transformation and from the extracted
> magnitude from the image.  If we then have the star's x,y position on the chip,
> then we can build a map of magnitude differences vs. position on the CCD.
>   If the flatfield is perfect, then the map should have some scatter, but
> should essentially be zero all across the chip.  If the flatfield is in
> error, perhaps by a gradient in declination, then there should be a
> systematic pattern in the difference map.

Unfortunately a poor flat field is not the only source of a gradient in
declination.  I have noticed in both my and Tom's images a difference in
focus between the high and low declination edges of the frame.  For a
fixed size aperture a broader PSF means a relatively lower measured
magnitude so one side of the frame may systematically measure lower than
the other side.

We also need to be careful in which Tycho stars we use.  Some of the
stars have pretty crude magnitude measurements compared to the stars in
the Hipparcos catalog.  I have been investigating the Tycho catalog and
find that only 20% of the entries have both sigvt and sigbt less than
0.050 magnitudes.  Using all the Tycho entries may give poor results
compared to using a selected subset.

I have been trying to determine if such systematic errors exist in Tom's
image set.  If I select a subset of the Tycho catalog with sigvt and
sigbt both less than 0.050 then I get a scatter of 0.065 magnitudes
between the Tycho catalog V measurements and several TASS V
measurements.  This is true if I use either the PSF aperture or a 7x7
circular aperture.

What is really strange, and so far incomprehensible to me is the
following.  If I measure the variation between PSF aperture measurements
on different nights I get about 0.020.  The same is true for a 7x7
aperture.  So TASS magnitudes are self consistent.  However, both
magnitudes have a variation of about 0.065 with the good Tycho catalog
stars.  If, however, I average the PSF and 7x7 aperture magnitudes and
compare those values with the Tycho catalog I get a variation of 0.020! 
Three times better than either alone.  If I look at the data I see that
the error between the Tycho magnitude and the TASS magnitudes is
opposite for the circular and PSF aperture measurements!  I am
completely baffled at this point since the code looks like it treats
both apertures identically.  I'll keep looking for the systematic error
but if anyone has an idea I'm all ears.

>   If the night is photometric, then one can use many frames of data in building
> this map, to improve both the resolution and the signal/noise at each
> position.  With such a big field, it is important to make sure that the
> flatfield is 'flat' and repeatable to within a percent or so over the entire
> area.
> Arne
> --------------------------------------------------------------------
> Arne Henden                               Instruments/software/CCDs
> US Naval Observatory Flagstaff Station    Cepheids/photometry/IR
> P.O. Box 1149                             ftp:
> Flagstaff, AZ 86002-1149                  Voice: (520)779-5132
> aah@nofs.navy.mil                         FAX:   (520)774-3626