Refcat Results

[Tom Droege <tdroege2@earthlink.net>]

Attached are some plots for consideration by the experts, and others that 
might like to look at interesting plots.

These plots were made from the DS-24-3 data set.  It contains the Orion 
belt stars in one corner of the fields.  For new lurkers, this data set 
covers the field 49 times with each image displaced from the previous image 
by 1/3 degree in x or y.

The reference catalog used was big_refcat.dat edited in declination to 
cover the area of interest to save lookup time.

One image pair at a time was put into the input directory and the process 
run through to conclusion.  After the do_ccdproc step the images were 
smoothed using an 8 x 8 piecewise grid.  Light box flats were used.

The 49 individual .cal files were then matched to the refcat.dat 
file.  Only the stars in both are plotted.  Each star can appear in the 
plot up to 49 times.  In practice there are very few stars that appear in 
the output more than 40 times.

We take the difference between the refcat magnitude and the magnitude 
determined from the pipeline and plot it vs the refcat magnitude.  This is 
done for V data in vcat.png and for I data in icat.png.

I point out one star previously mentioned.  This is the star at 81.853 
-0.586.  It appears on the I plot at mag 10 and +2 difference.  On the v 
plot this star is in the main stream at V mag 10.857 and a difference of 
0.155.  Note that the grouping for this star is relatively tight in 
I.  There are 20 or so measurements in the grouping.  Thus our precision 
appears to be OK, it is the accuracy that we need.

One notes that the V data is pretty symmetrical around zero as one would 
hope.  The I data is not symmetrical, and is biased toward tass stars being 
brighter at low magnitudes and fainter at high magnitudes.  (Readers are 
warned that I always get signs wrong on such things.)

OK, it is not yet clear to me that this is a big source of error.  While 
the one star is 2 mags off the mean, there are 800 stars typically in the 
solution, so I suppose we divide 2.000 by 800 to get a 0.0025 mag effect 
depending on whether this one star is in the solution.  But there are a lot 
of pretty large differences.  Comments are welcome.

Tom Droege

icat.png

vcat.png