another check on quality of Mark IV ensemble photometry

[Michael Richmond <richmond@stupendous.cis.rit.edu>]

  (apologies if you have seen this before -- I had E-mail issues yesterday)


   I've mentioned before my plan to reduce the Mark IV photometry
 by breaking it up into "patches" of roughly 1 degree on a side,
 and then considering the data within each patch as an ensemble.
 Tech Note 101 describes the idea and gives a few initial findings.
 
      http://stupendous.rit.edu/tass/technotes/tn0101.html
 
   I have finished running a first pass of the ensemble photometry
 code on most of the Mark IV data.  Below is a table which summarizes
 the properties of the resulting ensembles.   The quantity of interest
 is the "scatter around mean ensemble magnitude"; that tells us how
 _precise_ the Mark IV photometry of a single star is from one night
 to the next to the next.
 
   Caveat: to appear in this analysis, I required that a single star
 be measured at least 5 times in both V and I.  
 
   In the table below, I break all the data into bins by the magnitude
 of stars.  Within each bin, I list 
 
                N            number of stars in this bin
          
              unclipped      mean and stdev of the scatter within each bin
 
              clipped        ditto, after 1 round of 3-sigma clipping
 
              median         median of scatter within each bin
 
              iqm            mean and stdev of all scatter values 
                                   between the 25'th and 75'th quartile
 
 
 
 
 #  min   max  N        unclipped      clipped      median    interquartile
 #                      mean  stdev    mean  stdev              mean   stdev
 #----------------------------------------------------------------------------
       V-band
 
    7.0   8.0  1398   0.032 0.016    0.030 0.012    0.029     0.029  0.005 
    8.0   9.0  4098   0.038 0.025    0.037 0.015    0.035     0.035  0.006
    9.0  10.0 10887   0.043 0.026    0.041 0.016    0.038     0.039  0.006 
   10.0  11.0 26699   0.049 0.031    0.046 0.017    0.043     0.044  0.006 
   11.0  12.0 63549   0.067 0.035    0.063 0.019    0.061     0.061  0.007 
   12.0  13.0 72528   0.110 0.042    0.105 0.030    0.103     0.104  0.012 
   13.0  14.0 80510   0.161 0.056    0.155 0.045    0.153     0.154  0.018 
   14.0  15.0  2290   0.231 0.079    0.225 0.069    0.223     0.225  0.029 
 
 
 #  min   max  N        unclipped      clipped      median    interquartile
 #                     mean  stdev    mean  stdev              mean   stdev
 #---------------------------------------------------------------------------
       I-band
    7.0   8.0  19210  0.045 0.039    0.039 0.023    0.033     0.035  0.008 
    8.0   9.0  46670  0.035 0.030    0.030 0.017    0.026     0.027  0.006 
    9.0  10.0 115141  0.034 0.028    0.031 0.016    0.028     0.028  0.006 
   10.0  11.0 276427  0.037 0.026    0.035 0.014    0.032     0.033  0.005 
   11.0  12.0 628596  0.049 0.023    0.047 0.015    0.046     0.046  0.006 
   12.0  13.0 789607  0.077 0.031    0.073 0.024    0.072     0.072  0.009 
   13.0  14.0 111809  0.118 0.050    0.112 0.039    0.111     0.112  0.015 
   14.0  15.0    371  0.275 0.096    0.273 0.085    0.275     0.274  0.034 
 
 
   You can find a figure which illustrates some of these columns
 at the URL
 
       http://spiff.rit.edu/richmond/temp/scatter.gif    (GIF)
       http://spiff.rit.edu/richmond/temp/scatter.ps     (postscript)
  
 
   The bottom line is that the typical scatter left over after
 doing the ensemble photometry is about 3 percent at best, and increases
 to around 12 percent at the faint end.
 
   This is right along the lines of earlier work I and others have
 done, so no big surprise.  This just confirms that the results
 remain when you consider all the data.
 
   Very slowly, I am making my way through this sort of analysis.


                                  Michael Richmond