latest news on Mark IV pipeline

[Stupendous Man <richmond@a188-l009.rit.edu>]

  I've spent the past few days adding one more major feature to the 
Mark IV pipeline: "bad regions".  I noticed that some of the 
nights on Disk Set 18 feature nasty ice crystals in the I-band
camera (though some do not).  The crystals show up very nicely
in the flatfield images, so I've written a new XVista program ("mask")
which finds them, and modified another program ("phot") so that
it checks each star against a list of bad regions and marks those
which fall too close to one.  This should allow me to 
 
       a. see how large errors caused by ice crystals are
       b. discard any measurement near an ice crystal, if desired

  Also, one minor change.  There was a problem with the "master"
flatfields I created: because I used the true median of a (small)
number of night sky images, the master flatfield always ended up
with a significant fraction of pixels which were IDENTICAL to
the same pixels in the night sky images.  For example, if 10 
images were used to create a flatfield, then about 1/10 of the 
pixel values in the master flat would be the same as the pixel
values in each target image.  Later, when the master flat was used to
flatten the target images, about 1/10 of the pixels would end 
up being divided by themselves (in effect), and the resulting
corrected frame would have a VERY sharp peak in the distribution
of pixel values: about 1/10 would all have exactly the same 
value.  It looked okay to the eye, and (probably) didn't make
any difference in stellar positions or magnitudes, but really
screwed up my attempts to find a "sky" value.  

  So, I modified the pipeline so that it now creates median
flat (and dark) images by using the "interquartile mean"
rather than the median.  That is, it finds the 25'th and 75'th
quartiles of the pixel values at some position, and then calculates
the mean of all the values in between those limits.  
The big benefit is that the output value is (in general) _not_
the same as any input value.  When I use this routine to create
master flats, and then flatten target images with them, the
distribution of pixel values in the corrected frames is nice
and smooth.  

  It's time for me to stop fiddling with software and start
reducing some data -- especially since I leave for the AAS meeting
in, um, five days :-/  I won't have time to put together a kit
of the new pipeline before I leave, sorry.  But I'll do it after
I shall have returned, around mid-June.   Warning: there are some
minor changes to the output file formats.  

  Thanks very much to the people who have been testing out the 
current version of the pipeline: Michael Sallman and John McKendry 
have both sent me bug reports.

  I've looked at data from each of the Disk 18 CDs, and it looks
to me as though the best nights are

           18b, 18e, 18g, 18h, 18i

I _think_ that some of the images on disk 18e may be missing
FILTER values in the FITS headers; could someone else check on this?

                                              Michael Richmond