Re: Database error estimates

[Stupendous Man <richmond@stupendous.cis.rit.edu>]

>>   The values stored in the database are calculated for
>> each instrumental magnitude extracted from each image.
>> They are based on simple photon statistics: an aperture
>> of radius N pixels contains a total signal of E counts

  Chris asked: 

> Is the aperture fixed at N pixels?  

  Yes.

> Wouldn't it be better
> if it were fixed at a constant angular diameter? 

  No, I don't think so.  A constant angular diameter
would mean a radius which gets _smaller_ (in pixels) towards
the corners.  The FWHM tends to get larger (in pixels)
towards the corners.  So an aperture of fixed
angular size would give the same number of
electrons in the background for all stars (good)
but provide a smaller fraction of the total stellar
signal for stars near the edge (bad).

  The constant-pixel-size aperture gives a larger
background contribution for stars near the edge,
but also contains a less-variable-fraction of the
stellar signal across the frame.  I think, on the
whole, that's better.

  Tech Notes 51, 52, 59 and 63 show in various ways
how the PSF shape and size change across a Mark IV 
frame, and some effects on the photometry through
a fixed aperture.  
 
> If someone would send me the terms of an X,Y to RA,.DEC
> Nth order transform I'll work out the size of the error
> I'm sure this transform must be a byproduct of the pipeline
> and the numbers are available.

  I do not have these transformation values handy;
all my old Mark IV analysis data was lost when some
disk drives died a while back.  


                                 Michael


=====
Chris Albertson
  Home:   310-376-1029  chrisalbertson90278@yahoo.com
  Cell:   310-990-7550
  Office: 310-336-5189  Christopher.J.Albertson@aero.org
  KG6OMK


		
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