Re: Tech Note 66: Degree of Crowding on Mark IV images

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

  Tom discusses the problems of blending:

>  For example, one evening we might measure a star 
> sitting in the center of a pixel, then next evening it might be sitting at 
> the junction of 4 pixels.  The result is an entirely different looking 
> PSF.   This is not so much a problem for measuring a particular star, since 
> photons are mostly conserved by our detector. ( I am not even certain about 
> this.)  But a star close to the measured star may contribute more or less 
> photons to the measurement depending on how the two stars are located on 
> the pixel grid for successive measurements.

  Exactly right.

> One way to detect this, is to keep raw data around each measured star.  We 
> might keep 7 x 7 pixels.  Suppose we measure 250 stars per square degree, 
> or 10M stars.  Two filters and 100 measurements per star gives 200 
> GBytes.   Not an impossible number.   This is "only" 400 CD ROMs.
>
> Given this data, we could stack the 100 images for each measured 
> star.  This would bring problem near by stars out of the noise by a factor 
> of 10(?) where they could be identified. 

  I think that a better way to deal with this is to look up 
suspected blends in

         - the digitized POSS 
                 a catalog of stars from scanned plates is already
                 available, and has been for some time, at the 
                 U of Minnesota APS:
                       http://isis.spa.umn.edu/

         - USNO A2.0

         - 2MASS
                 http://www.ipac.caltech.edu/2mass/

  But we'll see ...

                                           Michael Richmond