astrometry

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

  Tom writes:

> I think we should not be very concerned about astrometry.  I think our
> primary astrometry concern is how well we can separate stars that are
> close.  We do not really care where they are.  But we do care if we have
> one star or two close stars.  Of course we care to a few arc seconds so we
> get the identification right.  

  Arne seconds that notion:

>   TASS is not an astrometric survey.  Don't worry if
> you only get arcsec coordinates; try to get the best
> possible photometry instead.

  I agree with both of them.  Other surveys have already measured
(or are about to measure) positions for stars the Mark IV will
detect, more accurately than the Mark IV can.  I don't think it's
very important to spend weeks of work in order to increase the
precision of positions from, say, 1.0 arcsec to 0.5 arcsec. 

  Unless, of course, it's a fun project for someone.  I enjoy 
thinking up and implementing clever ideas, and I certainly
don't want to discourage someone else from giving astrometry
a shot.

  The important thing is to provide enough information so that
others can identify each object we detect in other surveys.
As Tom points out,

> I think our
> primary astrometry concern is how well we can separate stars that are
> close.  We do not really care where they are.  But we do care if we have
> one star or two close stars.  

  Absolutely.  A sharp core to the PSF will help to separate close
pairs, but my guess is that the primary issue here is finding
a robust algorithm for merging observations from different nights.

    - what if 2 stars are detected one night, but only 1 (merged)
               image on another night?

    - what if 2 stars are detected one night, but only 1 (single)
               star another night (if one star fades, for exampl)?

    - what if 2 stars are detected one night, and 2 stars are detected
               another night, but the astrometric solutions on the 
               two nights are sufficiently different that the 
               detection of star A on night 1 matches up with the
               detection of star B on night 2?

  I don't have the answers...

                                            Michael Richmond