Re: photometric vs. non-photometric

[Chris Albertson <chrisja@jps.net>]

Jure writes:
> It would be useful to match all detected objects to a catalog ...

I think we do this.

Before the first Mk III data is imported, we "seed" the catalog with
Arne's "tassm16" data.  That is we creat a TASS catalog with a few
million stars in it all with "number of observations" set to zero.
The tassm16 list is a subset of the FAAST catalog which has good
astrometry.  As I remember, we use the FASST astrometry until
the "number of observations" reaches some threshold then we
compute and use a mean.  The idea was to mimimize the "wondering"
that Michael describes below which would happen if you compute
the mean using a very few observations.  That said, we likely
do have a little of the problem Michael describes but I don't
think it is that bad.

I have an another idea to partially explain the scatter.
Stars "close" to other strars may be rejusted by the detection
algorithum.  One why to check this is to get both an image
and the starlist and over-plot a circle on the image for every
line in the starlist.  Look at the result.  Any uncircled stars
were not detected.  I did this looking for another problem a
couple years ago. I did notice we miss quite a few stars that
my eye could detect.


Stupendous Man wrote:
> 
>   Andrew Bennett has noticed the uneven coverage of 'tenxcat':
> 
> > A cursory examination of tenxcat shows that some sources
> > were measured once and some 40 times. More detailed
> > examination shows that this wide scatter is:
> > 1) Not confined to weak sources.
> > 2) Not confined to the edges of the survey area.
> > 3) Not just the result of the uneven RA coverage.
> > Indeed the scatter persists even for small selected
> > areas well within the most heavily covered area and
> > with the exclusion of weak sources.
> 
>   I suspect that some of this scatter is due to the matching
> of sources from night to night.  Images from each night are
> reduced locally, yielding list of stellar positions and brightness.
> When we import them to the database, each position is compared
> to positions of stars already in the database: if a new
> source is within 15 arcseconds of an existing source, then
<SNIP>
-- 
   --Chris Albertson             home: chrisja@jps.net        
     Redondo Beach, California   work: calbertson@logicon.com