Re: Data reduction methodology for V-I colors

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

  Sorry to be so late in answering these questions, but it's the first
week of classes ...

  Richard Miles asked:

> I can't understand why you are overlooking the Hipparcos stars.  I shall be
> using this dataset even though my fields are much smaller than TASS' (1.6
> sq.deg vs. about 16 sq.deg).  Do note that the Tycho photometry was
> something of an afterthought exploiting the star tracker on board the
> spacecraft.  The Hipparcos photometry dataset (Hp magnitudes) was the prime
> photometric mission and is well calibrated against Landolt and Cousins.

  There are density main reasons that I chose to use the Tycho-2 catalog
instead of the Hipparcos catalog for photometric calibration:

        a) magnitude range: most of the stars in the Hipparcos 
                    catalog are brighter than V=9, whereas
                    the Tycho-2 catalog goes down an extra magnitude
                    or two.  I thought that the extra overlap 
                    with the Mark IV range (from about 7-12)
                    would be good
 
        b) the Tycho-2 catalog contains measurements made by the satellite
                    in two passbands, a B-like one and a V-like one.
                    This allows one to extrapolate an I-like magnitude;
                    it won't be very good outside a limited range of 
                    color, but it's better than nothing.  The Hipparcos
                    catalog contains only measurements made in a single
                    passband by the satellite itself; there is a
                    value for (V-I) color, but this is a mix of 
                    direct measurements and estimates from other 
                    information. I thought at the time that a set
                    of homogeneous measurements would be better.
                    I'm wondering now if that was the right decision.

  The fact that Tycho-2 has LOTS more stars helps, too; if one discards
some stars due to nearby companions, or wierd colors, or bad measurements,
one is more likely to end up with a decent number of "good" stars left over 
if one starts with Tycho-2.  

  However, if I were to start all over again, I might choose to try
the Hipparcos catalog as an input set for photometry.  Richard's
suggestion is a good one.

  Robert Bradbury asks:

> It would seem that one might get a handle on some of the
> scatter and/or background questions by taking some images
> of the same region of sky at different locations (e.g.
> the same stars early in the evening and late in the
> evening) when you have to point the scopes in almost
> opposite directions. 

  Yes.  This would be a good test to run.  

> Then one really wants a "best" ever image of any particular
> field of view (or sub-regions of a field of view).  There
> might be two or three of these, e.g. clearest night,
> quietest night (greatest atmospheric stability) and
> coldest night (least atmospheric background).

  Tom and I have both tried to compare a "best nights" approach
to an "all nights" approach.  I think it's fair to say that I see
promise in it (but I haven't gone far enough to say for sure),
while Tom finds little difference.  

> Finally, is any attempt being made to look at the same
> field(s) at the same time by multiple locations? 

  Tom's cameras are the only ones currently sending data
into the database.  I'm one of the guilty parties: I have a
Mark IV unit with a malfunctioning RA axis.  I just haven't
had time to fix it ... sigh.  Apologies.


                                        Michael Richmond