Re: Data reduction methodology for V-I colors

[Thomas Droege <tdroege2@earthlink.net>]

Thinking about it, I would like to restate my position.

Michael Richmond writes:

>   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.  

I think I would now state:  There is something else going on that prevents
one from seeing much difference between a good night and a bad night.  By
bad night I mean hazy or thin clouds, etc.. Or putting it differently, the
something else is a larger effect than the difference between a bad night
and a good night.  

Tom Droege


> [Original Message]
> From: Stupendous Man <richmond@stupendous.cis.rit.edu>
> To: <tass@listserv.wwa.com>
> Cc: <mwrsps@rit.edu>
> Date: 9/8/2004 4:47:29 PM
> Subject: Re: Data reduction methodology for V-I colors
>
>
>   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
>