Re: Working on Data

[Tom Droege <tdroege2@earthlink.net>]

Michael,

The V plot is very interesting.  Is this the whole data set or a sub set of 
it?  Hmmmm!  This could be the result of doing a planar flattening because 
of some sky/local/area brightness that was wrong??  Or just caused by the 
way light reflects off the structures on my roof.  This is more likely for 
TOM1 data.  TOM2 and TOM3 are up above everything, and so should not see 
any reflections.

Tom Droege

At 06:24 PM 7/9/03 -0400, Stupendous Man wrote:

>   Tom wrote:
>
> > I have been working on the data.  That is why I have been quiet.
>
>   I, too, have been working on the data.  I agree with Tom that
>one can identify "bad" nights and portions of nights.  I also
>agree that doing so does not solve all problems.
>
>   I've been concentrating on the scatter among measurements of
>bright stars.  It should be much smaller than we actually measure.
>Could it be due to errors as a function of position on the chip
>(which one could loosely term "flatfielding")?
>
>   I think that the answer is "yes" in V-band (which is where the
>scatter at the bright end is most annoying), and "no" in I-band.
>Take a look at these pictures of residuals as a function of position
>on the chip (for a very particular subset of all star -- details
>in an upcoming Tech Note):
>
>      http://spiff.rit.edu/richmond/temp/gamma_V.gif
>      http://spiff.rit.edu/richmond/temp/gamma_I.gif
>
>   Note the clear systematic pattern in V-band residuals, and lack
>thereof in I-band residuals.
>
>   My plan is to define a correction to V-band as a function of position
>on the chip, apply it to stars, and re-calculate mean and stdev
>of measured magnitudes.  If I'm right, it might reduce the scatter
>at the bright end of the V-band.
>
>   After I do _that_, I'll look into moving the Mark IV measurements
>from the Tycho V,pseudo-I system onto the standard Johnson-Cousins V,I
>scale.  Arne's extra calibration data should help.
>
>   I'm thinking that _eventually_, I'll be able to sift through the
>database, select a subset of all the measurements, apply a set of
>corrections, and produce a "nice" catalog.  It will be analogous
>to the Mark III dataset and the "tenxcat" catalog.  That's my
>current goal: creating a list of stars with decent (V,I) magnitudes
>on the standard scale over as large an area as I can.
>
>   This will probably _not_ be especially relevant to those who
>want to look through the Mark IV data for small-amplitude variable stars.
>Large-amplitude variables are already obvious, as a number of
>people have shown -- just look at the growing lists on the Wiki.
>I hope that some other people will consider ways to massage
>the data for that purpose.
>
>                                          Michael