Re: Working on Data

[Tom Droege <tdroege2@earthlink.net>]

Michael and all,

Another question about the V tilt is whether it is constant over time.  The 
V lens has been worked on several times.  When I put in the flocked paper, 
the lenses were completely changed.  It would be interesting to look at 
this BFP and AFP  -  before and after flocked paper.  Possibly this is a 
problem in the lens that has now been fixed.  Or possibly the lenses now 
have a problem they did not have before.  Sigh!

In general these things seem to take a lot of data to study.

Then again, there is always going to be something wrong.  Best if we just 
learn to "correct" data as well as is possible and live with it.  Data with 
errors of up to 0.12 mag still might be useful.  I wish it were better, but 
we may have to face reality.

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