Re: A New Record

[Patrick Wils <patrickwils@yahoo.com>]

Hi Tom,

> Some of you seem to think you see "bad" nights in the tass data.  I
> also
> "think" I see bad nights.  But I am yet to prove it.  I have done
> many
> hours of computation to try to select data that might be better.  
> 
> Here is an example of one of the things I did. Let us suppose that
> there
> are bad images. Bad images should result in measurements away from
> the mean
> for fixed stars. Most stars are fixed, so most stars in the data base
> are
> fixed. If we look at all the stars in the data base and select points
> away
> from the mean for each star, then if the above theory is true  more
> of
> these points should be from "bad" images than from "good" images. Now
> take
> all the bad points and sort them by image. Compute the percentage of
> "bad"
> points in each image and use this to make a cut on all the data. That
> is
> throw out all the images that have a high percentage of bad star
> measurement by the above definition. Now look at the quality of the
> result. 
> 
> I have done this and no cut I can find improves the quality
> significantly as measured by a mag, sigma mag plot.  

This means that most of your data is good, and that the bad data does
not influence the overall quality.  That is, the average magnitudes of
stars and their precision will not change much by throwing out "bad"
nights.  
On the other hand, when you look at the individual data points of a
star, these bad points do matter, because they distort the light curve.
 

For NSV 8866 (the plot you sent to the list some time ago with a faint
Ic point) this faint Ic point deviates nearly 5 sigma from the average
(of 60 points), while all other points deviate less than 2.5 sigma.  

When you look at stars in the vicinity of NSV 8866, you will see that
for that particular image, Ic deviates much from the average for a lot
of stars (whilst V agrees fairly well with other images).  This may be
a statistical "issue", but still, chances are very small that a large
number of stars deviate much on the same image, unless you have
millions of images.  Clouds, fog, or statistical conspiracy may all be
explanations, but in my opinion the image should not be used for the
study of individual stars.

Patrick


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