Data Processing (1) - further thoughts

[Martin Nicholson <Martin@crozet.demon.co.uk>]

In message <199912281901.OAA00720@a188-l009.rit.edu>, Stupendous Man
<richmond@a188-l009.rit.edu> writes
>
>  Martin Nicholson writes:
>
>> I have been matching up variable stars from the GCVS and the NSV
>> Supplement with stars in the TASS database.
>>  ... In the region I studied, R.A. 100 to 200, I
>> found a total of 50 matches of stars having a magnitude between 7.75 and 10.
>> These 50 stars had a median standard deviation of 0.032
>> ...
>> I then looked at the 3600 stars in the TASS data that also matched this
>> criteria. 1572 (c42%) had standard deviations equal or greater than that
>> of the known variables in the list. ...
>> This surely implies that many of these 1572 stars are also variable?
>
>  No, probably not.  The measurements in the Mark III database 
>have uncertainties which are quite large (compared to many past
>surveys of stellar brightness).  The brightest stars have a 
>typical standard deviation of about 0.03 mag; that means that
>a _constant_ star, one which was always exactly mag V = 8.00,
>would appear in our database with numbers like this:
>
>          7.99, 8.03, 8.01, 8.00, 7.97, 8.01,   etc.
>
>  The uncertainties in the Mark III measurements increase with stellar
>faintness.  If you look in Technical Note 56, specifically at
>
>    http://a188-L009.rit.edu/tass/technotes/tn0056.html#photometry
>
>you'll see a table and a graph which list the median of standard
>deviation for stars in the database.  
>
>  Only stars which have a standard deviation larger -- say, three times --
>than the median standard deviation, should be considered good 
>candidates for variability.  For example, the table shows that
>at mag V = 10.00, the median standard deviation is about 0.046 mag.
>Thus, a star with standard deviation, say, 3 * 0.046 mag = 0.15 mag,
>might be a good candidate for further research.
>
>                                                 Michael Richmond
>

I have now examined the 68 stars (in the region RA 100-200) where there
is a match between a TASS star and an identified variable in the GCVS.

Of the 68 -

18 had a SD less than the median value in TN56
50 had a SD greater than the median


Of these 50 -

32 had a SD greater than twice the median
11 had a SD greater than three times the median

This means that if I confined my study to those stars >3 times the
median value I would only find 16% of the known variables.


This is not of course to say that SD is not a useful indicator of stars
to study, far from it, simply that taken on its own it is too crude. Not
least because some of the stars with a very high SD have one rogue point
spoiling an otherwise straight line.

Martin Nicholson
American Philatelic Society number 181632
 
http://www.crozet.demon.co.uk/ebayweb/topical.htm - Topical stamps
http://www.crozet.demon.co.uk/ebayweb/country.htm - Country collections
http://www.crozet.demon.co.uk/civilwar/cards.htm - Civil War Cards