[vsnet-chat 7253] Re: [vsnet-obs 53846] Re: data format correction

[arne]

gvnn64 at libero.it wrote:
>>Likewise, as mentioned, use of USNO-A (or any astrometric catalog,
>>including UCAC) for photometry is highly discouraged; the photometry
>>is basically a placeholder so that field recognition can be accomplished.
> 
> 
> Hi Arne,
> I would like to know your opinion about this:
> 
> http://www.iac.es/galeria/mrk/comets/USNO_Landolt.htm
> 
> According the Author, the USNO-A2 is not so bad for "semi-accurate" (say, 0.1 to 0.2 magn. accuracy) photometry. What do you think?
> 
> Thanks,
> Giovanni Sostero
> 
> 
We can have a long discussion about USNO-A photometry; I was involved
in the actual process of the catalog creation.  There are significant
systematics - from field to field, the zeropoint changes and within
a field you get optical distortion causing global variation.  Even
without those problems, photographic photometry is only accurate to
0.2mags or so per star.  Note also the author's comment:  "there is
a significant fraction of stars with much larger errors".  I can
show you fields where there are a full magnitude zeropoint error.

Unless you know what you are doing, or have no other choice, stay
clear of using USNO-x or UCAC for photometric calibration.  If you
must use these catalogs, then be sure to provide the identification
and magnitude used for your comparison star so that your data can
be adjusted at some later date when the proper magnitude for that
star is known.
Arne