[vsnet-chat 1110] Re: USNO A1.0 comparisons

[Taichi Kato <tkato>]

Re: [vsnet-chat 1108] USNO A1.0 comparisons

Dear Eric,

> Can one apply this equation only for certain star fields or can one use it
> throughout the entire sky? You gave an example for the SW UMa and IR Gem
> field.  Are there any restrictions for the range of b-r?

   I have not studied them yet, though I have been producing USNO-based
charts for preliminary usage.  I presume very red (having large b-r) stars
should be avoided, since they probably lie out of the range of my earlier
calibration.  However, the true V magnitude being likely expected something
between b and r magnitudes, this approximation may not be so bad.

   I have recently examined the field of Nova Oph 1998 (example of a southern
and crowded field).  The comparison between PSF-photometry-based magnitudes
and the USNO magnitudes (using that equation) has yielded the following
results:

   1) USNO seems to be becoming imcomplete around V=18.5-19

   2) Against 'true' V magnitudes, there exists a tight and straight ralation
for the faintest USNO magnitude (converted to V).  Single stars in USNO
seem to be well-calibrated down to mag 19.

   3) However, a considerable number of USNO stars have brighter V magnitude
(up to 2 mag) than CCD-measured V magnitudes.  This is probably caused by
blending, in such a crowded field, on the photographic plate.  The same
tendency is clearly seen in GSC.  By carefully rejecting "too bright" stars,
USNO V magnitudes derived from that equation seem to be used as to a level
as GSC magnitudes are used for preliminary comparison in visual observations.

Regards,
Taichi Kato

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp