[vsnet-chat 4563] Re: Photometric problem using USNO-A2.0

[Seiichi Yoshida <comet@aerith.net>]

Dear Dr. Sumner and Dr. Skiff,

> Subject: [vsnet-chat 4538] Re: [vsnet 2103] Photometric problem using USNO-A2.0

> 	Do not use USNO-A2.0 as a source of photometric data, it IS NOT 
> a photometric cataloge. The USNO-2.0 magnitudes are representative 
> only, and even though the internal accuracy is sometimes quite high, 
> the external accuracy is abysmal.

I agree with you. (As described in another mail,) Actually, we
corrected the zero-point using Tycho Catalog in case of the example
image in the MISAO Project activity. We did not report the magnitude
based on the USNO-A2.0 (R) except when there are only few Tycho stars
in the field.  The document is just an example to show the bad
accuracy of magnitude in USNO-A2.0 (R) for CCD observations.

> 	There is no photometric catalogue of faint stars presently 
> available. The best would be the GSPC-II data set, of which a 
> preliminary version, known as GSPC2.1, is available at:
> 
> http://vsnet-gsss.stsci.edu/gspc/GSPChome.htm

Well, I have tried to use the GSPC 1 for photometry. But the number of
stars are too small. The Tycho-1 or Tycho-2 Catalog contain much more
data. So when we cannot correct the zero-point because there are a few
Tycho stars in the field, we could not use the GSPC instead of it. 

In general, people take images of the target object and images of
photometric stars. However, in the MISAO Project, the images with no
photometric stars images are also welcome. It depends on the image
observer's original purpose. So sometimes, the situation mentioned
above occurs.

> This is 2.4Mb (expands to 12.5Mb) and contains only stars where the nominal
> errors (which seem slightly underestimated) are < 0.05 mag. in all colors.

The file size seems much smaller than the Tycho-1 or Tycho-2
Catalog. So i guess the situation does not change between the GSPC 1
and GSPC 2.

Best regards,

--
Seiichi Yoshida
comet@aerith.net
http://vsnet.aerith.net/

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