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[vsnet-chat 4541] Re: Photometric problem using USNO-A2.0



I'm copying this response to the TASS maillist as well
since Yoshida-san posted to both groups.
  The photometric problem mentioned at
    http://vsnet.aerith.net/misao/pixy/result/usno-photometry.html
seems to be a non-issue to me.  From what I can read, they
are using USNO-A to calibrate CCD photometry with R-magnitude
values from 5.7 to 14.9, mostly clustered around 10th magnitude.
At the bright end, USNO-A magnitudes are not to be trusted
_at all_.  Remember, USNO-A comes from digitized POSS-I plates,
where any star brighter than 14th magnitude is saturated,
brighter than 12th magnitude has diffraction spikes, and certainly
by 6th magnitude has haloes.  I am also pretty confident that
the match between, say, the 11.8mag CCD star and the 14.9mag USNO-A
star is wrong.  You should not be using USNO-A for stars brighter
than 12th. A more appropriate catalog is Tycho-2, which is not
saturated and has photoelectric values which are much more accurate
than photographic ones in any case.
  Second, I doubt that Kadota-san's CCD has enough dynamic
range to give good magnitudes at R=5.7 as well as R=14.  Somewhere
the detector is saturated; somewhere the signal/noise is so low
that the photometry is suspect.  You can't use a 6th magnitude
star to calibrate photometry at 16th magnitude as suggested
by their tables.
  Finally, the unfiltered CCD magnitudes cannot accurately be
compared to a E-bandpass plate.  That is probably the reason
for the comment on the web page:
>At a first glance, we can see that many stars are lacked in the USNO-A2.0
>in this field. In addition, it is hard to make match between
>the stars on the image and stars on the chart.  We can make match
>easily between only a few very bright stars.
and also the reason why the magnitude discrepancy for
MisV1111 exists; the CCD is probably measuring I-band response
for this red variable (and it may have been faint at the POSS-I
epoch anyway). I don't see where stars are lacking in
their example, just different brightness than what appears on the CCD image.
This will often be the case in obscured regions such as
the Galactic Plane.
  Certainly USNO-A photometry is poor, but give it a chance and
use it where it is appropriate.  Also, as Brian Skiff mentioned, you
can usually find a nearby photometric sequence to calibrate the
photographic values for the field of interest if you need
better results.
Arne

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