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Re: Adjusting comp stars?

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  • mlinnolt
    Maik, Yes I agree the lack of color information in GSC1.2 contributes a lot to problem. The new GSC2.2 has 2 bands for most stars. Have you considered this
    Message 1 of 4 , Sep 1, 2003
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      Maik,

      Yes I agree the lack of color information in GSC1.2 contributes a lot
      to problem. The new GSC2.2 has 2 bands for most stars. Have you
      considered this catalog to supersede GSC1.2 in the interim?

      Unfortunately, the 2 bands are the old photographic J and F, which are
      only approximately like Johnson B and Cousins Rc, respectively. I
      believe the USNO A/B catalogs do convert these J/F to B/R, but GSC2.2
      does not. Once you have B/R, there is a decent linear formula that
      converts them to V. The problem is the GSC2.2 is in the raw J/F.

      So, from the viewpoint of color issues, it seems the USNO A/B catalogs
      are more suited for visual use than GSC, because the V conversion can
      readily be applied.

      Mike

      --- In comets-ml@yahoogroups.com, Maik Meyer <maik@c...> wrote:
      > Hi Mike,
      >
      > > The assumption being that Tycho2 is more accurate than GSC1.2 and
      that
      > > there is a systematic common offset within local areas of the
      > > catalogs.
      >
      > I am not sure that the offset is systematic even within a small
      local
      > area. I think the offsets depend mainly on the spectral class of the
      > stars. In any case I would not do any corrections on my own. Either
      use
      > the bad HS
    • Seiichi Yoshida
      Dear Mike Linnolt, ... Please see the John Greaves s research on the color and color conversion from USNO-B1.0 at:
      Message 2 of 4 , Sep 1, 2003
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        Dear Mike Linnolt,

        > So, from the viewpoint of color issues, it seems the USNO A/B catalogs
        > are more suited for visual use than GSC, because the V conversion can
        > readily be applied.

        Please see the John Greaves's research on the color and color
        conversion from USNO-B1.0 at:

        http://www.aerith.net/astro/color_conversion/JG/USNO-B1.0.html

        His conclusion is:

        So, it is no great surprise that the same result occured in the
        standard deviation for V as calculated from B1 and R1 against
        Johnson V, namely 0.5.

        <snip>

        In other words, V calculated from USNO B1.0 B1 and R1 are no better
        than using GSC1.x magnitudes directly, the latter itself being a
        normally decried practice.

        In summary: V magnitudes from B1 and R1 are really not suitable for
        any use, except possibly when there is no other alternative
        whatsoever.

        I think you already know the poor accuracy of magnitude in
        USNO-A1.0/2.0. You can see the example at:

        http://www.aerith.net/misao/report/general/usno-photometry.html

        Best regards,

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