Re: Adjusting comp stars?
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.
--- In email@example.com, Maik Meyer <maik@c...> wrote:
> Hi Mike,
> > The assumption being that Tycho2 is more accurate than GSC1.2 and
> > 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
> 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
> the bad HS
- Dear Mike Linnolt,
> So, from the viewpoint of color issues, it seems the USNO A/B catalogsPlease see the John Greaves's research on the color and color
> are more suited for visual use than GSC, because the V conversion can
> readily be applied.
conversion from USNO-B1.0 at:
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.
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
I think you already know the poor accuracy of magnitude in
USNO-A1.0/2.0. You can see the example at: