[Aavso-photometry] Where can I find info on accurate magnitudes?

[barbharris1]

Thanks for your quick reply Arne.  I swear you must sit it front of your
computer 24/7 waiting to answer questions!!

-----Original Message-----
From: arne [mailto:arne at aavso.org] 
Sent: Saturday, April 14, 2007 4:42 PM
To: barbharris1 at hughes.net
Cc: aavso-photometry at mira.aavso.org
Subject: Re: [Aavso-photometry] Where can I find info on accurate
magnitudes?

barbharris1 wrote:
> First, I would like to preface my question with the statement that I
am
> a beginner.  I took some images of GW Lib in blue filter since the
star
> was so bright.  The comparison star sequence that was posted on the
web
> site did not list the 9.4 star that I wanted to use as a comp star
(this
> was the closest in mag to the variable).  When I went on line to the
> SIMBAD astronomical data base site to look up the Tycho magnitude for
> this star (TYC 6766-1577-1) it listed a BT mag of  9.736.  Does the T
> stand for Tycho and is it ok to use this magnitude for my comp star?
> Also, when I looked under recent observations for this star in AAVSO,
an
> observer from 4/13 (CWT) has the same Tycho star identified but has
the
> mag as 9.706.  Is there someplace where there is other magnitudes for
> Tycho stars listed or is this a typo by him?  Is this the most
accurate
> place to get the magnitude info if AAVSO does not have the magnitude
> info?
>  
> 

My calibration of GW Lib from USNO-Flagstaff was designed to cover
the star near quiescence (about 17th magnitude).  For this reason,
all bright stars are saturated on my calibration frames.

Then GW Lib has the audacity to go into outburst!  So the brightest
comparison stars, such as the 9.4 star, only have marginal photometry
from other sources.  Your best bet for the time being is from the
Tycho2 catalog, which Barbara has found.  The problem is that the Bt/Vt
magnitudes are close to, but not exactly, on the Johnson UBV system;
that is why they have the "t" subscript.  There are transformation
equations to transform Bt/Vt into B and V, that work reasonably well
if the star is normal, not reddened, and bright enough to have good
signal/noise with that satellite survey - usually, brighter than V=10.5.

The 9.4 star, TYC 6766-1577-1, has Bt = 9.736 and Vt = 9.396 from
the Tycho2 reprocessing; the original Tycho catalog gave
Bt=9.706 and Vt=9.389, which is where CWT got his values from.
The Johnson magnitudes for this star are V=9.36 and B=9.66, and
I'd use 9.36 and 9.66 for reporting data.  Whatever magnitude you
use for this comparison star, please report it in the comments field
until such time as we provide a chart/sequence with that star
identified.

A single source for all-sky photometry does not yet exist, and you
have to weed through the multitude of possible answers at this time.
We expect to calibrate the GW Lib field on the bright end from Sonoita
in a week or two, providing BVRcIc calibration, but the weather in
Arizona has been abysmal for all-sky photometry this spring and I would
not hold my breath for a quick solution.
Arne

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