Re: New Draft: NSV 10892

[Michael Koppelman <lolife@bitstream.net>]

I got an Ic filter so I decided to grab some data of this star 
to compare with TASS. My results are very preliminary, but using 
the I equation below with Tycho magnitudes, I got a minimum at 
9.01 or so. The TASS data has the minima around 9.1 and fainter. 
TASS V-I is 0.6 or so while Henden and West got V-I about 0.7. 
My V-I is almost exactly 0.7, too.

I also grabbed data from the August data set for this field and 
compared the TASS calculated I magnitudes with the magnitudes 
from the equation below. These also seemed to be about 0.1 mag 
fainter more often than not.

I'm probably just verifying what was said below, but it seems to 
me that the I magnitudes trend to the faint side.

FWIW.

Cheers,
Michael Koppelman


On Thursday, August 22, 2002, at 09:04  AM, Stupendous Man wrote:

>   Yes, it applies a first-order color term in both passbands.
> However, you must remember that the I-band magnitude from the
> pipeline is calculated in a very indirect manner:
>
>        - start with Tycho Bt, Vt magnitudes for reference stars
>
>        - calculate Johnson-Cousins versions the magnitudes:
>
>                V  = Vt + 0.008  - 0.0988*(Bt - Vt)
>                I  = Vt - 0.039  - 0.9376*(Bt - Vt)
>
>        - measure instrumental v, i magnitudes on images
>
>        - fit an equation of the form
>
>                I  =  v  +  a   +   b * (v - i)
>
>   So, yes, the pipeline is trying to account for differences between
> the TASS I-band filter and the Cousins I-band passband .... but the
> basis for its accounting is not really a Cousins I-band magnitude.
>
>   I suspect that Tom is right, too: the current scheme will yield
> results which differ by ten percent (?) or so on different nights,
> because there is a different mix of Tycho reference stars.