[vsnet-chat 5862] Re: GSC vs Tyc2

[Michael Koppelman <lolife@bitstream.net>]

You make a lot of good points, Stan, but I think things are not as bad 
as you describe provided people are using comp stars of similar color. 
If you choose Tycho comp stars of similar color to your variable star, 
untransformed V magnitudes should be very much in the ballpark since 
the slope is the same for both variable and comp stars. The slop in the 
Tycho magnitudes introduce a zeropoint shift in the neighborhood you 
describe (0.05 - 0.10) which does not, in my opinion, make the data of 
"no value at all" -- it can be very accurate differential photometry 
but with an absolute photometric error in the neighborhood of 10%. In 
many cases, the differential information is all that is needed and the 
absolute values can be corrected if there are other simultaneous 
transformed observations (a la the CBA).

Cheers,
Michael Koppelman
http://vsnet.lolife.com/astronomy/

On Wednesday, January 8, 2003, at 01:05 PM, Stan Walker wrote:

> I notice that a number of people use CCDs with a V filter and publish 
> (and
> presumably submit these somewhere) without transformation. The V 
> filter I
> use has a 7.5% slope against V-R and I presume that others are 
> similar. So
> values determined without transformation corrections are probably 
> 3-10% out.
> Add to this the uncertainties introduced by using poor standards and 
> it's
> doubtrful that there is any value in the measures. Bad observations are
> extremely frustrating and any method which encourages these is not 
> good.

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