[Aavso-photometry] V filter only - transforms possible?

[Michael Koppelman]

Very interesting, Arne. I admit that I am surprised this is your  
advice! It is an interesting idea, the more I think about it, to plot  
V-v vs. B-V and correct the systematic difference. I was assuming that  
the application of the transform would introduce too much uncertainty  
given that we are adding both a transform uncertainty and the  
uncertainty from the assumed B-V's. But if you do know the color of  
the target star, I can see how you could correct your zeropoint a bit.

The reason I'm surprised is -- it's complicated and easy to screw up  
transforms. Also, if people sometimes do and sometimes don't do the  
transform, we lose the homogeneity of their data set (although you  
suggest a good method for undoing it). The transforms are also  
unnecessary if you have color matched comp stars.

But I defer to your expertise in the matter. I agree in principle that  
the better the zeropoints are in the database, the better.

M.


On Dec 19, 2008, at 7:55 AM, arne wrote:

> I recommend determining your transformation coefficient, and applying
> approximate transformation if you do not have the ability to take
> two-filter data.  In the "Notes" field, include (1) the transformation
> coefficient you used, and (2) the color index you assumed for the  
> variable.
> These two pieces of information give the researcher the option of  
> removing
> your transformation and applying a different correction method if they
> want to.  The "Mag Error" field should also change when transforming,
> but that should be the subject of another email.