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

[Michael Newberry]

This only works approximately if you know or assume the color index of the 
target object. So for followup observations of known objects, I can see it. 
I have been considering the general photometry case where you don't know the 
B-V of the target. In that case, you are talking about systematic errors on 
the order of 0.05 to perhaps 0.1 magnitude in some cases, independent of 
brightness.

Michael Newberry

----- Original Message ----- 
From: "Michael Koppelman" <lolife at bitstream.net>
To: "arne" <arne at aavso.org>
Cc: <aavso-photometry at mira.aavso.org>
Sent: Friday, December 19, 2008 9:46 PM
Subject: Re: [Aavso-photometry] V filter only - transforms possible?


> 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.
>
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