[Aavso-photometry] Transform coefficients and two-filtermeasurements

[RICHARD MILES]

Just echoing Michael N.'s comments, an extreme case in point is the example 
of measuring the V magnitude TC of an unfiltered system, where the effective 
wavelength can be close to V (as is the case for some Sony CCD chips). 
Here, the TC can be close to zero for say G-type stars but as you measure 
stars redder or bluer than this the TC can become increasingly positive or 
indeed change sign and become increasingly negative.  So this calibration 
does not yield a straight line but instead a very pronounced parabola! 
Adding a V filter to the system converts this parabola to a straight line 
having a slope of 0.04.

Thinking in terms of b-v and B-V, I agree that this doubles the no. of 
degrees of freedom than for the single filter case.  For each filter, there 
is both a contribution from the mismatch in both the shape of the bandpass 
as well as the difference in the mean effective wavelength.  I would add 
that the B filter calibration is more likely to show some change in the 
value of the TC for stars over a wide range of color than is the V filter 
calibration.

Cheers,
Richard Miles

----- Original Message ----- 
From: "Michael Newberry" <mnewberry at mirametrics.com>
To: "Michael Koppelman" <michael at slackerastronomy.org>
Cc: <tom.krajci at gmail.com>; <aavso-photometry at aavso.org>; 
<tom_krajci at tularosa.net>
Sent: Thursday, July 23, 2009 6:48 PM
Subject: Re: [Aavso-photometry] Transform coefficients and 
two-filtermeasurements


> Hi Michael,
>
> As I mentioned, there are two issues:
>
> 1) differences in effective wavelength, which gives rise to the 
> transformation coefficient being different from 1.0,  and
>
> 2) differences in bandpass shape.
>
> Just by knowing whether the filters are bluer or redder (the effective 
> wavelength) does not give you a definitive answer. Mismatches in the 
> shapes of the bandpasses between your system and the standard 
> filter/detector system can give nonlinearities in the transformation.
>
> If the overall bandpass shapes are exactly matched, then a small 
> transformation coefficient results, and the nonlinearities are 
> "negligible". The main problem is the shapes of the bandpasses, which 
> cannot be exactly matched to what Johnson and Cousins used. But the 
> closer, the better.
>
> One thing our study showed was that having a small transformation 
> coefficient (because the effective wavelengths are closely matched) DOES 
> NOT guarantee linear transformations across the range of spectral types.
>
> To definitively answer the question, short of doing a full synthetic 
> photometry analysis for your filters and CCD like we did, you would need 
> observe stars across the range of spectral classes and determine 
> transformations from them, then compare the coefficients you get. Or, in a 
> nutshell, for the purposes of 0.02 magnitude photometry, just get the 
> bandpass shapes and effective wavelengths as close to the system as you 
> can.
>
> Michael Newberry
>
> ----- Original Message ----- 
> From: "Michael Koppelman" <michael at slackerastronomy.org>
> To: "Michael Newberry" <mnewberry at mirametrics.com>
> Cc: <tom_krajci at tularosa.net>; <tom.krajci at gmail.com>; 
> <aavso-photometry at aavso.org>
> Sent: Thursday, July 23, 2009 11:24 AM
> Subject: Re: [Aavso-photometry] Transform coefficients and two-filter 
> measurements
>
>
>> Thank you Dr. Newberry.
>>
>> But to Tom's original question, I don't think there are 4 degrees of 
>> freedom, there are only 2.
>>
>> So, not this:
>>
>> - B filter bandpass is bluer than ideal
>> - B filter bandpass is redder than ideal
>> - V filter bandpass is bluer than ideal
>> - V filter bandpass is redder than ideal
>>
>> This:
>>
>> - B-V is bluer than ideal
>> - B-V is redder than ideal
>>
>> Yes?
>>
>> M.
>>
>>
>> On Jul 23, 2009, at 12:07 AM, Michael Newberry wrote:
>>
>>> Are you asking if it is possible for the graph of b-v vs B-V to have 
>>> curvature? It so, the answer is yes---it depends on the filter/ detector 
>>> bandpasses. The transformations are linear over the range  of spectral 
>>> types if the bandpasses are an exact match to the  standard system. But 
>>> as the filters move off the standard  bandpasses, in wavelength and 
>>> shape, nonlinearities creep in.
>>
>>
>
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