[Aavso-photometry] transformations

[Steven Orlando]

----- Original Message ----- 
From: "Steven Orlando" <sorlando at sorlando.com>
To: "arne" <arne at aavso.org>
Sent: Monday, July 14, 2008 12:39 PM
Subject: Re: [Aavso-photometry] transformations


> Arne,
>
> You mentioned catalogues, which current catalog is the best for 
> photometry? Which catalogues currently being developed will the best for 
> photometry?
>
> I know that for some catalogues, like Tycho-2 and 2-MASS, there are 
> conversion formulas to convert to the Standard BVRI system.
>
> Steve
> ----- Original Message ----- 
> From: "arne" <arne at aavso.org>
> To: "Wm. Mack Julian II" <mack-julian at cableone.net>
> Cc: "aavso-photometry" <aavso-photometry at aavso.org>
> Sent: Monday, July 14, 2008 9:38 AM
> Subject: Re: [Aavso-photometry] transformations
>
>
>> Wm. Mack Julian II wrote:
>>> Since its monsoon time here in New Mexico I've been reading stuff on 
>>> BVRcIc and how to get my measurments on the standard system. I have BVR 
>>> filters and just ordered a I based on a message from Arne saying its 
>>> easier to compute v-i than v-r since there is a greater color 
>>> difference.
>>>
>>> Message to group from Arne from Feb 16 2006
>>>
>>>> (B-V) and (V-Ic) are the two color indices that most professionals
>>>> use.
>>>> They have about the same size, and give the most range as a function
>>>> of star temperature.  (V-Rc) is has a much smaller size (about half
>>>> of
>>>> the other two indices), so transformations are less accurate.
>>>> V and Ic have completely separate passbands, so transforming to
>>>> other systems is cleaner.  Ic is less affected by dust and
>>>> atmospheric
>>>> extinction than is Rc.  Ic is easier to measure for red stars.
>>>
>>> Bah, hit send before I got to my question. I think I understand that to 
>>> find transforms you plot:
>>>
>>> B-V vs b-v and take 1/m to get Tbv
>>> V-R vs v-r and take 1/m to get Tvr
>>> R-I vs r-i and take 1/m to get Tri
>>> V-R vs V-v and m is Tv
>>> R-I vs R-r and m is Tr
>>>
>>> is there a Ti? Would you normally report using a standard mag say V and 
>>> V-I as color inidice something like B and B-V?
>>>
>> There is a Ti, just as there is a Tx for any bandpass x.  Normally, I'd
>> use (V-Ic) vs. (Ic-ic) and then the slope is Ti.
>>
>> Now, "normal" report.  Professionals typically always measure V, and
>> then report all other bandpasses in color index form: (B-V), (V-Rc), etc.
>> This is because the V magnitude sets the base brightness of the star, and
>> the color indices tell you something about the physics of the star: its
>> basic spectral type, any reddening, unusual brightness in some filter, 
>> like
>> Rc, indicating a bright emission line, etc.  Any other filter could be
>> used for the base magnitude, but professionals have standardized on V
>> for the Johnson/Cousins system.
>>
>> The AAVSO has had the tradition of reporting V, and as people started 
>> using
>> other filters, the database was just extended to include B, Rc, Ic. 
>> Reporting
>> each filter's measurement separately has the advantage of properly
>> timestamping each observation.  If you use Ti to transform to Ic, for
>> another example, you get a better error estimate than if you separately
>> transform V and (V-Ic) and then form Ic from Ic = V - (V-Ic).
>>
>> On the other hand, having the magnitudes from a BVRI dataset reported
>> separately makes it difficult to determine upon download which measures
>> are part of the dataset.  That is why we added the "Group" field in the
>> Extended Format - with this field, it is possible to recreate the
>> appropriate color indices.  Reporting each filter individually also
>> tends to promote untransformed submissions, and I'm going to be
>> changing that approach and pushing transformation shortly.
>>
>> Professionals and amateurs alike tend to be pretty sloppy with respect
>> to photometry.  Unfiltered observations are assumed to be V-like or 
>> R-like
>> and reported as if they were filtered measures; use of USNO-A photometry
>> and colors is often made, and even without reporting which USNO-A star(s)
>> was used as the comparison; extinction is ignored; transformation is 
>> ignored;
>> I've even seen professional reports that were not flatfielded.  It is not
>> difficult to get high-quality photometry; you just have to be attentive
>> to details.
>> Arne
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>