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

[Michael Newberry]

Jim,

My point is not whether you can get approximate color coefficients but 
rather the point of getting transformed magnitudes, which is what getting 
the transformations is all about. To me, the question as whether you can do 
transformed photometry using one filter. As far as scientific measurements 
go, you'd best be sure to add a huge asterisk after every such measurement 
because what is being reported is neither "clear" nor instrumental 
magnitude, but rather a pseudo-transformed system magnitude containing a 
systematic error that is a function of color index. Whenever those 
magnitudes are used for science, you don't want someone assigning them the 
same trust they would a magnitude based on transformations and observations 
using 2 filters.

On the other hand, you also might just guess the transformation coefficients 
using mean seasonal extinction for your site, altitude, or location, and 
color terms based on the mean wavelength of your V filter and the CCD's 
response curve. When you apply these rough transformations to target 
objects, what do you use as its color index (since you haven't measured it)? 
You've measured the object in only one filter, the V filter in Tom's case. 
So the transformed magnitude either has to involve a guess for the object's 
B-V (or V-R, whichever way you do it) else your magnitude will contain a 
systematic offset relative to the actual V magnitude. The systematic error 
will be a function of color index. If the target star is a variable of known 
type and you know about what the B-V should be in this phase, or if you know 
its spectral class, then you can use that estimated color index along with 
your seasonal/mean extinction coefficients. Of course you can get a 
magnitude this way, but what does it mean? Say you report 12.25 +- 0.06. 
What is its significance? The error being reported does not accurately 
reflect the true distance this measurement may be from the actual system 
magnitude. In the reported magnitude you have approximation piled upon 
approximation, involving extinction and color index, added to an "accurate" 
instrumental magnitude that you've determined in a well-defined way. The 
goal of good experimental technique is to minimize systematic errors, 
leaving only random errors. So the question is, do you really want to do 
this? To me, the answer would be no---I would get a second filter.

Michael Newberry

----- Original Message ----- 
From: "Jim Jones" <nt7t at centurytel.net>
To: "Michael Newberry" <mnewberry at mirametrics.com>
Sent: Thursday, December 18, 2008 4:55 PM
Subject: Re: [Aavso-photometry] V filter only - transforms possible?


> Sure it's possible.  See Henden's  "The M67 Unfiltered Photometry 
> Experiment" in JAAVSO Vol 29, 2000.
>
> With some encouragement from Arne a couple of years ago, I found my V 
> transform with fairly good results.  The problem is using the transform 
> without reliable B-V data for both stars.
>
> Jim Jones
>
> Michael Newberry wrote:
>> No, not possible.
>>
>> Michael Newberry
>>
>> ----- Original Message ----- From: "Tom Krajci" <tom_krajci at tularosa.net>
>> To: <aavso-photometry at aavso.org>
>> Sent: Thursday, December 18, 2008 3:48 PM
>> Subject: [Aavso-photometry] V filter only - transforms possible?
>>
>>
>>> I have a CCD/scope rig with only a V filter screwed into the CCD 
>>> nosepiece. No filter wheel, no other filters.
>>>
>>> Here's the $64 question.  Is it possible to determine transformation 
>>> coefficients for this setup with only a V filter?
>>>
>>> If I do differential photometry of a Henden calibrated field (V 
>>> magnitudes and color indices are known for the comps, *and* the target), 
>>> can I transform to V magnitudes?
>>>
>>> What if I take images of ye-olde standard field, M67, and compare my 
>>> instrumental magnitudes with the catalog magnitudes?  What sort of 
>>> correlation/transformation coefficient will that give me?  (If it's not 
>>> a true transformation coefficient, then what is it called - a pseudo 
>>> transform?)
>>>
>>> (I know that if I use this V-filter-only rig on uncalibrated comps and 
>>> targets, i.e. V mags and color indices are unknown,...then there is no 
>>> way I can transform.  Therefore I'm looking for 'easy' target fields 
>>> that I can work to higher precision...hopefully with a proper transform 
>>> to V magnitudes.)
>>>
>>> I'm just trying to get the most out of this V-filter-only system.  (And 
>>> if I can't transform to V with this setup...how close can I get?  How do 
>>> I report the magnitudes?...as untransformed?)  (And if I can't transform 
>>> to V with this setup...what do I need to acquire?...a B or R filter?)
>>>
>>> Thanks in advance.
>>>
>>> -- 
>>> -------------------------------------------
>>> Tom Krajci
>>> Cloudcroft, New Mexico
>>> http://picasaweb.google.com/tom.krajci
>>>
>>> Center for Backyard Astrophysics (CBA)
>>> http://cbastro.org/ CBA New Mexico
>>>
>>> American Association of Variable Star
>>> Observers (AAVSO): KTC http://www.aavso.org/
>>> -------------------------------------------
>>>
>>>
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>>
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