[Aavso-photometry] Differential transformation

[Arne Henden]

Radu Corlan wrote:
>>   Second, please don't use my normal field calibrations for
>>determining your coefficients.  Please use Landolt (and Cousins)
>>standards only.  While I am a careful observer, I will not guarantee
>>that my field calibrations are exactly on the Landolt system, and
>>if you use my calibrations, you basically remove yourself one more
>>step from being on the true standard system.  My field calibrations
>>are primarily for determining the magnitude and color zeropoints
>>for differential photometry.  I know I am pretty darn close, and
>>for rough values you will be fine.  For precise measures, a reviewer
>>will question the use of Henden photometry in this manner as systematics
>>have not been investigated.
> 
> I see this as a "lesser of two evils" situation. If i have to work in 
> non-photometric conditions, either using transformation coefficients 
> obtained on a different night or getting them at a significatly different 
> airmass (or widely spaced in time) is likely to introduce some systematic 
> errors. Are these less than the systematics of one of your sequences? 
> Maybe, maybe not. But, although i cannot prove it with data, i have a 
> strong hunch that the averaged sytematics of several sequences are pretty 
> small. 
> 
The problem here is that you, as an end user, do not know where
any systematics in my photometry might lie.  As a simple example,
some of my field calibration files have many nights of data; others
might only have one or two nights, and those nights might have been
marginal to start with.  That is why it is better to use published
photometry that was meant to be used for standard measures, and which
have been tested for the normal systematic variations such as color,
declination, RA, time_of_year, etc.  There are plenty of Landolt
standards, with several dozen fields that have multiple standards
that can be imaged simultaneously.  No need to use Henden photometry
for this, except as a place-holder to be replaced as soon as you
get the chance to observe true standards.
   Transformation coefficients remain constant over long periods of
time; I am not quite sure what you mean about not using coefficients
obtained on different nights since this is a very common practice.
In fact, the best coefficient determination method is to use the
handful of really good, photometric nights for your coefficient
determination and then average these determinations to give you
mean coefficients.  These can then be applied to future nights for
long periods of time.
Arne