[Aavso-photometry] Differential transformation

[Chuck Pullen]

Gier - Note that M67 has been extensively studied and there are a number of 
stars available in it that should meet the quality test and Arne's concerns.

Chuck

At 09:01 12/29/04 -0700, Arne Henden wrote:

>Chuck's answer is essentially complete, and Radu has an interesting
>technique, but I thought I'd toss in a couple of more comments.
>   First, even if you are using a single field, don't forget that
>you cannot always neglect extinction.  At high airmass, there is
>a significant airmass (and extinction) difference between the
>top and bottom of even a small field.  What is "significant" depends
>on the desired accuracy.
>   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.
>Arne
>