[Aavso-photometry] Flat Stuff

[Chuck Pullen]

Hi Steve - I'd heard that as well, but was corrected by Arne Henden.  Steve 
Howell cites 13 degrees east of the zenith for evening twilight flats from 
Chromey and Hasselbacher 1996 (PASP 108  944).  I'm not sure how much 
difference it makes unless you are trying to do extreme color stars or very 
high precision work in the mmag range.  Perhaps Arne will weigh in, though 
I know he's observing this week.  Also, to some degree, astronomers are 
like lawyers - ask a group of 5 of them a question like this and you'll get 
6 answers and 2 abstentions  :-)

What is important is that you do the best flats you can, and try to make 
sure you have no gradients across your images.

Chuck

At 10:35 1/18/05 -0500, Brady, Steve wrote:

>Hi Chuck--
>
>I remember reading somewhere that the optimum region for twilight flats is 
>not at the zenith but at an altitude of 75 degrees and 180 degrees from 
>the solar azimuth position. Thus, the sweet spot will be different for 
>each observer location and date. The article demonstrated that this spot 
>yielded the least sky gradient.  Bob Denny has implement this as a script 
>in ACP4.
>
>Clear skies,
>
>Steve