[vsnet-chat 5948] (fwd) Multi comp stars

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

(fwd) Multi comp stars

   This is the forwarded message (by permission of Dr. Berry) from AAVSO
discussion list.  Since many of you are using AIP for Windows for
scientific photometry, and some of you are planning or conducting
multi comparison star or (wide-field) ensemble photometry, including
PIXY program by Yoshida-san, the following message will be of interest
to many readers.  I invited Dr. Berry to this list for possible
communications on these photometry matters.  Yoshida-san, would you
please introduce your software and function/architecture in order to
illustrate one of the best achievements of this field?

Regards,
Taichi Kato
===

Hi guys--

When we wrote AIP for Windows, we based our photometry tools on the
most common "model" used by amateur astronomers, using one Comp,
one Variable, and one checK star. This is the basic model described by 
Hall and Genet for photoelectric photometry, and we used it because a 
large number of amateurs were already familiar with making measures
in CVCVCVC sequences.

We designed these tools to handle the types of data that our beta testers
had, namely sequences of images with large periodic errors in tracking,
occasional loss of one or more of the stars from the field, and less than
wonderful thermal control of the CCD temperature. We also included
tracking modes to allow field rotation for observers with alt-azimuth
mounts, and moving Variables to accommodate asteroid photometry. 

We also created an extractive photometry tool that finds and photometers
every star over some suer-set threshold in an image, gets data for seven
different aperture radii, and output the data in a file format that is file- 
compatible with the data analysis software in Arne Hendon's forthcoming 
book on photometry with CCD cameras.

But in the last year, we're had quite a few requests to support ensemble 
photometry, and would like to do so. My impression is that we'll be dealing
with a quite serious group of observers, and I'd like to take this opportunity 
to conduct a straw poll to see what characteristics we can expect of your
images, and what features you need.

Questions about your data:

Can we assume that those of you who wish to do ensemble photometry
have equatorial mounts, and we do not need to accommodate field 
rotation?

Can we assume that those of you who wish to do ensemble photometry
have mounts that track reasonably well, that is, with image-to-image
offsets not exceeding 2 or 3 pixels?

Can we assume that we do not need to support photometry of moving
objects because these will pass through most ensembles quickly?

Can we assume that if one of the stars in the ensemble drifts outside
the image, you would consider that image a reject? If not, how should
the ensemble magnitude be computed?


Questions about output:

What is the maximum number of stars that you would ever want in the
ensemble of comp stars?

Do you want raw instrumental magnitudes for every star in the ensemble
reported in the output data file? This would enable you to carry out
extensive cross checks between comp stars.

How should we compute the ensemble magnitude? Do you want a simple 
average, a sigma-weighted average, a median value, or something
else? 

Are there other features that you would absolutely have to have? What
features would you simply like to have, or want to have? 

You can send replies to me only, or reply to this group.

--Richard

Richard Berry
AIP for Windows
Handbook of Astronomical Image Processing
http://vsnet.wvi.com/~rberry
rberry@wvi.com

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