Re: TN-107

[Tass Mailing List <tass@mail.alembic.net>]

Date: Thu, 25 Oct 2007 17:20:40 -0400
From: arne <arne@aavso.org>
To: Tass Mailing List <tass@mail.alembic.net>
Subject: Re: TN-107

So it looks to me like your partial solution to the poor photometry
problem is to eliminate the crowded fields.  How are you deciding
if a neighbor is present?  You say all stars within 8 pixels of
another star, but are those stars identified from the TASS images,
or coming from another catalog like UCAC?  The better method is to
use a high-resolution catalog like UCAC or Tycho to pick unblended
objects.  You will of course eliminate most of the stars in the
database, as the majority will be (a) faint, where crowding is
more common and (b) near the plane of the Milky Way.  This is ok,
if it gets you better results.

What most of the exoplanet survey groups use is image subtraction
techniques to eliminate the blending problem.  There, you just
look at what varies and throw the rest of the stars away.  It is
a complex operation, and works best on systems that stay stable
(not trailed one night or out of focus the next), and you always
want to work with clear nights so that you don't have differential
extinction across a frame from clouds.  My guess is that some
fraction of the noise floor is due to transparency variations, and
if you could further select to remove nonphotometric nights, you
might get better results.  In other words, you have lots of nights
and plenty of data.  Pick out the 10percent that is really good
and work with that.
Arne