Re: TN-107

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

Date: Thu, 25 Oct 2007 20:00:21 -0400
From: Arne Henden <arne@aavso.org>
To: Thomas F. Droege <droege@fastmail.fm>
Cc: tass@tass-survey.org
Subject: Re: TN-107

On 10/25/07, Thomas F. Droege <droege@fastmail.fm> wrote:
> Hi Arne and all,
>
> I am using the tass reduced data.  Reading the TN, you will see that the
> fields range from 70 to 700 stars per sq degree.  The "cleaning process"
> removes about half the stars in the most crowded fields and 10%  or so
> in the least crowded.  I am removing stars that fall within an 16 pixel
> box centered on the test star.  When another star is found in the box,
> both are removed from the data set.
>
> I don't understand why you propose that referencing a high resolution
> catalog will make an improvement.  The tass pixels are 7.5 arc seconds
> and we generally get 1 to 2 arc second positions when compared to high
> resolution positions.  The PSF is typically 2-3 pixels.  We do use a
> selected set of the Tycho2 catalog in the pipeline that produces the
> positions.  About 150 Tycho2 stars are used for each 16 square degree
> image for comparison.  These are selected to be in the tass magnitude
> range.
>
The reason I suggested this is because, if you want to find blends,
it helps a lot if you start with a higher resolution image.  Two stars
that are one fwhm might never be split by TOMx, but because it
is a blend, will cause photometric error.  It is not a question of
astrometric quality, but of resolution.

> When you start running ARNE, I will look forward to seeing what better
> technique you find to process the data.  I have previously tried
> selecting the good nights with not nearly as much improvement.  One has
> to be careful or one can select down to nothing.  I find that what is
> left after this "cleaning" has lots of stars that look variable.  I am
> slowly getting everything set up and code snippets written so it is easy
> to search the data.  I hope to be able make this available so that
> others can have fun finding new variables from my data set.
>
> As I previously said, to much criticism, it is like "shooting fish in a
> barrel".  Is the AAVSO interested in lists of new variable stars?  Out
> of the process there will be a lot of measurements of existing variable
> stars.  These will be filtered values and will have error bars.  While
> not as good as professional measurements, the measurements should be a
> significant improvement over visual data.
>
The AAVSO has always looked for and accepted lists of proven
variables - that is what the Variable Star Index (VSX) catalog
is all about.  We also accept photometry on any known variable,
especially if it comes with accurate error estimates.  However,
what I want is real variables and not suspected variability.
I'm still concerned that so many of the stars in the TASS catalog
have large WS statistics, yet are likely constant.  Perhaps the
latest culling of blends will improve that situation.
Arne