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RE: Soliciting criticism
--- "Creager, Robert S" <CreagRS@LOUISVILLE.STORTEK.COM> wrote:
>
> Michael,
>
> Thanks for the prod. In setting up for the Welch-Stetson statistics,
> I was
> verifying the weighted mean, and it was occasionally very different
> from the
> normal mean. A little digging brought up a problem. SPS2X doesn't
> spew out
> wildly inaccurate magnitudes and error estimates, but sextractor can.
> I was
> not filtering out sextractor's far out values in one SQL statement
> (but I
> was in another), with the result of including magnitudes with values
> of >
> 100 (pretty faint), and an error magnitude of 99 being included. The
> weighted mean showed this error very clearly. I will be
> re-generating the
> sextractor graph later tonight. I had SPS filtering out large (> 1)
> error
> estimate stars, so those graphs are more constant, but still suffer
> from the
> same problem. These values appear about 10% of the time in
> sextractor
> (although sextractor generates more stars than SPS, so the total
> usable star
> count is nearly a wash).
>
> Chris,
>
> Nice pick up on SPS and the constant PSF. The scatter makes more
> sense to
> me now. SPS does calculate a constant, two dimensional (I believe)
> PSF from
> stars it picks. With your prod, I now understand this is what I was
> seeing
> in the SPS run logs. I may have a go at the central portion of the
> image.
> I'll wack out some software which clips the outer edges of the image,
> and
> re-run SPS and see what comes out. What if the image was broken into
> 9 (or
> 16) sub images, and each reduced separately? Painful, but possible.
> Anyone
> interested in seeing this done?
I thought of that too. But at least one PSF fitting software system
(daophot) allows for a spacially varent PSF. (That is a PSF that is
a function of X,Y) This is this same as breaking the image down in
to as many reasions as there are stars. Muchh better then 9 or 16.
SPS was written, I think with the goal of being easy to use.
Once you start allowing very complex PSFs to be specified you lose
that goal but if we are to break past doing 5% photometry we are just
going to have to beat every little detail into the ground and give up
on "easy". Still I think you should apply for Tom's software prize
twice as both SPS and sextractor do what Tom asked for:Hands off
reduction of multiple CDs.
Do you have a URL for SPS? I'd like to look at the code and see
how easy it would be to add a function like GetPSF(x,y) to be used
in place of the constant PSF. This could give doaphot some real
competition.
>
> Now, are there any packages which use non-constant two dimensional
> PSF
> models?
See above. daophot runs stand alone and as part of at least a couple
larger systems IRAF and Midas and likey others too. Midas being the
Euroean equivalent of IRAF. If you think sextractor has a large
parameter pace you ain't seen nothing yet...
See detail documentation at
http://iraf.noao.edu/scripts/irafref?daophot
and a user manual at
http://iraf.noao.edu/iraf/docs/daorefman.ps.Z
Again this stuff runs outside of IRAF if you want.
but IRAF/daphone only takes about an hour to setup unless it is your
first time.
=====
Chris Albertson
Home: 310-376-1029 chrisalbertson90278@yahoo.com
Cell: 310-990-7550
Office: 310-336-5189 Christopher.J.Albertson@aero.org
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