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------- Forwarded Message
Date: Wed, 22 May 2002 01:47:48 -0400
From: Maxim Markevitch <maxim@head-cfa.harvard.edu>
To: chandra-users
cc: wqd@gcs.astro.umass.edu, hermanm@space.mit.edu
Subject: Re: ACIS coord. accuracy
Hi Daniel,
> Maxim, Do you have any comment? What is the best plate scale anyway?
> 0.491" per pixel? I would suspect the major centroid uncertainty of
> an off-axis source is the asymmetric PSF.
> Daniel
>
>>> Subject: Re: PSF as a function of off-axis angle
>>> Date: Fri, 10 May 2002 15:54:25 -0400
>>> From: wqd@gcs.astro.umass.edu
>>>
>>> Eric,
>>> Thank you very much for your comments. I'd also guess that the
>>> counting statistics-induced errors (particularly in the ONC field)
>>> could significantly contribute to the discrepancy between the
>>> results derived from the ONC and CDF-N fields. In fact, I read your
>>> ONC paper and did an eye-ball fit to Figure 4. I may use the fit
>>> 0.2+1.4*(offaxis/8')^2 to give a rough estimate of the systematic
>>> position errors (~1 sigma) as the function of offaxis angle. But I'd
>>> certainly like to see a more thorough investigation of the problem,
>>> using both the real data and simulations. I hope that CXC would be
>>> able to work on this. It is very difficult for individual
>>> investigators like myself to devote a lot of energy on such
>>> problems.
Unfortunately, I wasn't subscribed to this list until now and its
messages stopped being archived for some reason (I have just sent an
email to admin about it), so I'm not sure what exact question is being
discussed. On your specific questions,
> What is the best plate scale anyway? 0.491" per pixel?
0.4920", if your data are processed after Oct 2001 using the new
cal. file (which implemented the new mirror focal length and the ACIS
pixel size). In older data, the plate scale is different, and the best
way to correct it is to reprocess the event file following the
instructions at http://cxc.harvard.edu/ciao/threads/geom_par/ (it has
links to a number of cal. memos if you are interested in details).
If Eric Feigelson's ONC paper you mention is astro-ph/0203316, as far
as I read it, their data used the the old coord. calibration available
at that time. As they also point out, wavdetect may introduce some
errors due to the PSF shape. The plate scale was derived using simple
centroids (a coordinate weighted by the number of counts within the
90% PSF circle). To get the best position accuracy, you need to use
the same simple centroiding (although you can detect sources by
wavdetect of course). I don't expect the coordinate accuracy of
wavdetect to be tested in the near future, although someday it should
be.
The present uncertainty of the coord. calibration should be of order
0.3" at the edges of the ACIS-I FOV.
Best wishes
Maxim
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