Dear Aaron and Doug,
The purpose of the CTI corrector is to regularize the event grades and
energies across the CCD. This helps to alleviate the problem of
spatially-dependent QE once a grade filter is applied (a non-trivial
effect at the top of S3) and, since the spectral resolution on S3 is
virtually independent of position, allows the use of a single RMF.
Doug is correct that any gain variations leftover after CTI correction
will affect the accuracy of your spectral fitting; note however that we
have gone to great effort to remove gain variations, including the
column-to-column gain variations present in all ACIS CCDs.
Thus the CTI corrector improves the spectral resolution even on S3
(especially above 2 keV); the CTI-corrected RMF matches this new
spectral resolution. It is our hope that these efforts, coupled with
the simplicity offered by a single RMF, will be useful for spectral
analysis of extended sources on S3.
I have not tried to run Alexey's background reduction code in
conjunction with the CTI corrector. It would have to be applied BEFORE
the CTI corrector, though, as it needs the 5x5-pixel event islands from
Very Faint Mode. The CTI corrector is only implemented for the
3x3-pixel islands from Faint Mode -- for Very Faint Mode it uses the
central 3x3 island and ignores the rest. Since the resulting events
are not actual events output from the CCD camera, we do not include the
whole event island (PHAS vector) in the modified Level 1 event list.
This is to avoid confusion -- we don't want people thinking that
CTI-corrected event islands are the same as real event islands (which
have readout noise, for example). It would be interesting to compare
the background suppression achieved by Alexey's code to that achieved
by the CTI corrector. I hope you will take the time to try that!
Thanks for your interest,
Leisa Townsley
> From lewisa@nabhas.ps.uci.edu Thu Aug 2 19:17:03 2001
> Date: Thu, 2 Aug 2001 16:23:15 -0700 (PDT)
> From: "Aaron D. Lewis" <lewisa@nabhas.ps.uci.edu>
> To: Leisa Townsley <townsley@ouachita.astro.psu.edu>
> cc: <chandra-users@head-cfa.harvard.edu>
> Subject: Re: ACIS CTI Corrector and CCD Simulator
> MIME-Version: 1.0
>
> Thanks for these products Leisa et al.,
>
> On your web page describing them, you state that for S3, only one RMF is
> needed, since the spectral response has little spatial dependence. I have
> extended sources covering the entire S3 chip, which I intend to divide
> into annuli for spectral analysis - can I truly use one RMF for the entire
> analysis?
>
> Also, I am interested in using Alexey Vikhlinin's background reduction
> code and I wanted to confirm my assumption that his code could
> be run following yours, and that it would be 'safe' to do so.
>
> -Aaron Lewis
> UC Irvine
>
>From dburke@head-cfa.harvard.edu Thu Aug 2 19:40:33 2001
From: Douglas Burke <dburke@head-cfa.harvard.edu>
Organization: Chandra Science Center, Smithsonian Astrophysical Observatory
To: "Aaron D. Lewis" <lewisa@nabhas.ps.uci.edu>,
Leisa Townsley <townsley@ouachita.astro.psu.edu>
Subject: Re: ACIS CTI Corrector and CCD Simulator
Date: Thu, 2 Aug 2001 19:40:26 -0400
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Aaron,
You might want to check out Mike Wise's presentation at the April CIAO
workshop, available from http://asc.harvard.edu/ciao/wrkshp_apr_ps.html
where he discusses issues concerning the spatial dependence of the response.
At the end of the day the decision as to whether or not you can get away with
one RMF depends on what level of accuracy you desire, but Mike's presentation
does show that the RMF does not vary largely over S3 (within about +-10% for
most of the chip, see presentation for further details).
Doug
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