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Viewing Calibration/9356
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From: bjm@star.sr.bham.ac.uk
Subject: XMM PSF off-axis clarification
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Date: Wed, 22 Oct 2003 14:25:59 GMT
From: bjm@star.sr.bham.ac.uk
To: xmmhelp@xmm.vilspa.esa.es
CC: bjm@star.sr.bham.ac.uk
Subject: XMM PSF off-axis clarification
Full_Name: Ben Maughan
Submission from: (NULL) (147.188.32.177)


Hi Helpdesk,
 I've been looking through some earlier messages about the PSF and its off-axis
variation, and I'd just like to check that I understand the situation
correctly.

 1) The recommended way to calculate encircled energy radii for a PSF at some
off axis angle is to use the the information in the KING_PARAMS extensions of
the files XRT1_XPSF_0005.CCF, XRT2_XPSF_0005.CCF and XRT3_XPSF_0004.CCF, and the
equation (10) of EPIC-MCT-TN-011. Is this still the best method, and should the
files XRT1_XPSF_0006.CCF, XRT2_XPSF_0006.CCF and XRT3_XPSF_0005.CCF now be
used?

 2) According to helpdesk message 7320 "There appears to be surprisingly little
change in the encircled energy fraction as you move off axis for all
of the cameras." and Figures 9 & 10 in the users handbook
http://xmm.vilspa.esa.es/external/xmm_user_support/documentation/uhb/node18.html
showing an off-axis dependence of the 90% encircled energy radius are
incorrect.
 Is this still the case?

 3) Does this method correspond to the results achieved by using View ->
encircled energy (PSF) in calview, with accuracy=EXTENDED? The plots produced
with different accuracy levels disagree strongly.

 4) To generate an accurate image of the PSF at some off-axis angle and energy,
one must use calview, correct? Which accuracy level is recommended? From my
experiments, only MEDIUM (corresponding to scisim simulated PSFs) PSFs replicate
the arc-like distortion of the PSF at large off-axis angles. What is the opinion
of the experts?

 I hope you don't mind clarifying these issues - with these different methods,
and accuracy levels, the picture is a little confusing!

 Thanks for the help,
 Ben


Reply 1

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From: Matthias Ehle <xmmhelp@xmm.vilspa.esa.es>
To: bjm@star.sr.bham.ac.uk
Subject: Re: XMM PSF off-axis clarification (PR#9356)
Date: Thu Oct 23 13:43:13 2003
Dear Ben,

we agree that it might be quite confusing for the user of calview to
decide which accuracy level should be used for which purpose.
We plan to make a technical note on this available in the near future.

Now moving on to your questions:
> 
>  1) The recommended way to calculate encircled energy radii for a PSF at
some
> off axis angle is to use the the information in the KING_PARAMS extensions
of
> the files XRT1_XPSF_0005.CCF, XRT2_XPSF_0005.CCF and XRT3_XPSF_0004.CCF,
and
the
> equation (10) of EPIC-MCT-TN-011. Is this still the best method, and should
the
> files XRT1_XPSF_0006.CCF, XRT2_XPSF_0006.CCF and XRT3_XPSF_0005.CCF now be
> used?

The difference between the two CCF versions is only the validity range for
the off-axis angle: The old files were valid only up to 12' whereas the new
versions can be extrapolated to indefinity. According to the experts,
the method you use is still the best one. 


>  2) According to helpdesk message 7320 "There appears to be surprisingly
little
> change in the encircled energy fraction as you move off axis for all
> of the cameras." and Figures 9 & 10 in the users handbook
> http://xmm.vilspa.esa.es/external/xmm_user_support/documentation/uhb/node18.html
> showing an off-axis dependence of the 90% encircled energy radius are
> incorrect.
>  Is this still the case?

Yes, the UHB figures are based on early measurements/simulations and we need
to update them: the off axis dependence of the encircled energy fraction is
indeed 
very small. Thanks for your note!

>  3) Does this method correspond to the results achieved by using View
->
> encircled energy (PSF) in calview, with accuracy=EXTENDED? The plots
produced
> with different accuracy levels disagree strongly.

Yes, using the PSF with EXTENDED accuracy (i.e. the king model) is fine here.
 
>  4) To generate an accurate image of the PSF at some off-axis angle and
energy,
> one must use calview, correct?

Yes, I don't know of any other way.

> Which accuracy level is recommended? From my
> experiments, only MEDIUM (corresponding to scisim simulated PSFs) PSFs
replicate
> the arc-like distortion of the PSF at large off-axis angles. What is the
opinion
> of the experts?

Yes, according to the experts, the MEDIUM is linked to maps produced by SciSim
and only these show the arclike structure, the HIGH method links to a 3
Gaussian
fit model (which is good for any spatial extraction), whereas the EXTENDED
king fit model is the best description (but works only for circular
extractions).
Both the HIGH and EXTENDED model are only 1-D models so they will produce PSF
maps which are symmetric. Btw, the LOW model is out of date and only maintained
for backward compatibility of the SAS. 

Cheers,
 Matthias

Matthias Ehle 
XMM-Newton SOC 
User Support Group


Followup 1

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Date: Thu, 23 Oct 2003 15:08:37 +0100 (BST)
From: Ben Maughan <bjm@star.sr.bham.ac.uk>
To: Matthias Ehle <xmmhelp@xmm.vilspa.esa.es>
Subject: Re: XMM PSF off-axis clarification (PR#9356)
Dear Matthias,
 Thanks for the quick reply - that was most useful. Can I just clarify one
further thing...

> >  3) Does this method correspond to the results achieved by using View
->
> > encircled energy (PSF) in calview, with accuracy=EXTENDED? The plots
produced
> > with different accuracy levels disagree strongly.
>
> Yes, using the PSF with EXTENDED accuracy (i.e. the king model) is fine
here.
>

So if I want to find the 90% encircled energy radius at some off-axis
angle should I use extended accuracy in calview? The different accuracies
give quite different results. For EPN, theta=700" phi=0 and E=1500eV I
find,

Accuracy	90% encircled energy radius (")

Medium		32
High		70
Extended	65

Which of these numbers is correct? From what has been said previously, I
think the extended value is correct. However, if I want an image of the
PSF to use as a convolution kernel when fitting 2D surface distributions
at a large off-axis angle, I must use medium accuracy, as the other
accuracy levels give 1D PSFs, but it seems the encircled energy is very
wrong with medium accuracy. Is this correct, and can you suggest a
solution?

 Thanks a lot for the help,
 Ben

------------------------------------------------------------------------------
 Ben Maughan                           |
 Astrophysics and Space Research Group |  Email: bjm@star.sr.bham.ac.uk
 School of Physics and Astronomy       |  Tel:   0121 414 3721
 The University of Birmingham          |  Fax:   0121 414 3722
 Edgbaston, Birmingham, B15 2TT        |  Web:   http://www.sr.bham.ac.uk/~bjm
------------------------------------------------------------------------------




Reply 2

Resend
From: Matthias Ehle <xmmhelp@xmm.vilspa.esa.es>
To: bjm@star.sr.bham.ac.uk
Subject: Re: XMM PSF off-axis clarification (PR#9356)
Date: Fri Oct 24 09:22:02 2003
Dear Ben,

The EXTENDED mode should be used, a 90% radius of 65 arcsecs looks correct.
Although the numbers appear to be very different they are taken from a very 
flat part of the curve, so the MEDIUM model gives a 95% encircled energy at 
65 arcsecs which isn't so different.

To reiterate, the best (and only) 2-d model of the PSF that we have is MEDIUM
mode. The best model that we have for the encircled energy is EXTENDED mode.
Whether this is important depends on the science, for example in the pipeline 
source searching the MEDIUM mode PSF is used to find sources and also to 
calculate the encircled energy. This introduces an estimated 5% systematic 
error on source fluxes. There are plans to modify this so that the MEDIUM 
mode PSF is used to find sources but the EXTENDED mode to calculate the 
encircled energy which will reduce the systematic error.

Cheers,
  Matthias


Followup 2

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Date: Fri, 24 Oct 2003 11:50:10 +0100 (BST)
From: Ben Maughan <bjm@star.sr.bham.ac.uk>
To: Matthias Ehle <xmmhelp@xmm.vilspa.esa.es>
Subject: Re: XMM PSF off-axis clarification (PR#9356)
Thanks Matthias - that answers all of my questions.
 Ben

------------------------------------------------------------------------------
 Ben Maughan                           |
 Astrophysics and Space Research Group |  Email: bjm@star.sr.bham.ac.uk
 School of Physics and Astronomy       |  Tel:   0121 414 3721
 The University of Birmingham          |  Fax:   0121 414 3722
 Edgbaston, Birmingham, B15 2TT        |  Web:   http://www.sr.bham.ac.uk/~bjm
------------------------------------------------------------------------------



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