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From: Randall Smith <rsmith@head-cfa.cfa.harvard.edu>
Subject: Optical contamination from bright sources on EPIC
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To: xmmhelp@xmm.vilspa.esa.es
cc: rsmith@head-cfa.cfa.harvard.edu
Subject: Optical contamination from bright sources on EPIC 
Date: Fri, 20 Jun 2003 16:50:34 -0400
From: Randall Smith <rsmith@head-cfa.cfa.harvard.edu>
I have observations of a source (X Per) that was observed with the
medium filter, although the source has a magnitude about 6.5.  The suggested
limit is around magnitude 8-10 to avoid optical contamination.  

X Per is approximately a B0 star, so from the optical loading calibration
note (http://xmm.vilspa.esa.es/docs/documents/CAL-TN-0001-1-0.ps.gz) I 
get that a 0 magnitude star would give 7.83e3 ph/pix/sec in the MOS and 
1.26e5 ph/pix/sec in the pn.  Correcting these values to a 6.5 mag star, 
I get about 20 ph/pix/sec and 316 ph/pix/sec, respectively.  Including the 
relative frame times, this becomes:

MOS : 22 optical ph/pix/frame
pn  : 51 optical ph/pix/frame

Clearly, this will degrade the on-axis spectrum (which I have allowed for).  
My question is--will these optical photons create additional spurious events
all by themselves, or can they only change the energy of a measured event? 
And, how are these optical photons scattered?  Can I assume they will only 
affect the core of the image, or will they affect the radial profile beyond 
(say) 30''?

Many thanks,
Randall Smith



Reply 1

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From: Pedro M. Rodriguez Pascual <xmmhelp@xmm.vilspa.esa.es>
To: rsmith@head-cfa.cfa.harvard.edu
Subject: Re: Optical contamination from bright sources on EPIC (PR#8754)
Date: Tue Jun 24 09:04:47 2003
Dear Randall, 


> 
> I have observations of a source (X Per) that was observed with the
> medium filter, although the source has a magnitude about 6.5.  The
suggested
> limit is around magnitude 8-10 to avoid optical contamination.  
> 
> X Per is approximately a B0 star, so from the optical loading calibration
> note (http://xmm.vilspa.esa.es/docs/documents/CAL-TN-0001-1-0.ps.gz) I 
> get that a 0 magnitude star would give 7.83e3 ph/pix/sec in the MOS and 
> 1.26e5 ph/pix/sec in the pn.  Correcting these values to a 6.5 mag star, 
> I get about 20 ph/pix/sec and 316 ph/pix/sec, respectively.  Including the

> relative frame times, this becomes:
> 
> MOS : 22 optical ph/pix/frame
> pn  : 51 optical ph/pix/frame
> 
> Clearly, this will degrade the on-axis spectrum (which I have allowed for).
 
> My question is--will these optical photons create additional spurious
events
> all by themselves, or can they only change the energy of a measured event?


The effect is different for each EPIC camera

Since pn exposures use offset tables computed on-board before the science
exposure, the optical loading in this camera is already partially corrected
on-board, except for extreme cases where the optical loading is so huge as to
create telemetry problems or even saturation of the offset maps. For pn the sole
effect is to degrade the energy resolution, except in those extreme cases.

MOS cameras use fixed offset tables, and optical photons may indeed generate
false X-rays events. However, the processing chain for MOS is able to identify
such events caused by the optical photons. So, in the case of MOS cameras, both
effects (spurious events and energy resolution degradation) are expected.

> And, how are these optical photons scattered?  Can I assume they will only

> affect the core of the image, or will they affect the radial profile beyond

> (say) 30''?
> 

You may also want to read to the section in the Users HandBook about EPIC
filters ( http://xmm.vilspa.esa.es/external/xmm_user_support/documentation/uhb_2.1/node32.html)
where the following paragraph can be found:

"If these measures work perfectly, the above problems are minimised. The use of
a thick blocking filter capable of minimising the optical light contamination
for all scenarios will necessarily limit the softest X-ray energy response. Each
EPIC camera is therefore equipped with a set of three separate filters, named
thick, medium and thin. It is necessary for the observer to select the filter
which maximises the scientific return, by choosing the optimum optical blocking
required for the target of interest. At the GO's discretion a thinner filter
could be used. In theory, due to the peaked optical response, a similar PSF core
excising as used in pile-up cases (see . 3.3.9) might be applied to recover the
desired spectra. Note, however, that the optical PSF is smoother than the X-ray
PSF due to diffraction of optical light at the gaps between the XMM-Newton
mirror shells. This analysis method has not yet been tested and in any case will
only work for pn observations where related offset tables are available as well.
"

Sincerely, 

Pedro


Reply 2

Resend
From: Pedro M. Rodriguez Pascual <xmmhelp@xmm.vilspa.esa.es>
To: rsmith@head-cfa.cfa.harvard.edu
Subject: Re: Optical contamination from bright sources on EPIC (PR#8754)
Date: Wed Jun 25 08:03:15 2003
Dear Randall, 

It seems that optical loading is not most serious problem you have got in your
observations. On one side the count rate of the source seems to be so high as to
result in a significant pile-up in both MOS and pn.

Further, for pn, the offset map seems to be affected by X-rays loading and the
energy scale is (very) wrong for the central pixel. The rest of the PSF is
affected (to a lesser extent) up to approx 0.3'. So, the core of the PSF should
not excluded in the analysis of the data.


Sincerely, 

Pedro


Followup 1

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To: "Pedro M. Rodriguez Pascual" <xmmhelp@xmm.vilspa.esa.es>
Subject: Re: Optical contamination from bright sources on EPIC (PR#8754) 
cc: rsmith@head-cfa.cfa.harvard.edu
Date: Wed, 25 Jun 2003 16:43:37 -0400
From: Randall Smith <rsmith@head-cfa.cfa.harvard.edu>
> Dear Randall, 
> 
> It seems that optical loading is not most serious problem you have got in
your
> observations. On one side the count rate of the source seems to be so high
as
> to result in a significant pile-up in both MOS and pn.
> 
> Further, for pn, the offset map seems to be affected by X-rays loading and
the
> energy scale is (very) wrong for the central pixel. The rest of the PSF is
> affected (to a lesser extent) up to approx 0.3'. So, the core of the PSF
should
> not excluded in the analysis of the data.

Pedro,

Many thanks for the detailed followup--you've noted that the observation has
quite a few 'issues' for extracting the point source (fortunately, not the 
prime goal!)

Just to clarify, though--on the pn, the core of the pn out to 0.3' or 20'' 
SHOULD be excluded, due to the offset map problem.  And on the MOS, the optical
photons will cause false X-ray events in the core and will also be scattered. 
If I understand correctly, in the MOS core there may be enough optical photons
to
cause false events, while the scattered photons out beyond about 20'' will at 
worse cause energy shifts of a 3-10 eV.  

My goal with the observation to extract the X-ray halo, starting about 60'' 
from the core, so as long as that region is at most lightly affected, all will
be well (once I understand the XMM PSF from 60-600'', a separate problem!).   

Thanks again,
Randall Smith





Reply 3

Resend
From: Pedro M. Rodriguez Pascual <xmmhelp@xmm.vilspa.esa.es>
To: rsmith@head-cfa.cfa.harvard.edu
Subject: Re: Optical contamination from bright sources on EPIC (PR#8754)
Date: Thu Jun 26 12:00:19 2003
Dear Randall, 

> 
> Just to clarify, though--on the pn, the core of the pn out to 0.3' or 20''

> SHOULD be excluded, due to the offset map problem.  And on the MOS, the
optical
> photons will cause false X-ray events in the core and will also be
scattered.

> If I understand correctly, in the MOS core there may be enough optical
photons
> to
> cause false events, while the scattered photons out beyond about 20'' will
at

> worse cause energy shifts of a 3-10 eV.  
> 
> My goal with the observation to extract the X-ray halo, starting about 60''

> from the core, so as long as that region is at most lightly affected, all
will
> be well (once I understand the XMM PSF from 60-600'', a separate problem!).
 

> 

It is not my intention to introduce more confusion, but you may want to read a
recent message in the helpdesk about XMM PSF:
http://xmm.vilspa.esa.es/xmmhelp/incoming?id=8765;user=guest

Note this message can be moved from the "incoming" folder to somewhere else
(most likely, "EPICMOS" or "EPICpn" folders)

Sincerely, 

Pedro

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