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From: Georgiana Ogrean <g.ogrean@jacobs-university.de>
Date: Thu, 22 Mar 2012 10:40:10 +0100
Subject: Exposure time of combined counts images
To: xmmhelp@sciops.esa.int

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I just noticed that the combined images created by the ESAS task "comb"
take the exposure time of MOS2. However, if let's say I'm only combining
MOS images, then I think the exposure time of the combined image should be
the average of the MOS1 and MOS2 exposure times. Am I missing something, or
is the EXPOSURE keyword not handled properly for merged files?

Thanks,

Georgiana

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I just noticed that the combined images created by the ESAS task "comb=
" take the exposure time of MOS2. However, if let's say
I'm on=
ly combining MOS images, then I think the exposure time of the combined ima=
ge should be the average of the MOS1 and MOS2 exposure times. Am I missing =
something, or is the EXPOSURE keyword not handled properly for merged files=
?<br>

<br>Thanks,<br><br>Georgiana<br>

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Reply 1

From: Ignacio de la Calle <xmmhelp@sciops.esa.int>
To: g.ogrean@jacobs-university.de
Subject: Re: Exposure time of combined counts images (PR#49807)
Date: Thu Mar 22 14:07:20 2012

Dear Georgiana,

This behaviour is correct. You can have a look at the documentation of
the comb task:

http://xmm.esac.esa.int/sas/current/doc/comb/node3.html

The end result of mosaicking is to have an image where all the input 
exposures are normalized in some way so that the image looks smooth. 
For that, ESAS determines the relative band count rates for each exposure 
to that of the MOS2 with a medium filter. The exposure map of each
exposure is then scaled by the count rate ratio. Thus, each exposure's
contribution is transformed to what the MOS2 medium filter would
observe. 

I will come back to you with the other pending question you had
later.

Regards,

Ignacio de la Calle
XMM-Newton SOC

> --e89a8f3bb0cfe79bd204bbd1b2ad
> Content-Type: text/plain; charset=ISO-8859-1
> 
> I just noticed that the combined images created by the ESAS task "comb"
> take the exposure time of MOS2. However, if let's say I'm only combining
> MOS images, then I think the exposure time of the combined image should be
> the average of the MOS1 and MOS2 exposure times. Am I missing something,
or
> is the EXPOSURE keyword not handled properly for merged files?
> 
> Thanks,
> 
> Georgiana
> 
> --e89a8f3bb0cfe79bd204bbd1b2ad
> Content-Type: text/html; charset=ISO-8859-1
> Content-Transfer-Encoding: quoted-printable
> 
> I just noticed that the combined images created by the ESAS task "comb=
> " take the exposure time of MOS2. However, if let's say I'm on=
> ly combining MOS images, then I think the exposure time of the combined
ima=
> ge should be the average of the MOS1 and MOS2 exposure times. Am I missing
=
> something, or is the EXPOSURE keyword not handled properly for merged
files=
> ?<br>
> 
> <br>Thanks,<br><br>Georgiana<br>
> 
> --e89a8f3bb0cfe79bd204bbd1b2ad--
> 
>