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From: tangsk@astro.umass.edu
Subject: How to match the observation and Blank-sky file according the atthk.dat ?
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Date: Fri, 25 Jul 2003 18:36:01 GMT
From: tangsk@astro.umass.edu
To: xmmhelp@xmm.vilspa.esa.es
CC: tangsk@astro.umass.edu
Subject: How to match the observation and Blank-sky file according the atthk.dat ?
Full_Name: shikui Tang
Submission from: (NULL) (128.119.51.250)


Dear Dr.

    I am doing a diffuse X-ray emission region and I am using the Blank-sky file
compiled by  people at Birmingham. The Blank-sky file in the the Detector
co-ordinate. I can project the Blank-sky file onto the sky region but cannot add
the effect of the attitue of the observation. The point direction do change
during one observation(but not dithering like Chandra does?). So there is a
problem to subtract the background from the Blank-sky file in the same region.
Is there other way to work around?

   By the way, how to construct an event file in the Detector co-ordinate which
can be display by ds9?
    Thanks a lot!



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From: Matthias Ehle <xmmhelp@xmm.vilspa.esa.es>
To: tangsk@astro.umass.edu
Subject: Re: How to match the observation and Blank-sky file according the atthk.dat ? (PR#8852)
Date: Mon Jul 28 07:28:23 2003
Dear Shikui Tang,

you wrote:
> I can project the Blank-sky file onto the sky region but cannot add
> the effect of the attitue of the observation. The point direction do
change
> during one observation (but not dithering like Chandra does?). 

Just checking if I got the whole message: you say you have an observation
(a single exposure?) in which the XMM-Newton pointing direction is
significantly
changing? Which ObsID are you talking about? - this is quite strange as
XMM-Newton is known to be very stable during a single exposure. No, we are not
doing any dithering during the esposure.

> So there is a
> problem to subtract the background from the Blank-sky file in the same
region.
> Is there other way to work around?

The blank-sky files are used to describe the spatial and spectral properties of
the EPIC background. The spatial properties are NOT depending on the sky
position
but on the detector coordinates (e.g. vignetting, instrument intrinsic
background).
So you can perform your background analysis in fact using the same spatial
extraction regions for your source and for the blank fields in detector (!)
coordinates DETX/DETY. You do not need to go to sky coordinates.

But if you want to skycast the blank field event files onto the sky position of
your target field, please, see http://www.sr.bham.ac.uk/xmm3/ - alternatively
use the SAS task attcalc.

>    By the way, how to construct an event file in the Detector co-ordinate
which
> can be display by ds9?

See e.g. our SAS Users' Guide available at
http://xmm.vilspa.esa.es/external/xmm_user_support/documentation/index.shtml
and there Section 4.8 Generating EPIC images:
you just have to use xcolumn=DETX ycolumn=DETY

Cheers,
Matthias.

Matthias Ehle 
XMM-Newton SOC 
User Support Group

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