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From: liu@mpe.mpg.de
Subject: position of optical-axis on the EPIC CCD
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Date: Thu, 13 Sep 2018 15:37:33 GMT
From: liu@mpe.mpg.de
To: xmmhelp@sciops.esa.int
CC: liu@mpe.mpg.de
Subject: position of optical-axis on the EPIC CCD
Full_Name: Teng Liu
Submission from: (NULL) (193.147.152.102)


Dear XMM-Helpdesk,

According to my understanding, the relative positions of the mirror and the CCD
are fixed, so that the optical-axis position should always fall on the same
position of the CCD (the same DETX,DETY). But I checked the optical-axis
position using the XCEN,YCEN keys from the header of exposure map, and I noticed
that the position of the optical-axis varies by ~10 arcsec among different
observations. Could you tell me why does it vary?

Best regards,
Teng Liu


Reply 1

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From: Rosario Gonzalez Riestra <xmmhelp@sciops.esa.int>
To: liu@mpe.mpg.de
Subject: Re: position of optical-axis on the EPIC CCD (PR#83947)
Date: Fri Sep 14 12:41:18 2018
Dear Teng Liu,

I have forwarded your question to the EPIC calibration experts. Please find
below  their answer.

Regards,

Rosario Gonzalez-Riestra
XMM-SOC User Support Group
------------------------------------------------------------------------

The exposure map records the pixel with the highest exposure value in the
XCEN/YCEN keywords.

This isn't necessarily the same position as the optical axis and can in fact
vary from observation to observation due to being calculated each time. The
vignetting function is very shallow at low off-axis angles.

It is best to use the RAW pixel calibration values in the XMM_MISCDATA_0022.CCF
file, keywords OPTICS_X and OPTICS_Y and then use the eccordconv program to
convert to the needed coordinate system if you really need the optical axis
value that the SAS uses.



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From: Teng Liu <liu@mpe.mpg.de>
Date: Sun, 16 Sep 2018 09:39:52 +0200
Subject: Re: position of optical-axis on the EPIC CCD (PR#83947)
To: Maria Diaz Trigo <xmmhelp@sciops.esa.int>
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Dear XMM-SOC group,

Thank you very much. But I still have some questions:
1. Why do the OPTICS_X and OPTICS_Y change? They changed for a few times
among these CCFs:
XMM_MISCDATA_0011.CCF  XMM_MISCDATA_0014.CCF  XMM_MISCDATA_0017.CCF
XMM_MISCDATA_0020.CCF
XMM_MISCDATA_0012.CCF  XMM_MISCDATA_0015.CCF  XMM_MISCDATA_0018.CCF
XMM_MISCDATA_0021.CCF
XMM_MISCDATA_0013.CCF  XMM_MISCDATA_0016.CCF  XMM_MISCDATA_0019.CCF
XMM_MISCDATA_0022.CCF

2. The OPTICS_X and OPTICS_Y do not look right for MOS1.
From XMM_MISCDATA_0022.CCF, I get:
EMOS1 OPTICS_X 305.
EMOS1 OPTICS_Y 291.
EMOS1 OPTICS_CCD 1.
Taking OBSID 0677610140 as an example, converting RAW 305,291 to image
coordinates:
ecoordconv imageset=P0677610140M1S001_ima_4exp.fits x=305 y=291 ccdno=1
withcoords=yes coordtype=raw
I get: IM_X: IM_Y: 345.4 335.8
But viewing this exposure map in ds9 using a rainbow colormap, the center
can be easily identified by eye as the center of some annuluses. It should
be 353.3 334.3 in image coordinates. The image has a bin size of 4 arcsec.
Therefore, there is a separation of 32 arcsec.

3. Why do you have to re-calculate XCEN,YCEN in the exposure map rather
than using OPTICS_X,OPTICS_Y directly? Is it because that OPTICS_X,OPTICS_Y
are not always correct?

Best regards,
Teng Liu

On Fri, Sep 14, 2018 at 2:41 PM Rosario Gonzalez Riestra <
xmmhelp@sciops.esa.int> wrote:

>
> Dear Teng Liu,
>
> I have forwarded your question to the EPIC calibration experts. Please find
> below  their answer.
>
> Regards,
>
> Rosario Gonzalez-Riestra
> XMM-SOC User Support Group
> ------------------------------------------------------------------------
>
> The exposure map records the pixel with the highest exposure value in the
> XCEN/YCEN keywords.
>
> This isn't necessarily the same position as the optical axis and can in
> fact
> vary from observation to observation due to being calculated each time. The
> vignetting function is very shallow at low off-axis angles.
>
> It is best to use the RAW pixel calibration values in the
> XMM_MISCDATA_0022.CCF
> file, keywords OPTICS_X and OPTICS_Y and then use the eccordconv program to
> convert to the needed coordinate system if you really need the optical axis
> value that the SAS uses.
>
>
>
> This message is intended only for the recipient(s) named above. It may
> contain proprietary information and/or
> protected content. Any unauthorised disclosure, use, retention or
> dissemination is prohibited. If you have received
> this e-mail in error, please notify the sender immediately. ESA applies
> appropriate organisational measures to protect
> personal data, in case of data privacy queries, please contact the ESA
> Data Protection Officer (dpo@esa.int).
>
>
>

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<div dir=3D"ltr"><div dir=3D"ltr"><div dir=3D"ltr"><div
dir=3D"ltr"><div di=
r=3D"ltr"><div dir=3D"ltr"><div dir=3D"ltr"><div
dir=3D"ltr"><div dir=3D"lt=
r"><div>Dear XMM-SOC
group,</div><div><br></div><div>Thank you very
much. B=
ut I still have some questions:</div><div>1. Why do the OPTICS_X and
OPTICS=
_Y change? They changed for a few times among these
CCFs:</div><div><div>XM=
M_MISCDATA_0011.CCF=C2=A0 XMM_MISCDATA_0014.CCF=C2=A0 XMM_MISCDATA_0017.CCF=
=C2=A0 XMM_MISCDATA_0020.CCF</div><div>XMM_MISCDATA_0012.CCF=C2=A0
XMM_MISC=
DATA_0015.CCF=C2=A0 XMM_MISCDATA_0018.CCF=C2=A0
XMM_MISCDATA_0021.CCF</div>=
<div>XMM_MISCDATA_0013.CCF=C2=A0 XMM_MISCDATA_0016.CCF=C2=A0
XMM_MISCDATA_0=
019.CCF=C2=A0 XMM_MISCDATA_0022.CCF</div></div><div
dir=3D"ltr"><br></div><=
div>2. The OPTICS_X and OPTICS_Y do not look right for
MOS1.</div><div dir=
=3D"ltr">From XMM_MISCDATA_0022.CCF, I get:<br></div><div
dir=3D"ltr"><div =
dir=3D"ltr">EMOS1<span style=3D"white-space:pre">
</span>OPTICS_X<span styl=
e=3D"white-space:pre">	</span>305.</div><div
dir=3D"ltr">EMOS1<span style=
=3D"white-space:pre">	</span>OPTICS_Y<span
style=3D"white-space:pre">	</spa=
n>291.</div><div dir=3D"ltr">EMOS1<span
style=3D"white-space:pre">	</span>O=
PTICS_CCD<span style=3D"white-space:pre">
</span>1.</div><div>Taking OBSID =
0677610140 as an example, converting RAW 305,291 to image coordinates:</div=
><div dir=3D"ltr">ecoordconv imageset=3DP0677610140M1S001_ima_4exp.fits
x=
=3D305 y=3D291 ccdno=3D1 withcoords=3Dyes
coordtype=3Draw<br></div><div dir=
=3D"ltr">I get: IM_X: IM_Y: 345.4 335.8<br></div><div>But
viewing this

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