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From: Zhongxiang Wang <wangzx@space.mit.edu>
Subject: XMM positional accuracy?
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Date: Fri, 19 Jul 2002 13:49:41 -0400 (EDT)
From: Zhongxiang Wang <wangzx@space.mit.edu>
To: xmmhelp@xmm.vilspa.esa.es
Subject: XMM positional accuracy?
Dear XMMhelp,

We are doing the study on one X-ray source which has been observed
by both Chandra and XMM. One problem we find is the big difference     
between the two positions derived from Chandra and XMM. The XMM
position has the offset from the Chandra position more than the XMM
positional uncertainty (4-5 arcsec). Therefore the XMM position is
definitely not consistent with the Chandra position (about 6 arcsec offset).

We notice that there is the document describing the astrometry of 
the Chandra observations at 
:http://cxc.harvard.edu/cal/ASPECT/celmon/index.html, which seems
to us that the Chandra position is reliable.

We asked the NASA Xmmhelp desk, and Steve Snowden gave us the following
suggestion
---------------------------------------------------------
You might want to
send a question to the SOC helpdesk.  They should be able to 
give you an estimate of the probablility for a 6" offset, my
guess is that it will not be vanishingly small.  Give them the
dates/times of your observation so they can verify that there is
nothing special about that time.
--------------------------------------------------------------------

So the observation time is 2000-12-28 14:10:41-16:31:07.0 
and the observation ID is 0112780401. Could you help us to see
if there was something special for this observation?

Thanks!
Zhongxiang

Reply 1

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From: Maria Santos-Lleo <xmmhelp@xmm.vilspa.esa.es>
To: wangzx@space.mit.edu
Subject: Re: XMM positional accuracy? (PR#5580)
Date: Tue Jul 23 17:21:53 2002
Dear Zhongxiang

Sorry for not having answer before, but it took us some time
to investigate the problems related to the observation you
mention, i.e. 0112780401, performed during XMM-Newton 
revolution 193

We have not a unique answer , but at least we can give you
some clues on how to interpret your data. 

There is indeed either a pointing or attitude reconstruction
problem for observation 0112780401. 

This can be verified by you , looking at the 'Spacecraft 
Attitude History File', included in the ODF, and with name:

0193_0112780401_SCX00000ATS.FIT

(for a description of this file, please look into the 
'Data File Handbook' at: 
http://xmm.vilspa.esa.es/external/xmm_user_support/documentation/index.shtml
)

this fits file contains, among other, three columns that give the 
difference between reconstructed and commanded viewing direction 
RA, DEC and PA. The file includes data for the whole revolution, 
so you should select the time period covered by your observation. 

How to interpret the information in this file might may not be 
easy for people not used to it. You can plot the difference 
between reconstructed and comanded viewing direction as a function
of time. A quick look to this plot shows that: 

1- the mean value for the pointing error is quite large !!: 
   with an offset of about 10.5 arcsec
2- the oscilation is quite large as well: values spread from 
   8.5 to 12.5 arcsec 

both facts are by far much larger than typical ones for most 
XMM-Newton pointings

In adittion to this we have also checked:

3- the RAWX and RAWY pn coordinates of the bright target, they are offset
  from the nominal position for on-axis pointings. This could simply be
  explained by uncertainties in the true position of the X-ray source. 
  However, we have computed, from the RAWX and RAWY position and the 
  nominal pointing (i.e. assuming perfect pointing), the sky coordinates
  for this bright source, and they differ from what one measures in the
  calibrated event list:

  from RAWX and RAWY (nominal pointing) 
     RA=10:50:07.2 DEC=-59:53:26.0
  from the event list (quick estimate 'by eye') 
     RA=10:50:06.2 DEC=-59:53:17.5 

4- the star tracker field of view: it is very crowed


The reason(s) of the above problems and discrepancies can be 

- the pointing is correct but there is an error in the attitude
  reconstruction : in such a case, the coordinates you measure in the event
  list are not correct

- error in spacecraft pointing, but attitude reconstruction is good: then
  the coordinates you measure in the event list are the correct ones


In general, the offsets like the one found in this observation (point 1
above) are NOT real attitude pointing errors of the XMM star tracker 
but instead are errors that affect the attitude measurement accuracy 
(AMA). They are therefore an erroneous indication of the true pointing 
error w.r.t. the target attitude, i.e. the first interpretation is 
usually the correct one. 

We have forwarded this question to the Flight Dynamics team, to see whether 
they can give more ligth on it. However, I cannot tell you whether a 
clear answer can be given soon.

With best regards
 
Maria Santos-Lleo, 
XMM-Newton SOC 
User Support Group


Reply 2

Resend
From: Maria Santos-Lleo <xmmhelp@xmm.vilspa.esa.es>
To: wangzx@space.mit.edu
Subject: Re: XMM positional accuracy? (PR#5580)
Date: Thu Jul 25 15:45:56 2002
CC: mjlt@star.le.ac.uk
Dear Zhongxiang

I have already the response from the Flight Dynamics group
and also the way to solve the problem. 

They confirm that the problem was indeed due to the quality of the
attitude reconstruction. Since the time this observation
was performed, they have very much refined the method they 
use in these particular cases where they were having problems.

They have therefore regenerated the attitude files and sent
them to me. In a following e-mail from my private 
account I will send you a file called 

0193_0112780401_SCX00000RAS.ASC

what you need to do is to reprocess yourself the files. 

Please follow the next steps:

- copy the 0193_0112780401_SCX00000RAS.ASC in the same directory
where you have the ODF files.

- rename the file 0193_0112780401_SCX00000ATS.FIT that is in your
ODF to something else, or even remove it. 

- if not done yet, set all the SAS variables (SAS_DIR, SAS_PATH, 
SAS_ODF, SAS_CCFPATH ) and start SAS

- if not done yet, run cifbuild and after it has finished, set 
SAS_CCF to the result of cifbuild (ccf.cif, file)

- run odfingest (even if you did it before, this is to get the 
new attitude file in the sas summary file instead of the 
previous one)

- re-run the sas 'pipeline' tasks, e.g. epchain or epproc 
to re-generate new event file

the coordinates in this new event file should be the 
correct ones.

With best regards

Maria

P.S. the correct attitude file will be used to generate 
new ODF and Pipeline products that will then be ingested in 
the XSA archive, however, this may take some time. Maria Santos-Lleo, 
XMM-Newton SOC 
User Support Group

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