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From: McHardy I.M. <imh@soton.ac.uk>
Subject: OM fast mode
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From: McHardy I.M. <imh@soton.ac.uk>
To: Jacobo Ebrero <xmmhelp@sciops.esa.int>
Subject: OM fast mode
Date: Wed, 1 May 2019 16:15:54 +0000
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Reply 1

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From: Nora Loiseau <xmmhelp@sciops.esa.int>
To: imh@soton.ac.uk
Subject: Re: OM fast mode (PR#84912)
Date: Fri May  3 16:41:18 2019
Dear Dr. McHardy,

Some information about problems in the observations is available in the Data
Product Screening Report included in the pipeline file 
PP082461*PPSMSG000_0.ASC. It is also shown in the Quality Report available in
the XSA Archive (http://nxsa.esac.esa.int/nxsa-web/), in the  Results TAB,
clicking on the magnifying glass besides the ObsesvationID. 

Please find below some comments by the OM Calibration Scientist:

Best regards,

Nora

................

1) from the quality reports, 0824610101 (rev 3482) and 0842610401 (rev 3492) are
OK (at least no issues flagged for fast mode).
For 0824610201 (rev 3485), report says source was mis-centred after exposure 115
(as indeed it is) due to a hardware problem.
For 0842610301 (rev 3491), report says target was out of the window due to a
ground segment problem (you also noted there was a problem with a bad
centroiding table).

2)  For 0824610101, the source is generally well centred - all PSF correction
values are 1.0 while the source rate drops from around 1.0 to 0.6, so this might
be a real variation.

In 0824610201, the marked shift in position after exposure 115 is accompanied by
a notable reduction in the PSF fraction but while the PSF fraction declines by
around 25%, the source rate drops by 40-50%. I would be surprised if the rate
was badly wrong even in the latter exposures of this observation, though there
are hints of an anti-correlation between rate and PSF correction in places.
 
0824610301 is obviously not recoverable.

For 0824610401, the source is slightly off-centred at the start and drifts to
the lower right corner over the observation. The source rate seems to increase
fairly systematically, from 5.2 to 8 cts/s while the PSF correction factor
changes from 1.0 for exposures 052-063 to ~0.996-0.96  for exposures 064-074,
i.e. the count rate increases by 54% while the PSF correction factor changes by
only 4%, so potentially incorrect PSF values can not explain the rising count
rate. Could this be a real variation?

3) A single field acquisition (FAQ) observation is made at the start of an
observation (for some modes but nominally 'always' for fast mode cases). Any
subsequent drift is not corrected because tracking does not get applied to fast
mode data - the window is not shifted on board during fast mode. There is drift
history information from the tracking stars that can be/is applied by the SAS
but this applies only within an exposure so if the source drifts within the fast
mode window from exposure to exposure, this 'long-term' drift is not corrected. 
At present, there is nothing that can be done about that without some form of
repeat field acquisition. That the source remained centred in 0824610101 may
simply be fortunate, and less so in 0824610401 - and there were other problems
with the other two observations as noted above.

4) I think there can be small offsets of the source at the start of an
observation, due to the pointing accuracy of the spacecraft and the accuracy of
the field acquisition
solution, and, of course, the accuracy of the target coordinates (given the same
values for each observation, variations are presumably down to the first two
points).

5) For future observations, I don't think there is currently anything that can
be done to improve the centring, other than the possible aim of performing
repeat FAQs through the observation but this has time impacts of course. Dummy
mosaics is another way (essentially also results in new FAQs) but again comes
with time penalties/inefficiencies.

6) The count rates for the target are obtained through aperture photometry. If a
source is off-centred such that part of the source wings are outside the fast
mode window, the rate should be corrected for the missing PSF (as quantified in
the PSF_FRAC keyword in the time-series header (check also the PSF_CAPP value is
T)). Thus, in principle, there is no need to make further corrections. There
are, however, cases where this can break down. In particular, if the source is
close enough to the edge that a substantial portion of it is outside the window,
the centroiding will be inaccurate and therefore the PSF correction unreliable.

7) It is possible for the rates in individual bins to be negative because
background subtraction is performed. 

..................



============
Hi Jacobo, Jan et al,

We recently did some observations of NGC4395, proposal 082461.

There were 4 orbits.

REV  | Visibility start        Visibility end       (sec)

3482 | 2018-Dec-13 06:49:28 -> 2018-Dec-14 15:32:48 (117800)
3485 | 2018-Dec-19 06:24:18 -> 2018-Dec-20 15:14:17 (118199)
3491 | 2018-Dec-31 05:38:41 -> 2019-Jan-0114:30:21 (118300)
3492 | 2019-Jan-02 05:30:18 -> 2019-Jan-03 14:23:38 (118400)

Sometimes the AGN was within the 10x10 arcsec fast mode box, and sometimes is
was near the edge and for one orbit (3491) it wasn't in the box at all.


3482  In box all the time

3485   Sta

Message of length 6046 truncated

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