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Status of standard pipeline pointing products: "simple" ground-based improvements (prior to SPG 13.0)

Scope of applicability

Please read carefully!

Important! if your observation has been processed with SPG 13.0 or later, the pointing information has been derived from gyro-reconstructed attitude data. Please refer to the page on improved pointing information from ground-based processing using gyro-reconstructed attitude.

The STR focal length corrections are ONLY applicable to ODs in the range [1, 857] and [859, 865].

  • If your data retrieved from the Herschel Science Archive (HSA) have been processed with SPG V11.1 or later, all the corrections have been applied to the observations and no manual action is required.
  • If your data retrieved from the Herschel Science Archive (HSA) have been processed with SPG V10.0.3 (or later) AND OD > 320, all the corrections have been applied and no manual action is required.
  • If your data retrieved from the Herschel Science Archive (HSA) have been processed with SPG V9.1.0 (or later) AND 320 < OD < 762, all the corrections have been applied and no manual action is required.
  • In any other case, please contact the Herschel Science Centre Helpdesk for support and refer to the old TWiki page

Introduction

On OD 320 the temperature of the star-tracker (STR) CCD was lowered, to mitigate the effect of hot flickering pixels. As a side effect this change slightly the focal length of the star tracker, hence introducing a shift in the reconstructed astrometry of Herschel maps when the distribution of tracking stars was not homogeneous in the STR field-of-view, up to 7-8 arcsec in the most extreme cases. A new model was uploaded on-board on OD762 to correct for this. For the interim period ODs 320 to OD761 new first order pointing products have been generated at HSC to correct this problem.

Below an example for a star (UZ Tau) in OD684, on the left the current HSA level 2 blue channel (70 microns) maps and in the right processed with the new pointing product. The absolute astrometry offset is reduced from 7.5 arcsec to 1 arcsec.

More details are provided in the Release Note v1.0

A copy of the test report can be found here New pointing products test report (PDF)

UZ_Tau.jpg

Here is a SPIRE example: PSW map GammaDra from OD450 with/without improved pointing correction: although from the circle looks like there isn't any significant offset change, it is possible to appreciate the (tiny) difference in flux distribution in the pixels.

GammaDra_450_PSW.png

Additional corrections included (SPG v10.0.3 & v11.1)

The standard products generated with SPG v10.03 include a residual correction to compensate the for the difference between the 1D correction uploaded in OD 762 and the 2D linear correction uploaded in OD 866. In SPG v11.1, a correction for the small focal length error existing in the early phases of the mission ( OD < 320) has been included as well.

README information - first-order corrected pointing products

MSP, 27th July 2012 v1.0

Background information

A main component of the Attitude Control and Measurement System (ACMS) is the star-tracker (STR; two units present on-board Herschel, prime and backup). The STR is a small optical CCD camera plus an image processing unit that, starting from an image of the sky, extracts the attitude information measured with respect to the J2000 inertial reference system and delivers it to the Attitude Control Computer (ACC). At the start of the mission, the STR CCD reference temperature was 20 deg C. An effect that was observed along the first months of S/C operations was the sporadic occurrence of distortions in the scan map patterns accompanied by variations in the tracking angular rate (the so-called 'speed bumps'). These events were eventually tracked to the existence of transitory 'warm pixels' (not flagged as 'hot' ones in the defective pixel table). In order to reduce the number of warm pixels, the STR CCD reference temperature was lowered in operational day (OD) 320 from 20 to -10 deg C. The 'speed bumps' were eventually corrected, but at the price of augmenting the STR plate scale errors that produced systematic but boresight-dependent offsets.

The new (low) temperature setting had a negative impact on the S/C absolute pointing accuracy due to increased plate scale errors in STR. In OD 762, a simple, 1D correction (i.e. the same linear correction factor in the Y and Z STR axes) was uploaded to the S/C ACMS and the performance was substantially improved. In OD 858 an enhanced 2D correction (different linear factors in the Y and Z axes) was successfully tested; the change was made permanent in OD 866. Eventually, in OD 1011, a full STR focal plane (FP) distortion correction was made operational. Moreover, in OD 1032 the STR catalogue was purged by removing 73 'dubious' stars from the tracking list.

Several groups within the Herschel Ground Segment are currently working in the a posteriori improvement of the S/C attitude, coordinated by the Herschel Pointing Working Group. This is specially important for observations obtained in the period OD 320 - OD 761, as explained above.

The pointing products produced by the software described in this note intend to provide a first, yet crude correction of the systematic, boresight-dependent effects introduced by the STR plate scale errors.

Processing steps

The current processing software applies the following steps:

(a) Load the STR guide stars' catalogue 
(b) Open the pointing product and read records in a loop. For each attitude sample:
   b.1 Read filtered attitude and STR quality flag (number of stars tracked)
   b.2 Get the catalogue information fro stars within the FoV of the STR at the current S/C filtered 
       attitude: X,Y,Z, magnitude, and trackability status.
   b.3 For trackable the stars within the FoV, determine the direction vectors using the
       STR "focal length" correction factors provided by Flight Dynamics.
   b.4 Determine the tracked stars by means of the same algorithm used in the STR (but limited to a maximum         
   of nine stars, i.e. the "interlacing mode" is not implemented) 
   b.5 For the tracked stars, given the direction vectors and the inertial
      (reference) vectors, determine the best attitude using the "q-Method" (Davenport 1968).
   b.6. Replace the filtered quaternion by the modified quaternion computed by the q-method.

The applied correction does not add noise to the attitude samples since is a purely geometric one. When the correction factors are set to 1.0, one recovers exactly the input attitude (within ~1.0e-10 arcsec).

In the current implementation, the processing script is not distributable to end- users since it requires the proprietary STR catalogue.

Limitations

(a) Only up to 9 stars are used, i.e. no interlacing is considered
(b) The stars' selection algorithm uses the same logic as the STR. But there is no check that the stars used within the script are the same as those actually used for tracking. In particular, if one of the 73 bad stars was used for tracking, it will replaced by a different one in the script.
(c)  The 2-D, linear correction is implemented (rather than the full one).

Future improvements

The software described here is just a first step in a long-term strategy to improve the S/C attitude estimation and hence the astrometry of the science products. It is expected to have further releases of the current software that will include things like:

- New STR star selection algorithms based on the guide stars actually selected by the ACMS - Implementation of the interlacing algorithms - Implementation of the STR full FP correction

Future software tools will include a more sophisticated processing based on batch attitude estimators. The construction of such software is currently being started.

STR-induced offsets for observations in the period OD1-319

The files linked below list the expected STR-induced offsets at the nominal RA, DEC coordinates (i.e. those entered as "target" coordinates in HSPOT) for all observations in the range OD 1-319, in comma-separated format (readable in Excel, Open Office, TOPCAT and many other common tools).

STR-induced offsets for observations in the period OD320-761

The files linked below list the expected STR-induced offsets at the nominal RA, DEC coordinates (i.e. those entered as "target" coordinates in HSPOT) for all observations in the range OD 320-761, in comma-separated format (readable in Excel, Open Office, TOPCAT and many other common tools).

  • offsets_HIFI.csv: STR-induced offsets at the nominal RA, DEC positions for HIFI observations in the range OD320-761

STR-induced offsets for observations in the period OD762-865 (excluding 858)

The files linked below list the expected STR-induced offsets at the nominal RA, DEC coordinates (i.e. those entered as "target" coordinates in HSPOT) for all observations in the range OD 762-865 (excluding OD 858, where the linear 2D correction was tested on-board), in comma-separated format (readable in Excel, Open Office, TOPCAT and many other common tools).

-- MiguelSanchez - 26 Nov 2013

-- MiguelSanchez - 08 Apr 2015

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Topic revision: r6 - 2015-04-08 - MiguelSanchez
 
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