Astrometry correction due to a thermoelastic drift

Rationale

Observations performed at the end of the Solar Aspect Angle (SAA) range (the so-called 'warm' attitude range, SAA in the 110 to 119.2 interval) could present a degradation of the pointing performance due to thermoelastic effects. These would include a larger APE and a pointing drift.

We have performed a study on this effect by calculating the Z-axis offset for a set calibration observations. While for STR2 temperatures below -15C the median offset presents a certain scatter around zero, for higher temperatures the behaviour of the Z-axis offset diverges. We have derived a linear correlation between the STR2 temperature and the Z-axis that aims at correcting the effect of such a thermoelastic drift.

We have developed a script aimed at correcting the astrometry offset produced by the thermoelastic effect. This should be applied to improve the pointing for observations performed at the 'warm' attitude range (STR2 temperature >-15C). We have applied the script to a set of calibration observations as a test, and checked that the general pointing is improved. The left panel on plot below shows the linear correlation between the STR2 temperature and the Z-axis used to correct this effect. The right panel presents the Z offset distribution before (green) and after (purple) applying the correction, the latter showing an improvement of the APE+ with respect to the original.

STR2_vs_offset.jpg warmbetaAPE.png

Script Usage

The script Astrometry Thermoelastic Drift Correction can be found in HIPE under the Scripts menu, in the General Useful scripts section. Any Herschel observation can be processed, although for observations performed in STR switch-over periods, those indicated in http://herschel.esac.esa.int/twiki/bin/view/Public/StrSwitchOversAndSvvResets, an informative output message is only returned. The script inputs that should be provided by the user (lines 260-263) are the following:

Input DescriptionSorted descending
workdir working directory
obsId observation id
instrument instrument used to perform the observation
changeWCS boolean (True/False) to define output: change in WCS vs new pointing product

Output

The user can select between two possible outputs using the changeWCS input parameter:

  • changeWCS = True: new Level 2 products where the offsets in RA y DEC have been used to adjust the WCS are created and saved to the working directory. These maps incorporate the thermoelastic correction, and are ready to be used for scientific purposes. Please note that this is only applicable to photometry.
  • changeWCS = False: a new pointing product that takes into account the updated rotation matrix/quaternion and corrects the filtered attitude is created and saved to the working directory. This product should be used by the user to reprocess the observation to correct for the thermoelastic correction. Please note that for spectroscopic observations, changeWCS is automatically set to False.

-- Pilar Esquej - 2016-12-01



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Topic revision: r6 - 2017-01-12 - PilarEsquej
 
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