POA: Revised Documentation for POA_CALFOS ST-ECF

Revisions to the handbook to accompany the final release are currently being prepared.

Here is the text from the HST Data Hand Book (pages 31-16 ff from Chapter 31: Calibrating and Recalibrating FOS Data) relating to the correction of the GIM problem revised so as to be consistent with 'poa_calfos' . Future versions of the hand book will contain this material.

31.4.5 Correction for image motions (OFF_CORR)

As with all spectrographs the ability to obtain correct wavelengths and clean line profiles is critically dependent on the meachanical, optical and electronic stability of the entire system. Several precautions where taken in the design and operation, but nevertheless the effects of image motions of various kinds affected almost all data obtained. On top of that, contributions by the sources discussed below are approximately of the same size, so that a coherent solution could only be found post-operational in the comprehensive model built into 'poa_calfos' and based on the analysis of the entire FOS archive.

The detector related image motions stem from the electron-optical design. The FOS Digicons accelerated single photo-electrons released in the photocathode and imaged them onto a 1-dimensional array of diodes where they were counted. The electron optics consisted of the accelerating E-field of 20 kV and a parallel (longitudinal = Z) B-field of 105 Gauss. The strip on the photocathode to be imaged onto the diode array could be chosen by application of currents to crossed pairs of large coils that added B-fields in X,Y direction (order a few Gauss). The whole structure was embedded in a tube made of "mu-metal" to provide shielding against external magnetic fields.

  • GIMP: Early on during SV in 1990 it was detected that the geomagnetic field (order 0.5 Gauss) penetrated through the magnetic shield of the red side detector at considerable strength (attenuation only about factor 10). As a result, the location of the electron beam moved about on the diode array along small ellipses (diameters of up to 1 diode = 4 pixel), orientation and dimensions depending on orbital path and attitude (pointing) of HST. Whether or not the blue side detector suffered as well from this geomagnetic image motion problem (GIMP) remained unclear at the time, but a factor 7 smaller effect was found to match the sparse data. The full effect of this is seen in all data obtained prior to 3-Apr-1993.
  • OBGIMP: On 5-April-1993 an algorithm was implemented in the FOS on-board computer using the X,Y deflection system to compensate for GIMP in 15 second increments (see ISR CAL/FOS 66). POA analysis has verified that this compensation was suboptimal for the red side (amplitude 4 pix) and about a factor 10 to large for the blue detector (amplitude .5 pixel). All data obtained after 5-April-1993 need a compensation for OBGIMP.

  • YBOFF: Because the X,Y deflection system used magnetic fields, the ExB electron drift (Lorentz-force) moved the electron beam in X-direction even if only a Y-direction field was applied. This effect went unnoticed and uncompensated for, so that the regular updating of YBASE values (i.e. the definition of locations of spectra on the photocathode) resulted in a stepwise function of X-direction offsets in time. Data obtained late in the FOS life suffer the most, with offsets of up to 6 pixels in late 1996. All data need this YBOFF compensation.

The optical bench of FOS was manufactured from carbon rods and invar joints, and therefore pretty insensitive to thermal effects. However, the wheel carrying the focusing gratings used by both, the red and blue side trains respectively, and its stepper motor drive have always been a matter of concern even before launch. The usual explanation (also carried into the scientific literature) of seemingly random offsets in dispersion direction in the FOS data base has been to assign it to non-repeatibility in the position lock mechanism. Since any under/over rotation would produce a dislocation of the spectra in a direction perpendicular to the dispersion direction long before an X-offset would be noticed, this was not a valid explanation. However ...

  • TMPOFF: The drive shaft of the filter-grating wheel motor apparently was able to exert lateral force onto the wheel, thus altering optical alignment between collimator, grating and detector to produce a deviation in the exit angle for a given wavelength as seen from a particular location on the photocathode (or diode array for that). This effect is very strongly correlated with the ambient temperature, in particular the one read from the FGWA motor sensor. The amplitude of the effect is 0.06 pixels per degree change in temperature, and since the general temperature in the HST aft shroud has increased by more than 10 degrees between 1990 and 1997 all data (red and blue side) need this correction.

Correction procedure

  • After conversion from counts to count rates and the correction for paired-pulse loss, the raw data is shifted in memory location according to the sum of offsets determined from all effects above. The correction is applied to the arrays of raw data, errors, data quality values.

    For spectrophotometric ACCUM, IMAGE and RAPID-READOUT modes a unique correction is determined for each data group or ystep based on the orbital position of the spacecraft at the mid-point of the observation time for each subintegration. While the correction is calculated to sub-pixel accuracy, it is applied as an integer value and is therefore accurate only to the nearest integral pixel. This is done to avoid resampling the data in the calibration process. Special handling is required for data obtained in ACCUM mode since each data frame contains the sum of all frames up to that point. In order to apply a unique correction to each frame, data taken in ACCUM mode are first unraveled into separate frames. Each frame is then corrected individually, and the corrected frames are recombined.

    The correction is applied by simply shifting pixel values from one array location to another. As a typical example, if the amount of the correction for a particular data group is calculated to be +2.38 pixels, the data point originally at pixel location 1 is shifted to pixel 3, pixel 2 shifted to pixel 4, pixel 3 to pixel 5, and so on. Pixel locations at the ends of the array that are left vacant by this process (e.g., pixels 1 and 2 in the example above) retain their unshifted values and are assigned a data quality value of 700.

  • The correction for the post-onboard-GIMP (post 5-April-1993) datasets involves first removing the original onboard correction and then applying the same correction as used for pre-onboard-GIMP data. The orbital position is not calculated from the orbital parameters in the science header (.shh), because these elements are those valid at SMS generation only (usually approx 7 days before the observation). Instead, 'poa_calfos' uses a highly accurate ephemeris program from NORAD that is based on daily updates of the elements of HST determined from the tracking of HST. Both, the onboard-GIMP effects and the original GIMP effect are then determined from appropriate models of the geomagnetic field (the 10th order version of the International Geomagnetic Reference Field, IGRF), and an elctron-optical model of the digicons, and scaled appropriately. The effect of YBASE value changes are determined with reference to the YBASE values in use for the dispersion relation. Aperture/mode specific biases are included. Finally, a weighted average is obtained from 7 different temperature monitors in the FOS container and converted into a temperture related offset. For details on the physical backgrounds see ST-ECF Technical Report POA/FOS 004. For a discussion regarding how the on-board GIMP correction was driven and how the effect can be monitored in the engineering data stream see ST-ECF Technical Report POA/FOS 003.

    The basic parameters for the OFF_correction are saved into the POA (group parameter) keywords, a comprehensive listing can be found here.

The released versions of 'poa_calfos' OFF_CORR correction do not resample the data (same behaviour as the previous 'calfos'). The data are shifted in the FOS X-direction (the dispersion direction); this shift is calculated with sub-pixel resolution, but applied as the rounded value whole number value. The 'pfos_pix2wav' tool in the v1.2.1 release now encorporates the sub-pixel aspect of the correction. This technique will not touch the FOS data itself, but apply the correction on a line list of pixel positions.

The offset correction (OFF_CORR) is not applied to target acquisition data and polarimetry data. The correction can be applied SPECTROSCOPY, IMAGE and TAPID-READOUT mode data while using 'poa_calfos', or more accurately, all the data that fit the POA processing criteria (link).


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