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32.3 Location of Image on Diode Array

This section discusses factors that affect the degree to which the photocathode image is accurately directed onto the diode array for readout. Factors which produce displacement of the photoelectrons between photocathode and diode are the commanded magnetic deflection (Y-bases) and GIM motion. Incorrect location of the image on the diode array could affect:

32.3.1 Y-bases

The amount of magnetic deflection required to direct photoelectrons from a designated portion of the photocathode onto the diode array for readout was characterized in Y-base units. Depending upon the ambient magnetic characteristics of the Digicon at the time of observation differing Y-base deflections could have been necessary to image the same portion of the photocathode onto the diode array. Pre-COSTAR calibration data to determine these optimum deflections initially showed that there was a trend with time in Y-base for all gratings with FOS/BL. These trends were not as clearly seen with FOS/RD.1 Post-COSTAR analyses showed that the change of Y-base deflection with time for all FOS/BL gratings continued, while the Y-bases for all gratings observed with FOS/RD remained randomly scattered.2 See Figures 32.1 and 32.2 which show the measured Y-base as a function of time for FOS/BL and FOS/RD dispersers, respectively. The dashed lines represent one departures from the solid line best linear fits to the data. The light, or dotted lines, represent typical peak-to-peak displacements due to FGW uncertainty. The horizontal "error bar" style lines in these figures indicate the actual Y-base values used.

The amount of scatter in the mean Y-base location increased after on-board GIM correction started in April, 1993 as frequent DEPERM commanding (clearing of the ambient magnetic field in the detector) was turned off for all but ACQ/PEAK exposures. The uncertainties in the Y-bases affected both ACQ/BINARY pointing accuracy and FOS photometric accuracy. The size of these uncertainties required that, lest positional and photometric accuracy be compromised, all ACQ/BINARY acquisitions be followed with an ACQ/PEAK stage for those cases in which science was performed in an aperture smaller than 1.0. The photometric quality of the data, especially for FOS/RD and the 1.0 and 4.3 apertures, could be compromised due to the fluctuations in the location of the spectrum (see the discussion in "Image Centering (Image Location) Factors" on page 32-22).

Furthermore, the shapes of the spectra on the photocathode were not linear, but had a curvature of +/- 20 Y-base units (the so-called "s-curve" of the disperser). Figures 32.3 and 32.4 show the scale of the s-curvature with formal one measurement uncertainties for each FOS/BL and FOS/RD grating, respectively. Therefore, the photometric effect of Y-base uncertainty was not a simple matter of losing light in a uniform fashion, rather due to the s-curvature of the spectra, the effect was also wavelength (and disperser) dependent.

No systematic temperature-related Y-base changes were expected in the relatively small (10 degree C.) FOS on-orbit operating range and none were ever observed.

Commencing in mid-1993, FOS Y-bases were updated approximately every six months. The updates were made so as to include the anticipated linearly extrapolated trending over the ensuing six month period from the date of update. With a few notable exceptions (the June 1992 to December 1993 period for all FOS/BL dispersers and the January 1994 to January 1995 period for FOS/BL G130H as seen in Figures 32.1 and 32.2) Y-base updates resulted in worst-case 10 Y-base unit deviations from the final linear trend determined from all of the Y-base measurements. Absolute photometric accuracy and, especially, spectrum shape, could be affected for large aperture observations made with these dispersers in the time periods mentioned above. The dates of all FOS Y-base updates are included in Table 32.3.

FOS Y-base PDB Update Dates



October 01, 1990

January 13, 1994

October 10, 1990

November 04, 1994

November 11, 1991

October 16, 1995

September 21, 1992

January 02, 1996

(FOS/BL G190H,

G270H, G400H only)

August 11, 1993

June 15, 1996

December 17, 1993

December 18, 1996

February 11, 1997

Summary of Y-base Uncertainties and Impacts

Repeated independent determinations of the same Y-base yield an external +/-25 Y-base unit (pre-COSTAR: ~0.14"; post-COSTAR: ~0.12") full-amplitude scatter for observations taken on the same day. This scatter is attributable to the effects of residual GIM (minimally) and, primarily, FGW non-repeatability uncertainties. The internal measurement error in any individual Y-base value is +/- 5 Y-base units (one ).

The photometric impact of an erroneous Y-base depends on the size of the error, the aperture involved, and the detector-disperser combination employed. On average, for post-COSTAR point sources, if we assume that a Y-base were known with an accuracy of only 20 Y-base units (approximately 1/13 diode height), for the large apertures (1.0") 3-5% of the light in general could be lost and up to 10% at certain wavelengths where the s-curvature is substantial. The light loss is larger, but harder to quantify, for extended objects and pre-COSTAR point sources (see FOS ISR 096).

There is essentially no photometric impact due to Y-base uncertainties for FOS apertures smaller than 1.0 as the displacements required to place the images of these apertures off the diode array are simply much larger than the observed uncertainties.

Y-base errors had no impact on measured positions in IMAGE mode observations made with the MIRROR. The brightness of objects within the field of view could be mis-represented, however.

For large aperture (1.0 and larger) observations, Y-base uncertainty is a prime contributor to photometric, especially spectral shape (color), uncertainties. Refer to Figures 32.3 and 32.4 to determine the spectral regions in which loss of signal is most likely to occur in your spectra. There is little or no photometric impact on apertures smaller than 1.0.

Remember, 256 Y-base units = 1.43" (pre-COSTAR) and 1.29" (post-COSTAR)

Figure 32.1: FOS/BL Y-bases as a Function of Time

Figure 32.2: FOS/RD Y-bases as a Function of Time

Figure 32.3: Normalized Spectrum Shape on Photocathode ("s-curve") : FOS/BL

Figure 32.4: Normalized Spectrum Shape on Photocathode ("s-curve") : FOS/RD

32.3.2 Geomagnetically Induced Image Motion (GIM)

Off-line geomagnetically-induced image motion (GIM) correction was needed only for data taken before April 5, 1993. For spectra taken later, the GIM correction was applied onboard the spacecraft.

As noted in the previous section, photoelectrons from the transmissive FOS photocathode were magnetically deflected onto the diode array. Due to insufficient magnetic shielding of the Digicon detectors, particularly for FOS/RD, the geomagnetic field affected where the photoelectrons landed on the diode array. The effective magnetic field experienced by the electrons depended on the location of the spacecraft in the geomagnetic field. A positional shift of the recorded spectrum due to the changes in the effective magnetic field occurred both parallel to the dispersion direction (x) and perpendicular to the dispersion direction (y). As of April 5, 1993, this geomagnetically-induced image motion (GIM) problem was corrected in real-time onboard the spacecraft through the application of a spacecraft position-dependent correction to the magnetic deflection that compensated (in both x and y) for the effects of the geomagnetic field. However, before April 5, 1993, there were no real-time onboard corrections for GIM.

The effect of the x-shift in the photoelectron impact point was to effectively shift the spectrum in the dispersion direction as a function of time. This displacement can be seen in data taken before April 5, 1993 by plotting the individual groups of raw data on a single plot (use the STSDAS grspec task) and noting the shift (in x) of the centroids of individual emission or absorption lines. In routine post-observation calibration calfos applies a correction for the GIM x-shift (as long as the OFF_CORR switch is set to "PERFORM") in the creation of the calibrated spectral data (.c0h and .c1h files).3

Pre-onboard GIM Correction

The post-observation GIM correction for a given readout segment in an observation is determined from the orbital position of the spacecraft at the mid-point of the observation time for the segment. To avoid resampling the data, and hence losing error information, the correction is applied as an integral pixel shift, although the accuracy of this correction is therefore +/- 0.5 pixel where each pixel is 1/NXSTEPS diodes (1/4 diode in the standard spectrophotometry modes).

In post-observation processing there is no way to correct for the photometric effects of the shift in y introduced by GIM. The y-shift caused the s-curve of dispersed light to move along the long dimension of the diodes, and as with Y-base error, could produce a wavelength-dependent loss of light off the edge of the array. The resultant time-dependent error in the overall flux level and in the shape of the spectrum was most severe for poorly-centered observations and for the large apertures. The typical size of uncorrected GIM motion was 0.15" or about 25 Y-bases. For well-centered observations with no Y-base error and the 4.3 aperture the error was <5%. You can use the models of light loss as a function of Y-base offset, wavelength, disperser, and detector given in FOS ISR 096 to estimate the effect of GIM motion on your observations.

Depending upon when the program was prepared for scheduling, some data taken after April 5, 1993 may not have had the onboard GIM correction applied. The header keyword YFGIMPEN will tell you if the onboard correction was enabled; if the value is "TRUE" then the onboard correction was applied.

Onboard GIM Correction

The onboard GIM correction is applied on a finer grid than is provided by the pipeline GIM correction on both the x and y axes, so that both wavelength and photometric effects of GIM are minimized. In the x-direction the onboard GIM correction is applied in units of 1/32 of the width of the diodes, while in the y direction the unit is 1/256 of the diode height (or Y-bases). The onboard GIM correction is calculated and updated every 30 seconds (please see FOS ISR 098 for complete technical details of the onboard GIM algorithm).

GIM Summary

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1 FOS ISRs 096 and 110.

2 FOS ISRs 133 and 152.

3 Before May 1991, calfos did not correct for GIM. All data in the HST Archive were reprocessed and given new header keywords after the GIM correction was added to calfos. Therefore, if your FOS data were processed before May 1991, you should retrieve the updated raw data from the HST Archive instead of recalibrating the data on your tape.

Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 01/14/98 14:55:10