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.
October 01, 1990|
January 13, 1994|
October 10, 1990|
November 04, 1994|
November 11, 1991|
October 16, 1995|
September 21, 1992|
January 02, 1996|
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 ).
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.
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).
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).
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.
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