HEIDI SOLAR ASPECT SYSTEM

Design of the Aspect System

•  With high-bandwidth,
•  With high-precision.
•  Grid-grid vector  to be provided with respect to sun center
•  At the time of  arrival of each X-ray photon.

• Lens/filter assembly mounted on the front grid tray
• Focussed an optical solar image onto a  detector assembly mounted on the rear grid tray.
• Both aspect system and X-ray optics would respond identically to telescope deformations

• Four inear diode arrays
• 1024 x 25 micron pixels
•  25 microns => 1 arc second

• The output of each array was scanned every 20 ms
• Analog thresholding used to identify the limb pixel.
• The telemetry included up to 4 limb pixel numbers
• Useful number of crossings = 3 or 4

• Accurate measurement of the relative locations of the ends of the array
• Initial measurement error estimated as roughly 25 microns
• Done before and after integration into the caniister using ECDS system
• Inconsistencies of about 60 microns found
• Reconciliation of the two data sets reduced internal inconsistencies to 48 microns
• Inconsistency is due in part to refraction in the sensor cover

• Programmed to obtain a rough aspect 25 Hz solution
• To provide input to the pointing control system
• Resulting aspect solution was satisfactory,
• But software problems with acquisition software,
• So used a Lockheed Intermediate Sun Sensor instead for tracking.

Overall Functioning

• In general the system functioned well,
• Providied arcsecond solar aspect 50 times per second.
• Occasionally (less than 1% of the time) the digital data transfer from the SAS was corrupted by noise,
• But analysis algorithms easily identified and rejected such occurrences.

• Quantitative evaluation of relative aspect performance is based on the fact that
  • Each readout not only locates Sun center,
  • But also provides one or two independent measurements of the solar radius,
  • Roughly speaking, the former is the half the sum of the two limb positions,
  • The latter is half the difference.
• A histogram of 13 minutes of Radius data
  • can contain up to 17,000 estimates of solar radius
  • and provides a good measure of system performance.
• Ground testing indicated that performance was limited by atmospheric seeing.
• In-flight performance was much better.
• Found discrepancy between pre-integration and post-integration measurements of SAS positions.
• Using pre-integration SAS positions, Four-limb crossing data yielded:
• 4 determinations of radius for each time, using 3 arms for each determination
• Initial analysis (including both 3- and 4-crossing data) showed a scatter of 0.95 arcseconds RMS in the solar radius histogram.
• Plot of radius R vs inferred sun-center x position (XC) without using -X arm
• Showed a flat distribution
• With FWHM about 1 arcsec
• Internal parabolic curves--an effect of quantization
• Plots of R vs XC using other 3-arm sets produced non-flat R vs XC
,
• For example, the plot of radius without the +X arm produced non-flat R vs XC
• The other plots showed similar bumps and curvature.

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19 March 1998 EJS HEIDI_SAS page 2

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• Attempted resolution of discrepancy between pre-integration and post-integration measurements of SAS positions.
  • Used redundancies to optimize the CCD pixel locations
  • Varied the estimated x-array position until the R(XC) curve flattened:
  • Optimum locations corresponded to positional errors of 65 microns RMS
  • Concluded that pre-integration measurements were more "correct".
• Summary:
  • The -X arm appears to be 'bad'-- inconsistent with other 3-arm data
  • RMS radius data was 0.5 asec (except for 3-arm data with -X arm)
  • Optimized SAS (without -X arm) resulted in 3-crossing histograms with 0.3 arcseconds RMS.
  • This can be compared to the value 0.3 arcseconds, determined by computer simulations, that might be obtained with a perfect system with precise 1-arcsecond (25-micron) sensor pixels.
  • It is especially noteworthy that this distribution was very 'clean' in that typically there were no 'bad solutions' in typical runs of 30000 estimates.
  • The residual errors can be attributed in part to random pixel-to-pixel variations in sensitivity.
  • In principle, the effects of such variations can be removed by statistical analyses given samples of 10^6 or more outputs.

• In terms of design, the use of 4 half-length rather than 2 full-length sensors had two undesirable effects: the effects of the 'gap in the middle' of the sensor were twofold:
• First, it reduced the field of view
• Second, it added 6 additional degrees of freedom to possible pixel locations.
• Another weakness in the design was
• that since the outputs of successive pixels were transmitted serially through a lengthy cable before analog thresholding was performed,
• cable dispersion was an uncertain factor
• which would systematically bias the limb determination.
• Digital thresholding at the sensor would alleviate this problem.
• In retrospect, additional care should have been taken both in
• mounting the sensors (whose positions were constrained only by normal dip sockets)
• and in prelaunch calibration of the sensor locations.
• Fitting the sensor locations as we did on the basis of inflight data, has the potential to
• cover up other problems
• and so introduce systematic errors.
• Once these steps are taken, performance could also be improved by
• interpolation of two or more sensor outputs at each limb,
• thereby overcoming limitations associated with pixel resolution.
• Inflight calibration of the relative sensitivity of the 4 arrays would also be highly desirable both to
• avoid systematic biasses in the solution associated with sensor to sensor differences
• and also to provide an alternative means of evaluating pixel-to-pixel variations.

In summary, the SAS performance, both qualitatively and quantitatively, would have provided

• Excellent support to X-ray observations
• Resolution of a few arcseconds.
• WIth the refinements suggested above, sub-arcsecond performance should also be possible.