SOI/GOLF Intercomparison Detailed Specifications

Team Coordinator: Rick Bogart


Observing the GOLF Magnetic Proxy in the SOI-MDI Structure Program

R. S. Bogart & R. K. Ulrich

v. 0.8 94.07.12

1. Observables

At each pixel in the full-disk mode (2" square): two observables are calculated from the five averaged line filtergrams F1 - F4, Fc, and the constant ADC bias according to eqns. 1-4 and 14-17 of the IPFR. One of these quantities, sensitive to spots and pores, is the Continuum Intensity proxy Ic (eqn. 17); the other, sensitive to faculae, is the Line Depth proxy Idepth (eqn 14). The Continuum Intensity proxy is the same observable as used by the Flux Budget part of the Structure Program (v 0.9). This specification only refers to the Line Depth proxy under the explicit assumption that the Flux Budget program will use an identical spatial and temporal filter for the Continuum proxy.

The TAI time of observation (center of time average) and the camera location with respect to the Sun (SOHO ephemeris) are also required.

2. Cadence & Duration

Observations to be made once per minute as part of the low-rate Structure Program, without interruption, from nominal commencement through nominal end of Structure Program (2 years on station).

3. Spatial Filtering

The full image 1024*1024 per pixel data are boxcar averaged into a 128*128 square array (8*8 pixels per averaged pixel). Bad pixels (dark current > 5 dn) are not ignored in the spatial averaging.

Two separate buffers for temporal filtering are maintained. In the current specification these buffers are spatially coincident. In future revisions it is possible that one buffer will be offset from the other by 4 pixels in each direction, so that it would only be a 127*127 square array, the 4 rows and columns at each edge of the image being stripped.

4. Temporal Filtering

The data are averaged weighted by a Gaussian truncated to 0 at tc +/- 12 min (thus, only 23 one-minute samples are included, and 2 accumulation buffers are required). The weighted averages of the Line Depth proxy are sampled concurrently with the weighted averages of the Continuum Intensity proxy once every 12 minutes for telemetry. The Gaussian weights, normalized to unity at tc, correspond to a standard deviation of 204 seconds (3.4 minutes) as follows:
tc		1.000000	tc +/- 6	0.210748
tc +/- 1	0.957669	tc +/- 7	0.120106
tc +/- 2	0.841129	tc +/- 8	0.0627770
tc +/- 3	0.677549	tc +/- 9	0.0300931
tc +/- 4	0.500553	tc +/- 10	0.0132301
tc +/- 5	0.339149	tc +/- 11	0.00533448
The actual integer weights will be adjusted to avoid overflow in the temporal sums (TBD).

The specified temporal window provides a low-pass filter with a smooth roll-off to a value of 0.1 (-20 db) at 1.7 mHz, 0.01 at 2.4 mHz,first zero at 3 mHz, and transmission < -60 db (0.001) for all frequencies > 2.8 mHz. With this filter no more than about 40% of the power below 1 mHz should be due to aliasing of higher-frequency p-mode power.

5. Calibration

No calibration (in the sense of correction of values calculated on board to take account of for example known drifts in the filter response function) is presently planned: the level 1 (calibrated) data as delivered are identical with the level 0 (extracted and sorted) data.

6. Data Organization

Averaged pixels lying wholly outside the maximum solar diameter are discarded prior to telemetry. The remaining averaged pixel values are arranged in a fixed stream, order TBD. The order of this stream is preserved as the data ordering through level 0 and any level 1 processing.

7. Data Representation and Precision

It is assumed that individual filtergrams are digitized as 16-bit integers normalized to values of 4095 for full-well, that the typical exposure value is one half of full-well (~2048) per pixel at disc center, and that the counting statistics are such that the shot noise to signal ratio per pixel at disc center for each of the accumulator quantities S_a, S_b, etc. is approximately 0.001, so that 10 bits are required to represent the signal portion of the quantity. Then the relative noise-to-signal per pixel for the Line Depth proxy should also be approximately 0.001.

The spatial averaging will reduce the noise-to-signal ratio in the resulting bin by the square root of 64, so that 13 bits will be required to represent the normalized average without noise. The temporal averaging involves averaging over an effective 8.5 samples (the sum of the Gaussian weights), implying a further reduction in noise-to-signal by the equivalent of 1.5 bits. The quantities to be transmitted should thus be represented as normalized 15-bit quantities prior to lossless compression.

The above numbers are conservative estimates, as they are based on disc center values. Away from disc center, the signal-to-noise ratio will be lower and the corresponding precision less. It may be possible to normalize the signal to 14 bits prior to compression. If the average number of bits per bin of compressed averaged data for the Line Depth proxy is 10.5, then the required telemetry is 175 bits per second.

8. Expected Future Revisions
Refinement and specification of clipping mask
Specification of requirements for flat-fielding, calibration; desiderata for tuning
Reflection of updated IPFR
Possible use of Gaussian spatial filter
Possible change in locations of spatial masks for dithering
Possible revision of observables
Possible extension of Structure Program

9. Notes, Exceptions

The Flux Budget Continuum Intensity observations of the Structure Program as specified in V 0.8 are also required for this program.

If the GOLF observations commence before SOHO is on station and if it is possible to do so, both the Continuum Intensity and Line Depth proxy observations should commence at the same time.

One or more campaigns may be planned to be run and analyzed prior to the final definition of the GOLF observable and calibration, in which case the initial observing sequence will be provisional.

It may be desirable or necessary at the level 1 calibration to reconstruct the data stream into images and map into another coordinate system, e.g. heliocentric.

Both the spatial and temporal filters discussed here differ from those in the IPFR. In particular the temporal filter is more complicated than the asymmetric filter of the IPFR.

10. Revision History

0.1 - 94.03.24:
Revision history added (10)
Changed baseline temporal filter from triangular to trapezoidal taper (4)
Calibration description reworded (5)

0.2 - 94.06.06:
Changed baseline temporal filter from trapezoidal to truncated gaussian (4)
Changed second observable from average of F2 and F3 to calculated line core intensity, less sensitive to Doppler shift and tuning (1)
Added suggested but unused criteria for throwing out bad or dark pixels from spatial averages (3)
Added suggestion to spatially average numerator and denominator of magnetic proxy separately (9)

0.7 - 94.07.06:
Advanced revision number to nearer 1.0 (10)
Changed observables from core filtergram and magnetic proxy to continuum intensity and line depth (1)
Added explicit tie to Flux Budget program (1, 4, 9)
Changed spatial binning from 10*10 to 8*8 and added requirement of separate buffers to allow for dithering in future revisions (3)
Removed irrelevant comment on averaging of pixels for magnetic proxy (3)
Changed sampling frequency from 20 minutes to 10 minutes (4)
Added section on future revisions (8) and renumbered
Removed superseded notes and exceptions regarding choice of observables (9)

0.8 - 94.07.11:
Changed sampling frequency from 10 min to 12 min, samples from 19 to 23, and Gaussian width from 3.2 min to 3.4 min (4)
Added section on data representation and precision (7) and renumbered
Updated references to Flux Budget specification version (1, 9)

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Page last revised Wednesday, 05-Jan-2000 15:02:30 PST