Parameters | ||||||||||||||||||
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LOC coordinates [P1_X:P2_X,P1_Y:P2_Y]
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Initial estimates | ||||||||||
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Some remarks
- this sequence is limited by the processing time up to a sequence duration of about 150 mins.
- below that duration, the data rate is more or less independant of the field size, and simply dictated by the compression rate of about 169 blocks/min (the telemetry buffer cannot saturate). The integration time equals the processing time, in free-running mode (as fast as possible). There might thus be slight fluctuations of the exposure, if there are small variations of the compression ratio. However, the tests show a very accurate cadence.
- above that duration, the subfield size should be reduced and the delta T mode should be used to reduce the duty cycle below 100%. The integration time is then larger than the processing time, and entirely controlled by delta T.
- In processing-limited mode, it is useless to increase the image size, as this will decrease the image cadence and leave the block rate largely unchanged (processing limited). It is useless to suppress the compression as the block rate will be multiplied by 2.5 (compression) and 1.3 (lower processing times > higher image cadence), from 59 blocks/min to 200 blocks/min. This would fill the buffer in 10 minutes.
Options | ||||||||||||||||||||||||||||
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195A sequence OPTION 1 (no clears, compression)
Parameters | ||||||||||||||||||
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LOC coordinates [P1_X:P2_X,P1_Y:P2_Y]
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Initial estimates | ||||||||||||||||||||
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Some remarks
- this scenario is identical to the default 304A shutterless scenario and is processing-limited: the telemetry bandwidth is underused (76%)
- Incidentally, at a 2.5 compression ratio, the processing capacity is 166 blocks/min and matches the telemetry capacity (66*2.5=165 blocks/min). However, the ability to progressively accumulate data in the telemetry buffer increases the total allowed telemetry flux, while the CCD readout overhead time reduces the number of image blocks produced per min. Whatever the sequence parameters, the telemetry will thus always be underused. Only when the compression ratio drops below about 2.1, then for long sequences, it becomes possible to fill the telemetry buffer.
- With this option, the exposure duty cycle is 100% (all photons are used)
- As there is no "clear" cycle, the exposure is determined by the cadence which is dictated by the field size (via the processing speed). The 13x13 subfield size (169 blocks) is a firm upper limit to keep the cadence around 60s and below 70s.
- For the 195 passband, the 68s effective exposure is very long compared to the default "synoptic" exposure. However, there is no overexposure with the current degraded response. A factor of 2 recovery (to 40-50% of initial response) would still be just below saturation. An hypothetical factor of 3 recovery (back to 60-70% of initial response) would lead to saturation over about 0.5% of the images.
195A/171A sequence OPTION 2 (no clears, compression)
Parameters | ||||||||||||||||||
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LOC coordinates [P1_X:P2_X,P1_Y:P2_Y]
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Initial sstimates | ||||||||||||||||||||
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Expected Counts: about 3.8x a normal synoptic exposure, with a degraded response at 20-25% of initial response (situation in Dec 2000).
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Some remarks
- derived from Option1, with a reduction of the exposure by a factor of 2, at the expanse of field coverage.
- the use of compression and no clears, allows a 100% duty cycle and a fast 38s cadence (vs 71s without compression).
- As the image rate is 1.86 larger than without compression but as the data flux is reduced at the same time by a factor 2.5, this option is no more telemetry-limited but is thus again underusing the telemetry (1.86/2.5=0.75). No middle solution!
195A/171A sequence OPTION 3 (1 clear, no compression, cadenced, short exposure)
Parameters | ||||||||||||||||||
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LOC coordinates [P1_X:P2_X,P1_Y:P2_Y]
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Initial estimates
Initial estimates | ||||||||||||||||||||
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Expected Counts: about 3.8x a normal synoptic exposure, with a degraded response at 20-25% of initial response (situation in Dec 2000).
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Some remarks
- This mode is derived from the OPTION4. It should be considered mainly for 171A sequences, due to the higher count rates in this bandpass (the exposure time should not exceed 30s to avoid saturation).
- In this case the image cycle (programmed delay) is fixed at 71s.
- The exposure time can be freely reduced below a maximum value of 40s.
- This scenario makes full use of the available telemetry (small margin!)
- On the other hand, the exposure duty cycle is below 60% (max) of the image cycle time. The "dead" time is nevertheless necessary to keep the exposure low enough.
195A sequence OPTION 4 (1 clear, no compression)
Parameters | ||||||||||||||||||
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LOC coordinates [P1_X:P2_X,P1_Y:P2_Y]
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Initial estimates | ||||||||||||||||||||
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Expected Counts: about 3.8x a normal synoptic exposure, with a degraded response at 20-25% of initial response (situation in Dec 2000).
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Some remarks
- Without compression, the cadence is strictly limited by the telemetry, which sets an upper limit to the subfield size: 90 blocks for a 120min sequence and a 70s max cycle time. Therefore, this mode allows on ly smaller subfields than with compression.
- this scenario uses the free-running mode (no fixed image intervals): the cadence is thus dependant on the commanded exposure time, which must be choosen within a narrow range around 40s for this subfield size. This exposure is smaller than in the compressed mode option. This may prove useful in case of saturation. Moreover, the use of one CCD "clear" halves the trailed readout background.
- This scenario makes full use of the available telemetry (small margin!)
- On the other hand, the exposure duty cycle is limited to 60% (max) of the image cycle time (40% of the photons are dumped).