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Hi Shaul,
I went through the TeamX instrument and mission reports. I agreed with most of it, but found some problems and some areas that need clarification to avoid mis-interpretation. Starting with the big items, we have:
1) Discussion of I&T is inadequate for a cryogenic instrument. The instrument report estimates $6.1M for I&T at 50% confidence (slides 31 & 32) but gives no detail. Is this a wrap? Is the percentage for I&T in family with previous cryogenic missions? The mission report says $1.9M for observatory-level integration and test (slide 131) and shows only 20 days for observatory thermal vacuum testing. Both are gross under-estimates. 60 days for observatory thermal vacuum testing would be more appropriate. Adding some discussion of chamber mods for sub-K instrument would also be a good idea.
2) The estimated heat loads for the cryogenic stages seem low to me. The instrument report slide 81 says 3.8 mW CBE for the load at 4.5 K. Does this include the heat rejected from colder stages by the ADR? I note that PIXIE has much larger heat leak at 4.5K despite a much smaller cold volume. There's also a discrepancy between the heat loads reported in slides 79 and 81 of the instrument report: Slide 79 shows 14 uW load on the 100 mK stage while slide 81 says 4.8 uW. PIXIE carried 20 uW at 100 mK, again with a much smaller cold volume. Do we have any info on the estimates from LiteBIRD or CORE? It would help to show PICO estimates as in-family with other mission concepts.
3) The instrument report was not clear about how the cryocoolers reject heat. The compressors need to be mounted on the rotating stage, and take a fair amount of power (339W CBE), but I couldn't find any mention about radiators for the cooler. Is the cooler radiator in the outermost V-groove?
4) I don't see the value in including detector design option 2 (mixed TES / MKID focal plane) in the TeamX instrument report. MKIDs are considerably lower TRL and there's no real discussion of the (limited) benefits. If we can show we meet the science requirements within the cost cap using the baseline design, I would leave it at that and not open potential cans of worm for lower-TRL designs.
5) I don't see any clear description of the various observatory modes. In particular, what is safe mode? The instrument report slide 53 discusses using thrusters to slew during safe mode. Why do we want to slew during safe mode? Keeping the spin seems reasonable, but I don't see any justification for slewing…
6) The mission wraps for systems engineering (mission report slide 36, WBS 2) seem a little light, particularly for a cryogenic mission. It's cited at $17.0M, or 4% of the total hardware costs in WBS 5+6. 5% would be more appropriate. Granted, it's only a $2M difference, but why skimp here?
7) There needs to be a better discussion of the contingency for cryo-cooler power. A given cooler design (e.g. a MIRI cooler) has a fixed max power. You can't run one at 110%. The margin and contingency for the cooler are kept on the thermal side: typically want 100% at this early stage. Thus, the cooler will show 100% margin/contingency vs the expected thermal load, but a fixed max power with no added contingency beyond the contingency already built in to account for the thermal load. This typically need to be spelled out to the cost folks. The instrument report slide 83 spells out 100% margin in thermal load PLUS an additional 70% contingency in power. This is too conservative, so perhaps we can use this to cut costs (in the cost model, anyway).
8) PICO plans to field a large cryogenic focal plane with lots of detectors. Can we provide any comparisons with current state of the art? For example, the instrument report slide 67 says there will be 21 detector modules at 0.3 kg/module, for a total of 6.3 kg at 100 mK. What's the heaviest existing focal plane assembly? At what temperature? Similarly, Slide 62 says the detector yield will be above 90%. What is the demonstrated yield for existing focal plane arrays?
9) The Cost rationale for WBS 7 (Mission Ops) in the mission report slide 41 should cite the simplicity of PICO ops: Single instrument with a single science observing mode. Science mode is simple spin/precession with no targets of opportunity or guest investigator program. No part of the sky is more or less interesting than any other.
There are also some relatively minor points:
10) The instrument executive summary (slide 9) should include the CBE (or maybe MEV) thermal loads at the critical 4K and 100 mK stages. It's hard to think about the coolers without knowing the loads.
11) Change “Mirror” to “Reflector”, throughout.
12) The instrument report slide 35 says that the cost driver for the cooler is the minimum required temperature. Nothing is said about total heat lift at minimum temperature. Should we say that the PICO expected heat lift is in family for the various coolers cited?
13) What is meant by the “negligible (1 nW)” power per detector in the instrument report Slide 67? The detector readout is considerably bigger than that …
14) the discussion of cosmic ray hits (instrument report slide 72) is confusing for anyone not already an expert. Need to state that the vast majority of Planck CR hits were to the detector frame, not the detectors per se. The PICO design eliminates such frame hits by heat-sinking frame to bath, as validated by Suzaku, Keck and SPIDER.
15) Three reaction wheels is OK, but provides no margin against single wheel failure. Four wheels is better (robust against single failure). Mission report slide 58 says design is tolerant to single wheel failure. It's not immediately clear to me how this happens.
16) The mission report slide 144 says that the instrument continues to observe during Ka-band downlinks. Is there a risk for science data loss at Ka band or harmonics? Given the 4 hr/day download, this could be significant. Can we provide an estimate of the leakage from the Ka-band transmitter to the Ka-band detectors?
Not all of these need to be addressed via the cumbersome TeamX response/rebuttal process, but we should fix what we can now and keep the rest in mind for the eventual PICO report.
Cheers,
-- Al