Shaul: Pointing Control is how well spacecraft points at a direction; Pointing Knowledge is how well we know where it points at; Pointing stability is how stable pointing is in terms of changing rate.
Ben: thinking about map to understand definitions of pointing knowledge, control and stability; that's how engineers talk about them.
PICO rquires 6 arcsec pointing knowledge, which about 1/10 of the smallest beam.
Shaul thinks we do not need more than 1 degree of pointing control.
Bill: pointing stability is associated with spin; we may also think about how stable spin axis is aligned with star camera.
Bill: points out having 2 or more scan modes would have been very helpful for Planck. Break time constant and beam shape degeneracies. Shaul: right now we fix on having only 1 scan strategy, but it can be discussed.
Jacques: 3 star sensors garuntees 1 pointing knowledge. This is the easiest and likely to be reasonable in cost. Worth just using 3.
* Jacques: Thoughts from CORE,
* Pointing control is to eliminate gaps in map. 24 ensures no gaps in single detector maps.
24 was technically doable for Core with reaction wheels. Should check if that is true for PICO, if the engineers don't scream, then stop worrying about it.
* Shaul: reaction wheels are current assumption for PICO
* A/I Shaul to check with Amy about 24 control with wheels.
I + T, lessons from Planck, options for PICO. (Tomo)
Plank tested mirrors + baffles, then focal plane and feeds separately
example of parameters that need to be tested in presentation page 5
tests at component and subsystem levels.
Jacques: Primary, secondary at different temperatures. difficult to test this on ground. Alignment may create issues.
Planck cold system test at CSR. Signal response test, but no beam mapping.
Photogrammetry of full structure to measure alignment, and align system. Was costly for Planck, facility needed to be adapated.
10 um interferometry of planck mirrors.
optics needed to be sufficiently polished for this test. may need to 'overbuild' mirrors.
also consider mirror material in cost, planck was CFRP.
focal plane testing at Saturne. criteria tested on page 13.
Comments for PICO:
page 16-17, cost drivers. 3 cyrostats (all large), plus full satellite sized chamber to test full system (cooling, etc.)
page 19, calibrations done for Planck shown.
Shaul: has Planck over tested or under tested any optics?
Bill: most valuable was cold photogrammetry of flight optics. Other tests were of interest, but not as critical.
Mirror material? Bill: Silicon carbide, lower mass solution than aluminum. Jacques: Core proposal used silicon carbide, but now in doubt. carbide is expensive, machining challenge, large thermal contraction. Aluminum is cheaper, easier.
Discussion on hold until next week due to time.