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CHLAT Sidelobes and observing elevation

03/10/2021

P. Gallardo

CMB-S4 Director’s Review - July 14th-16th 2020

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Observation strategy sims and minimum observing elevation

Measurement requirement was set of observing ~68% of the sky

For Neff sky area matters more than sensitivity

Using the baseline high cadence survey strategy with modulated azimuthal velocity, this appears to require observing at 35 deg elevation (see flowdown slides).

Here we highlight some of the concerns about this and why ACT chose to only observe at >40 deg elevation

We need to address whether smaller sky areas of fsky ~ 63% are sufficient

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CMB-S4 Director’s Review - July 14th-16th 2020

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Hardware limits for rate and acceleration (Reijo)

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Wider throw translates to more fsky, higher maximum scan rate and higher acceleration. E.g.:

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Base rate : 0.5 deg/s

Throw : 130 deg

Elevation : 40 deg

fsky : 0.64

Max az rate : 1.2 deg/s

Max az. accel : 3.0 deg/s^2

throw

CMB-S4 Director’s Review - July 14th-16th 2020

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Maximum acceleration limits the scan

Examples for base rate of 0.5 deg/s on sky, turnaround at maximum acceleration, 5deg FOV:

Elevation [deg]

Throw [deg]

Max rate (mount) [deg/s]

Max acceleration (mount) [ deg/s^2]

Stable scan [s]

Turnaround [s]

Fsky

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35

122

1.26

2.86

164

0.88

0.679

40

120

1.00

2.95

152

0.88

0.653

45

116

1.33

2.85

137

0.94

0.576

50

112

1.39

2.87

122

0.97

0.516

CMB-S4 Director’s Review - July 14th-16th 2020

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Observation strategy efficiency, sun avoidance and minimum observing elevation

Observation efficiency studied as a function of elevation and sun avoidance.

Efficiency decreases, but still remains fairly high for moderate sun avoidances of ~30 deg and 45 deg

Uniform depth areas at 40 deg elevation ~26,000 deg or fsky ~ 65% but still might fall a bit short of current measurement requirement

Update: Joel says current Neff forecasts based on 40 deg elevation

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el = 30

el = 40

el = 45

Plot by Haruki Ebina

el = 50

CMB-S4 Director’s Review - July 14th-16th 2020

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Observation strategy elevation and NET

NET rises sharply at lower elevations

Dropping from 40 deg to 35 deg causes ~5% increase in NET

=> ~10% decrease in mapping speed

=> Other science goals are degraded by mapping speed decrease

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NET normalized to 55 deg elevation

CMB-S4 Director’s Review - July 14th-16th 2020

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ACT sidelobes and pickup

Pickup:

  • ACT observes strong pickup that appears to be from the ACT ground screen between 30-35 degrees elevation
  • Pickup drops off quickly between 35-40 deg elevation
  • These two points drove the selection of observing elevations in the 40-48 degree range.
  • These points were considered in the ACT observing strategy

CMB-S4 Director’s Review - July 14th-16th 2020

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Sidelobes

Sun

Zemax prediction

https://phy-act1.princeton.edu/~snaess/actpol/sidelobes/20200612/

  • Past experience in ACT-SO: use ray tracing to get sidelobe hot spots
  • Diffraction also can be seen radially
  • Keep sun out of sidelobe regions

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CMB-S4 Director’s Review - July 14th-16th 2020

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CD sidelobes

Calculations assume reflective surfaces (should some be blackened or scattering?)

Defining the sun keepout region is needed to define observation strategies.

In orange: 30-45 degree circles

Strongest sidelobes within ~10 deg circle

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Plot from Gudmundson et al 21 10.1364/AO.411533

CMB-S4 Director’s Review - July 14th-16th 2020

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LAT Sidelobe and strategy next steps

Work with forecasting team to assess whether measurement requirement can be reduced to fsky ~ 62% or 65% to allow 40 deg elevation observations

Continue studying sidelobes and NETs to assess whether some parts of telescope structure should be treated differently (e.g. blackened or made to scatter)

Work towards finalizing both ground keepout (minimum elevation) and sun keepout regions.

CMB-S4 Director’s Review - July 14th-16th 2020