Agenda:

• Optics in ppt (UMN), Optics in pdf (UMN)
• Focal Plane Technologies (Jeff, Roger)

Action Items

Optics

• UMN reviews tradeoff study between open and cross Dragon. For fixed aperture and f# Open Dragone has 3/4 the DLFOV of the crossed system. Largely agrees with the result from the comparison shown a week before when they had different f#.
• How large is the stop in the current Open Dragone design?
  • Not clear. To be checked. Qi checked: It’s ~130 cm.
  • Loading needs to be calculated
• How cold does the stop need to be?
  • It’s easily calculable; to be done by UMN. this is part of the focal plane model that UMN is developing.
  • Different temperatures for the stop and optics box (e.g. 4K, 30K) require different cooling technologies, and they could probably lead to different costs. Thermal modeling and testing are needed to estimate the cost
  • The additional power/cost required to cool the stop and secondary to 4 K may not be significant relative to cooling just the focal plane.
  • Let's calculate the temperature we need, and in parallel develop a cost model.

• Cross Dragone VS Open Dragone
  • Open Dragone has no significant side lobes
  • Version with cold stop improves density of detectors on the focal plane
  • Brad discusses the sidelobe analysis that was done for epic. Analysis was done by Mark Dragovan, but can probably be repeated. For every pixel, rotate the beam with respect to fixed sky coordinates. Assume uniform coverage (within some resolution). Main beam was masked out. Assess the leakage from T to P due to the galaxy. Compare to the target B. Results: just barely good enough for B of 10 years ago.
  • UMN finds that for both cross and open dragon there is a 7 deg gain in alpha for 20 cm reduction in aperture diameter without deployable shields.
  • Need input on deployable shields.
  • What are 'good' scan angles? Kris said with high confidence that he would give a report next week on scanning strategy.

Focal Plane Technology:

Roger and Jeff post their answers to the questions from last week on last week’s minutes page. Below is a copy of the questions and some discussions people had today.

Q1: what is the plan for detector technology for the high frequency bands of the imager? - > 600GHz for Galactic Science; conservative choice is SPT-type polarization-sensitive absorber - Power consumption of read out is quite a large impactor considering the cost - Different read out systems are discussed, e.g. TDM, FDM and ASICS - 400 W or 1.1KW is probably too large - ASICS has a factor of 10 cost saving in power

Q2: are the distribution of colors for pixels as described by the worksheet reasonable? - 2-3 colors per pixel - maybe fit more bands in lower frequency - high frequency depends on f#, pixel size etc.

Q3: are the bandwidths assumed reasonable? - 25% is reasonable

Q4: what should we assume about beam sizes as a function of frequency, specifically for the high frequency bands? Is it reasonable to assume single mode coupling all the way to the highest frequencies? - “Technologies are there.”