DevCon: 22 May

Agenda:

Microlens Development (Reni, Tim, Mike)

Spectrograph Discussion (David, KG, Pat)

White Paper (Kyle; due 8 June)

• Science Case document
• Throughput curves?
• Budget priorities:
  • a) Microlens Development; Detailed Throughput optimization
  • b) Spectrograph Design:
    • Blue response
    • NIR arm
  • c) Mechanical Design: Observatory space and placement.
  • d) Focal Plane:
    • Positioners
    • GLAO corrected FOV
  • e) Science Trades: How to best engage?

Notes:

Attending: Kyle, Tim, Mike L., Nick, KG, Maureen

Microlens Development (Reni, Tim, Mike):

• Telecentricity vs. Bershady-Spread:
  • non-telecentricity into microlens is much less than a degree; deviation of pupil is small enough to stay in fiber; mismatch fiber axis and chief ray should only be a few hundredths of a degree; not going to generate any geometric FRD
  • Bershady spread (i.e., deviation of the chief ray from being exactly perpendicular to the fiber face, behind the microlens): good point, adding a second lens (from Reni's e-mail) makes construction more difficult and adds to throughput loss (3 air-to-glass contacts). Useful to compare throughput loss from this effect vs. impact of a second lens, will put in and see how much of a hit
• What about the difference between stability and throughput? Throughput should be stable, but will Bershady spread cause stability issues. Microlens defines how pupil lands on the fiber. Expect that far-field varies but near-field does not. Useful to look into, but not considered a "big ticket" item in terms of the throughput budget.
  • more concerned with larger throughput loss like astigmatism due to overfilling fiber aperture
• KG will look into throughput losses:
  • Use Tim's output to model beam shape.
  • convolve HST with telescope seeing and pass through system
  • how much is lost from the collector vs. how much is collected by adjacent fibers
• Future work:
  • how straight-forward is it to model full end-to-end system to show monochromatic spots on the detector (as a function of field angle)?
    • would need images of fiber scrambling
      • zmax models exist
      • missing element is how to model fiber injection and output
      • Pat's report for DESI is probably good enough
        • can have him invert what was done for DESI to reflect the pupil imaging system of the microlens

Spectrograph Discussion (David, KG, Pat)

• Push extremes of wavelength coverage of the DESI design

• Pat was able to get a sense of how easy it is to go blue/red

• UV transmission: fused silica 99+% transmission down to 0.3 micron, but one element with low transmission below 0.35; ~40% at 0.31; we would need to redesign the camera to replace that lens; also a triplet lens in the system that is filled with an oil that still transmits, but probably would want to use a different oil there. VPHs don't (currently) transmit there (XChange-WFOS uses VPHs and does transmit at 0.31, right?), but fused-silica etched gratings definitely transmit at 0.31 micron. Summary, would want to redesign if want to go to 0.31, but 0.34 would be doable.

• NIR (to ~1.4 micron): Slit head gets in the way for a new dichroic for 4th arm. Pat says 1-2 weeks of work to look into how to repack things for a fourth arm. Back-illumination for metrology causes issues; can we get away with actuator that doesn't require feedback? Summary, possible to redesign, but requires a few weeks of work.

  • Change current NIR arm to go redder? Not full wavelength coverage, but could get a different bandpass. We could get us to 1050. Is that interesting? What do we loose in the gap between the visual and red arms? Resolution chosen to split the OII line (at relevant redshift?).

• Mechanical design of cameras: keep in mind that we could have a grating exchanger to allow for higher resolution in a single order, but no camera articulation

White Paper (Kyle; due 8 June)

• Draft by end of week

• Need help with budget items and costing!

• Science Case document

• Throughput curves?

  • No single "certified" throughput for different instruments. Can have DESI throughput, but risky to compare to existing instruments. Would be good to have top-of-telescope estimate. Mirrors are aluminimum.

• Budget priorities:

  • a) Microlens Development; Detailed Throughput optimization
  • b) Spectrograph Design:
    • Blue response
    • NIR arm
  • c) Mechanical Design: Observatory space and placement.
  • d) Focal Plane:
    • Positioners
    • GLAO corrected FOV
  • e) Science Trades: How to best engage?

• Need to think through GLAO and engage with GLAO team

  • GLAO going through a funded study, not explicitly looking into FOBOS though
  • What would our focal plane look like; how do we want to spread GLAO-ready collectors?
    • Match FOV to JWST?
  • Relates to positioner development