NGAO System Architecture meeting minutes from the 27th Jun. 2007

Present in effigy: @ Keck - Neyman (CN), Wizinowich (PW), @ Caltech - Dekany(RD), Velur(VV), Moore(AM) (joined in near the end of the meeting) @ Livermore: Bauman (BB), UCSC - Gavel (DG)

Original Meeting agenda as per RD's plan:

Review and adopt subsystem candidate rankings
Address questions raised during initial evaluations

Meeting minutes:

1. The Phone number to call must be posted on the meeting page.

2. Optical relay design document is still to be written by BB.

3. CN and RD have been on top of what each other have been up to.

4. Sub-system pages have not been updated much since the last meeting.

5. UCSC meeting between AM, BB and DG:
AM, BB, and DG have made progress regarding pick off mechanisms and have gotten further along starting from AM's ideas presented as powerpoint slides in the previous meeting. AM presented a candidate arch. and talked about pick-off options. 

BB: Using Kick-bots is good for the TT sensors (LOWFS) and the d-IFU. Kick-bots are simple, stable and easy to implement. AM likes the idea because the kick-bot mechanism can be same pick off scheme for both the d-IFU(D-NIRI) and the LOWFS.

RD: Is a faceted pyramid better than a kickbot, what about pick off mirrors on translation stages with some throw on them? Implementation scheme and complexity are strongly dependent on OSM. Should the options be ranked based on the architecture and state specific advantages. The Kick bot makes things more generic. MCAO relay fed object selection mech. were not considered, and it seems like more emphasis was laid on the systems with 1" DM on voice coil type mounts and corresponding relays.

BB: The Indian Wells design gives some idea of packaging. It takes a big(ger) DM to get arcminutes of FoV as one ends up with pupil aberrations otherwise.

RD: Based on sky coverage calculations for different science/observing scenarios, J and H must be corrected to 10% Strehl levels or 375 nm (lambda/4) for the total WFE. tiny IR quad-cells that are positionable and faceted pyramids are still options for TT sensors. Both AM and RD think that there is no viable way of doing TT sensing without moving something. IR TTWFS with 4 Rayleigh GS is good enough for the D-NIRI case as the WFE is much more relaxed for the 120" science field. 

RD, VV and DG: One single IR detector will need to have to many pixels. But a patroling IR WFS (a 1RG or PICNIC are options).

Chris' candidate LOWFS must add in (in addition to- Quadcells, pyramid) the patrolling H1RG (solves the acq. problem) reading in quad-cell mode. it works for the TT, but not TTFAs.

Chris sent out a sys. arch mod (small one with costs) to twiki.

Chris didn't do detailed component costing and didn't account the OSM.

Action item :  Chris to check the ranking of the sub-system candidate architectures and add more notes.

6. Sub-system arch. criteria must meet requirements and must be cheap to operate - will be dealt with in detail on the 5th of July at a UCSC meeting. Attendees will be RD, VV, AM, DG, BB (and DLM??) in person and CN, PW by phone.

DG: DLM thinks we must think of the sub-system arch and the observing scenarios together and make sure all the considerations are taken care of. This may be the criteria for selection of OSMs (perhaps).

Anna preferred the whole system in front of her to see how her subsystem interacts with others subsystems, then she can rank the OSM mechanisms.

7. Not all sub-system architectures rankings are documented as excel sheets as of yet

CN, DG, RD: The sub-system arch xls sheet was iterated (only 3 or 4 subsystems were dealt) with. Others have to do their bit and bring all the subsystems together as a group hopefully all the subsystems evaluation is done properly. The evaluations have to revised based on the whole system. When do we start making candidate arch.

Two schools of thought: RD - need more homework on the subsystem arch .... DG/AM - we are do decide on the system arch as a whole and come up with candidate arch.

Action Item  - Brian, Don and Anna need to do their spreadsheets for sub-system arch.

8. LGS WFS case from Chris' excel sheet.

RD, DG, VV: We need to understand how radial CCDs work (w.r.t. the mechanism of charge storage, shifting and reading) to rank it better w.r.t. a regular CCD. They are definitely laser projection and pupil geometry specific. If lasers are centrally projected then they can be mass-produced from a single mask. Radial CCDs preclude from pyramid WFS and using multiple GS on the same detector. We need to read @ 1.5-2KHz on LGS WFS. A science grade detector for multiple beacons is not cheap. Uplink correction, laser clean up, open/closed loop operations etc. also affect things. LGS and NGS plate scales may be a little different for having different spot sizes. 

why is a standard CCDs and radial CCDs cost the same - not real costing was done. Sean is only building a quadrant. Once a mask is made then making 1 or 10 is only the cost of the foundry run, which is $250K. 128x128 can be bought of the shelf. If the detector is bigger we need development cost. radial CCD has a largest pixel scale of 4x6 pixels/subap. may be we can get 256x256 off-the shelf.

CN: Are we to add a 250K cost for development and 250K+ for the 1st unit? 

Action Item: RD will comment on Chris' costs

9. Option of using just Rayleigh guide stars:

RD's Intro: many beacons give a larger d0. In case of the d-IFU where diffraction limit is not important, does the residua tomographic error from a multiple Rayleigh laser system good enough to obtain productive science. A small tetrad of 5" radius based on D-IFU/d-IFU basis and detect on the same WFS would be the option to consider over 5".  

D-NIRI won't have pupil sampling of 64 across, it may need only 32 subaps The D-NIRI error budget is SO loose. The aim here would be to get are looking at 50% Strehls instead of 70%. The difference in cost comes from the fact that we need 150W of Na laser power to do the wide field case and could probably get away with 50W of power for most narrow field cases. The tipping point occurs when we need 100W (instead of 50W) for some science case. The budget allocated $4 Million/ 50W na-laser and $2.6M for all of the needed Rayleigh lasers (frequency tripled nd:YAGs).

Downside: Range gating will be needed for all WFSs and about 40 lasers are required on the whole. Detecting multiple beacons on the same WFS is an option if the # of subaps are lesser. Complexity from both of these needs to be accounted for in costing. 

Action Item: RD will do error budget, RF will do tomo, VV will do costing and complexity to compare a all Rayleigh GS system and a 150W Na LGS system.

Action item : RD will draft agenda for next Thurs. meeting.

Next week's meeting will be at UCSC (not Fremont, CA).

PW: Rankings can be made based on architecture if thats useful or the notes elaborated to convey the same message.