11/16/2007	NGAO Opto-mechanical design meeting 
In person: Don Gavel (DG), Chris Lockwood(CL), Rene Kupke (RK), Brian Bauman (BB)
On the phone/ video at Keck: Peter Wizinowich (PW), Sean Adkins (SA), Chris Neyman (CN)
On the phone/video at Caltech: RD

Note Action items are indicated in italics.
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CL & RK on Optical design and mechanical packaging

RK - We have a NEW optical relay design (180" field). This relay has a bigger K mirror to accomodate the larger field.

BB: Packaging forces us to have a two tier relay. The K-mirror and/ or OAP2 (and perhaps a bunch of other things) will end up interfering. If K mirror is moved ahead of focus then you can have a 1 tier system. 

RK : This is a 5 mm pitch DM and Rene wanted a 7mm pitch mirror - Everything becomes big but it helped the D-NIRI focal plane is nicer. The focal length of the first relay was larger ... and demag. to the 25.4 mm pupil so the package ended up being 3 x 3 m. The BB/ RK 1st relay is 2.3 m long.

VV - Why is the laser pick off first? Easier coatings is it?

AM - Can d-NIRI be fed up-looking? 

PW: Why can't we package things on 1 tier? RK - More field means packaging gets harder. 

AM (Shows the Zemax model to show the packaging constraints better), BB - I couldn't get the dichroics out of the way (120", 5 mm actuator).

PW - Gut feeling is that current Keck AO system like design may be able to accommodate the field and deal with dichroics, but this needs to be looked at carefully.

CL - Can't comment how hard packaging is until some some real structural work is done.

DG - The change is FoV (and atmosphere) is sudden and we have to have consensus before we start changing design using the new parameters.

SA - The second tier needs some real-meat and its not clear if this is easy. PW - the advantage of buying an optical bench is that we buy a lot of engineering with it (and save on FEA type analysis).

DG - 5mm pitch DMs (low hysteresis and high stroke) are available from CILAS? Do we care about hysteresis, as the DM is run in closed loop?
CN - Both 5 mm and 7 mm is available from Xinetics and at least one of the 2 from CILAS. CILAS' stroke specs may be better(, but need higher (400V) voltage to drive it).
PW - Sounds like its hard to make a decision about 1 tier and 2 tier. This needs more work. 
BB - What requirment is making us to go to a 1 tier system.

SA - do some work one the single level system. Pure technical (mechanical and access requirements) may determine whether a 1 tier 7 mm or a 2 level 5 mm is better?

DG - Field stability, pointing and tracking are the real issues. We can carry two designs forward, but that will make the deisgn process longer and more costly.

CL - I don't think split level design is as hard and it may well work. FEA isn't daunting and can be done.

Action Item - CL continues on the 2 level concept and RK can look at a 1 level design.

RD  explaining KAON changes and FoV requirements:

A new atmospheric model (Ridge model) was accepted in July(?). We used the historical data for the ground layer while the ridge data was used for the upper atmosphere. It was earlier decided by the NGAO team that during the worst 1/4th conditions we wouldn't operate NGAO in LGS mode. LGS system was to be designed for the top 3/4 part of the time. But, astronomers want the AO system specified for median conditions rather than for the best 3/4th condition. If we adopt median conditions  (r_0 = 16 cm not from 18 cm) based on the hybrid Ridge and historical data. This makes drives the laser power up!

Action Item: Claire must clarify what conditions need be specified for the atmospheric model. IIs a atmospheric model specified at median conditions using ridge data for upper atmosphere and historical data for the ground layer  acceptable?

RD - Direct financial implication is laser power. Re-optimizing other terms, most notably # of sub-apertures was done to keep the laser power within financial bounds for the best possible Strehls. Baseline will change from 90W out of the telescope to 150W for the new conditions if this compromise in # of sub-apertures was made and the TT stars have lesser Strehl.  Re-optimizing suggests that the science FoV must be larger than 120 but between 150 and 180, there isn't much to be gained at 180". The optimal field of view for meeting sky coverage requirements is 150".

DG - The optical designers will love this for BB's original design will work without opening up the angles etc.

RD - Tomography performance needs more simulations.

DG - The 174" radius LGS asterism is optimal for covering the cylinder for the TT stars. May be 150" and 174" have the same performance.

RD - If there is more tomography error in this configuration as the TT stars aren't as well sharpened, and so the Science image gets affected due to second order effects (like slower update rate).

PW - Allow 10% more space on DMs for foreshortening effects. K-mirrors are to be sized at 90% of the natural guide star field (at Zenith),  vignetting laser footprint may not be bad. 

DG - You never want to clip laser light.  Scattering issues can be catastrophic, a big LGS dichroic may be able to deal with it.

AM on OSM concepts and volume constraints:

DG - ADCs working over a single band are useable :(. Whole IR band may be possible.

VV - can't we not get 9 ADCs (1" size) on each d-NIRI channel?

PW/ AM - dispersion is approx same as spot size and d-NIRI works only one band at a time (as per science requirement).

Action Item: PW will look at dispersion #s for 1 band and calculate the dispersion at 60 deg.

RD: In the analytical error budget model, I assume an ADC design that reduces atmospheric dispersion by 20 times. It's the same for all channels in the current error budget sheet, in version 2.0 we can have separate #s if we deem that necessary.

PW checks dispersion #s: Across J band at 30 deg. - 50 mas and @ 60 deg. it is 150 mas.

DG/ BB : We need to cover the entire IR wavelength range and reduce the dispersion 20X. Can this be done?

BB: It takes a 2 glass ADC to correct all of IR at one time. Its a throughput issue, nothing else? For the GPI design, the trombone design can work with 100 mm separations. Coatings, cemented surfaces etc. induce more losses. 1/2 - 1 % per surface perhaps. 4 open air surfaces  (1/2%) and 2 cemented surface (2%) - total loss is 4%. 

Action Item : BB will try to design a J, H, K design (tweak GPI design for NGAO) to see if we can achieve 5 mas residual dispersion or better at 60 deg. with 4% loss (this loss needs to be fed into the EBS)

RD - The wavefront sensors see 21% total throughput which includes (5% is the atm. extinction). 3% loss for each flat and 4% loss for each transmission is assumed, this includes a window. Cleanliness of the meat locker may need to be tightened.

Action Item: What's the realistic throughput (BB can do it as he did it for GPI). Transmission vs. Clean room class? Will send a reference his GPI study by Nov. 26.)

Action Item: Fix the surface ordering issue as compared to KAON 499 (VV).

BB GPI  studies suggest that over 4 years in a class 1000 clean room, a 0.3% loss in a upward looking surface is typical. The loss is between 3X-10X smaller for a vertical optic. The are numbers assuming no maintenance. 

Action Item: Whether all of the d-NIRI channels will work at the same band at a given time?  (CM)

BB - the GPI ADC is designed for 0.92 - 2.4 um and has a residual dispersion of 10 mas at 50 deg. Zenith.  This is about a factor of 15 (not 20). The ADC uses  ~100 mm of motion to achieve this performance.

VV - We need it to have this performance (for TT) over just J and H bands

RK - The distance between the dichroic fold to the focal plane is  500 mm.

AM - Whatever the OSM mechanism, the vol. is going to be kind of the same.

OSM and d-NIRI volume requirements 

AM showed a first order Zemax model with one type of OSM. The design had 200 mm distance from DM to lens The distance from the articulated to the moving mirror is 400 mm. Independent of the method the size is roughly the same she points out.

DG - FoV (size) vs. how close can they be? What about the differential refraction (we don't worry about it with ADCs).

AM - 10" or more apart is the science requirement as I remember it. 3"x1" FoV pick offs @ 70 mas samples.
 
DG - Looks like OSM will fit into the space for all configurations, but can we fit the whole d-NIRI channel?

AM - 400 um is as small as a single slicer  can be made (Eikenberry concurs with this #). I use a f/120 beam to the slicer focal plane. The distance from the f/120 focus to image slicer is about 1.2 m long! Flexibility on sharing cameras is always there. We need additional things like cold stops.

OSM focal plane is 400 mm across in dia.  and the articulated mirror that folds the beam into the d-NIRI channel from the focal plane has to move ~150 mm.

Action Item: AM will make drawings for the OSM depicting all these details.

Preliminary volume estimates for CL:
d-NIRI - 1.0 m deep by 1 m dia.
OSM + d-NIRI - 1.5 m tall, dia. = 300 mm may be the ME packaging limit.

Action Item: Chris will talk to jim and decide on who assumes the role of "the agglomerator" position for the ME packaging.

AM - Other instruments are missing on the TSIP proposal and OSIRIS doesn't meet the science requirements for NGAO.

DG - We also need acquisition pick offs.

Action Item: Check pupil issue shifts (BB we know what the effect is). VV will talk to BB and PW  and come up with a solution and write it up.

DG - We have decided a 32x32 DM for all (TTFA and d-NIRI) channels.

PW : Keck AO is not telecentric, BB that may make a difference. If the pupil is a finite distance away, then moving the focus is a problem. NGAO is ~telecentric.

Acquisition pick off (CN)

Points to notes on http://www.oir.caltech.edu/twiki_oir/bin/view.cgi/Keck/NGAO/AcquisitionDesign
Three possible acqusition camera pick off positions are indicated as Acq. 1, Acq. 2, Acq. 3 on the document.
For point and shoot - we need LGS acq. as well.
Acq. 3 was prefered - don't exactly remember the reasons as to why.

DG - What's wrong with acq. 2?
VV - we have a TT procedure w/o cam. It has been elaborated on the WFS volume requirements document.
DG -OSIRIS currently has no acq. 
DG - We want a IR acq. camera option is good. 
PW - ADC and acq. don't have to be in the d-NIRI beam path at the same time.
CN/DG - mirror + 3 dichroics
CN - We need a mechanism that will change 400 mm diameter dichroics.

DG/ VV: TWFS issue where do we need it? Is having one in the NF relay enough?

RD: 23 mv star in 30" FoV (Spagna model). 50% probability  is Mv= 22.

PW - Sci. folks want a PSF spectrum (could use one of the d-NIRI channels). Mirror is the lowest possible background.

DG - We need 2 TWFS.

PW : TWFS can share light with a TT sensor.

DG : one on f/46 beam (23 mv stars) visible WFS

PW: Keeping image elongation and keeping the image motion small isn't the same thing.

DG - Bots have to actively track. Nods and dithers mean bots have to "jump" and track at precision. Fixed pupil and fixed image are still open issues.

PW - Dither stuff is written as requirements already. 

Action Item: Do we need a pupil fixed mode ? EC to ask science team, and if so upto what zenith angle?

PW/ RD/ DG: We still have to worry about digital offset at the TT trackers. If there are 2 ADCs for the NF and WF paths will cause image blur. ADC will shift the image, but image motion is known.  Will need to quantify.

DG - We must try to keep everything fixed during a astrometric observation. Things have to move during a dither! We need to have bots moving fast and precisely (mas scale).

PW: We must not blur the spot or drag the spot.

RD: There is a 0.25 mas/ min allocation for these effects in the error budget spreadsheet. So we must hold the TT sensors to this requirement.

DG : For e.g. requirement can be something like "A mechanism or a algorithm that maintains 0.25 mas of open loop TT measurement".

DG: How long an exposure can we deal with (pupil fixed and refraction affect this)?

Action Item: What do we do with the 2 ADCs in the NF and WF arms? (to BB) -- must flow down into TT(FA) TT mirror dynamic range and ADC encoders (seems plausible). - should be written down - discussion suggests taht its of the order of an arcsec.

Action Item: What's the effect of moving 2" on the TT sensor (VV) - How far can you move before it effects the WFE.

Action Item: Write the specs or and look at feasible mechanisms for 20"  dithers (AM)

Action ITem : LGS acq. mechanisms (CN/ VV)

Action item: IR cam, where does it go?