NGAO Alignment, Calibration, and Diagnostics mini-review

Questions and comments from Jim Lyke

KAON 719 Alignment Plan

Sec. 2.4 Alignment Requirements

• Focal plane alignment (x,y), 4th bullet: Does DAVINCI team know about instrument position requirement to 0.2 mm or a possible alignable coronographic field stop and has it been designed?
• Pupil plane alignment (x,y), 1st bullet: Does DAVINCI team know about this 75 micron requirement?
• Tilt (theta x, theta y), 2nd bullet: Would these off-axis aberations add to the WF error budget? Is there a rough calculation?
• General question: Have requirements for xyz stage for the kinematic instrument mounts been fleshed out? If not, please consider discussing with DAVINCI team. The kinematic mount designs will be produced during NGAO's detailed design phase.
• LGS WFS to AO bench
  • Focal plane alignment (x,y), 2nd bullet: Is this in the f/15 beam? If so, <0.2 arcseconds is <0.14 mm, which seems tight. What is the range of the fast TT mirrors? If 5", seems this could be relaxed by a factor of 2 without losing much dynamic range and still be smaller than median seeing. From the perspective of the LGSF this requirement is not challenging according to Thomas and does not drive the design (yes it is in an ~f/15 beam). The current fast tip-tilt mirror design has a range of 2" radius (although the requirement is only ~1"). We could loosen up this requirement if it becomes an issue.
  • Focal plane alignment (z): 2 mm is not critical if and only if the LGS WFS focus stage is not near a limit or end of travel. Agreed, a note has been added to the KAON "assuming the LGS WFS has at least +/-2 mm of extra range (otherwise this requirement will need to be tightened)".
• Calibration Unit Thomas says that all of these features are provided in the calibration unit design he will present at this mini-review.
  • Implies an on-axis point source at NGS focus, are there 3 off-axis point sources that can be fed into the LOWFS? Will these be available to use simultaneously with the LGS sources to simulate observing? This is a vital testing mode.
  • In addition to focus range to accomodate an ADC, phase retrieval (image sharpening) requires +/- 4 mm of defocus at f/15, psuedo pupil requires +/- 10 mm defocus @f/15
  • Is there a Na layer at 150 km? Can the 80 km conjugate track to 90 km?

Sec. 3.7 LOWFS to AO Relay Alignment

• Does aligning the LOWFS via the TWFS align all three LOWFS or just the one with the TWFS? Yes, given my assumption that the LOWFS have previously been well aligned with respect to the mounting plate. We will need to determine an alternate procedure if this is not the case.

Sec 3.10 DAVINCI Science Instrument to AO Bench Alignment

• Do you expect DAVINCI to have xyz adjustments for its kinematic mounts? Yes I do for the initial install. If all three kinematic mounts or their bases have these adjustments then we can adjust all 6 degrees of freedom.
• Why is there no pupil imaging mode for DAVINCI? It has proved very useful for K2AO. Need to check this with Sean but the basic premise is that this adds cost & complexity to the instrument. It would be good if you or others could provide the operational benefits of such a mode so that we can weigh the advantages against the cost.

Sec 3.11 AO Bench Warm Enclosure Testing

• What is the mechanism to align both science instrument detectors and the coronographic focal plane mask? The subsystem internal alignments are not discussed in this KAON. This is a question that will be relayed to Sean as the instrument lead.

Sec 4.2 Summit Alignment Preparations

• Who is responsible for deconstructing/decommissioning K2AO and NIRC2? The NGAO project.
• What is the proposed schedule? The proposed schedule will be in the development phase schedule (a draft version is posted but this is incomplete).
• What is the design load of K2 LNAS? How does it compare to the likely weight of the new AO enclosure, bench, and DAVINCI? We do not have these numbers yet. Producing the mass budget has been deferred to the detailed design. I don't expect the overall weight to be much more than for the current system.
• Are the design mods to K2 LNAS envisioned to be major? No. Some modest additional structure will need to be added to the existing steel platform structure to provide the new instrument defining point pads.

KAON 739 Calibration Plan

Overall Comments

• The NGAO team has the expertise required to calibrate an AO system as they have real world experience with other AO systems. The only concern is how developed are the open loop or "go-to" AO systems in terms of real world performance? I have no first hand knowledge of such systems so I cannot comment. The VILLAGES experiment was a 'go-to' system on a real telescope at Lick, it worked on real NGS, I'll send you the reference

Sec 2. Background

• I understand that the DD phase will explore "go-to" AO in more detail. If this proves unworkable, is there a way to retrofit closed loop AO? The SDR AO optical relay had a flat mirror at ~10Km that could have been retrofitted with a DM, to the best of my knowledge we've removed it in the current AO optical design. So I think NGAO is all-in with go-to AO.

Sec 3. Calibration Tasks

3.11 Acquisition Camera Pointing Origin

• Minor point but rotation center is defined as the optical axis and is one of many "pointing origins" for the AO system. It is also the origin of the focal plane coordinate system. I'll fix this oversight, and just call it optical axis.

Sec 4 Calibration Solutions and Methods

4.1 Radiometric calibration of instrument and AO detectors

• Just a word of caution that in astronomy, "radiometric" calibration is the treated as the exact response of a system to a given input of photons. The calibration unit as describe provides a flat field and arc lamps and not a radiometric calibration. I'll change this to flat field

4.2 Astrometric calibration of AO system and instrument distortion

• NIRC2 grid is inside the dewar, this will be outside the dewar. May need adjustments to verify the grid is perpendicular to the optical axis. I think this would be a issue in either case.

4.5 Calibrate LGS WFS response for 'go-to' operation

• LGS on sky is about 1.5-3.0 arcseconds depending on seeing. Will LGS calibration source also subtend a range of angular sizes? I believe that the calibration source could hold larger LGS sources if needed, also NGAO will use a center launch so the LGS should be smaller than we currently have on Keck II.

Calibration Unit Opto-Mechanical Design

Overall Comments

• Looks well thought out.
• Ability to provide NGS and LGS input sources will help calibrations and checkout without sky time.

Specific Comments

Sec 4.4 Flat field/spectral lamps

• First paragraph is cut-off mid sentence. What is the restriction on the integrating sphere?

Sec 6.1 Motion devices

• Obviously tight clearance near the elevation bearing. How would a technician access the AO rotator fold if it was stuck in beam?

-- PeterWizinowich - 09 Jun 2006