1. Feasibility studies for the MOBIE Acquisition, Guiding, and Wavefront Sensing (AGWFS) subsystem
Hu Zhongwen , Chen Yi,et al. (NIAOT)
Zhu Qingfeng, Zhai Chao,et al. (USTC)
May 25, 2011
04/09/2011
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2. Outline Introduction of MOBIE
Top-level requirements of AGWFS subsystem
Project Team & Schedule
Feasibility studies

3. WFOS Requirements vs. MOBIE Design
Requirements/goals Realized in MOBIE concept
Wavelength range: 0.31 –1.0µm – 1.1µm
Field of view: >40.5 arcmin arcmin2 (4.2’ x 9.6’)
Total slit length ≥ 500 576 (9.6 arcmin)
Image quality:
– fwhm ≤ 0.2 (imaging) 0.1µm band < 0.2”
– fwhm ≤ 0.2 (spec) any , no re-focus < 0.2 (preserve resolution)
Spectral resolution:
– 1000 < R < 5000 for 0.75 slit R = ~1000, ~5000, ~8000
– Complete -coverage at R~ complete or select orders
Throughput ≥ 30% (all ) > 40% down to 0.30 µm (est.)
Sensitivity: limited by photon stats for t>300s (high transmission design)
Field acq: <3 min/mask, <1min/single obj. (addressed in CDP)
01/19/2011
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4. MOBIE Design Concept - Optical
Two color channels, each with direct imaging and three spectroscopic modes
TMT FOCAL SURFACE & MOBIE SLIT MASK (~550 x ~1250 mm)
RED FOLD
COLLIMATOR (~1m x ~1.8m)
DICHROIC
RED DETECTOR
BLUE DETECTOR
RED CAMERA
BLUE CAMERA
GRATINGS
X-DISPERSION PRISMS
01/19/2011
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5. MOBIE Design Concept - Mechanical
STRUCTURE
ENCLOSURE
ADC
CABLE WRAP
CARRIAGE
COLLIMATOR
01/19/2011
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6. MOBIE on TMT
01/19/2011
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7. Functionality of MOBIE AGWFS subsystem
Acquisition
Check the telescope pointing
Guiding
Maintain a stable telescope tracking
Nodding
Wavefront sensing
Low-order wavefront error correction
Acquisition & Guiding can be performed by the same camera, they are different only in term of function/mode.
Acquisition mode is only used for checking the telescope pointing. Open-loop
Guiding could be both open-loop or close-loop. In the close-loop, the guiding star should always be in the guiding field.

8. Top-Level Requirements
Acquisition (mainly required for TMT early operations) :
20 arc sec diameter FoV
Guiding:
0.5 Hz closed optical guide loop bandwidth  5 Hz guider frame rate
To meet the overall requirement of 0.05 arc sec (rms) image position uncertainty
Allocation for guiding is 10 mas (rms) for guide error, or PSSN=0.999
Support nodding and offset:
1 [10] arc sec nods along slits in 10 [20] seconds with 80% duty cycle
30 arc sec offsets maximum (in any direction)
Wavefront sensing:
30 s typical exposure time
Degrade telescope point source sensitivity (PSSN) by at most
Unaveraged turbulence contributes a factor of after 300 seconds
Remaining factor of corresponds to 75 nm RMS WFE
Sky coverage (Relaxed, FOV 3 sq arcmin)
95% for galactic pole guide star densities
Must be split into 97.5% for guider and for AO WFS if separate stars are used
Given the design requirements below the limiting star magnitude for the guider to achieve the 50 mas measurement error at 5Hz is mV~=23.6 (TBR). This value is in fairly good agreement with the MOBIE OCDD value of mV=23. The OCDD analysis assumed 10-second integration times with “bright moon” conditions. This analysis used a 5Hz integration time and a background of 20.65mV/arcsec2.
Conversion from image jitter (guide error) to PSSN: PSSN ~= 1 – alpha*sigma^2; while sigma is the RMS jitter in mas and alpha is a coefficient of 9.12e-6.
PSSN describes the photometric efficiency of the system.

9. Project team
NIAOT(Nanjing Institute of Astronomical Optics & Technology) :
Hu Zhongwen, Chen Yi , Wang Jianing ,Dai Songxing , Hou Yonghui ,Tan Zheng
USTC(University of Science and Technology of China): Zhu Qingfeng, Zhai Chao, Liu Zhigan
NAOC(National Astronomical Observatories)
UCSC(University of California, Santa Cruz) ：
Bruce Biglow
2019/4/6
04/06/2011

10. Schedule of the Phase
2019/4/6
04/06/2011

11. Main issue & objective for feasibility study
Explore all potential solutions
Demonstrate potential possibilities
Detect possible problems
Doing first-order trade-offs
Find near optimized solutions instead of going deep into design
So would not “lose the way”
Solutions should be applicable & versatile
Simple & applicable
Leave variations for further optimization

12. FOV complexity Inside FOV of ADC & TMT Outside FOV of MOBIE
Preserve space for GLAO WFS
Avoid collision – best outside slit mask robot region,possible collision with spectrograph camera.
Perform Acquisition,guiding & wavefront sensor function

13. Structure space Complexity

14. Optics complexity Option 1: large optics
Fixed imaging sensor(s) for acquisition and guiding
Patrolling wavefront sensor
Option 1a: two suits of patrolling mechanics
Patrolling imaging sensor for acquisition and guiding
Option 2: large optics
Fixed imaging sensor for acquisition
Patrolling wavefront sensor and separate guider, splitting the light from a single star
Option 3: Need more investigation
Patrolling wavefront sensing provides both wavefront sensing and guiding functions
Additionally, either a Shack-Hartmann wavefront sensor or a curvature wavefront sensor could be considered for the wavefront sensing function

15. Summery of the work done
1. Explore shared FOV concept
Preserve or even make better top-level performance
Relax optics complexity: X large expensive optics
Relax FOV complexity: X crowded FOV
Reduce structure complexity
2. Explore flexible & eliminate improper patrolling schemes
Explore a few optically available schemes to relax mechanical complexity: X possibility of collision
Need further feasibility investigation & iterative work with MOBIE structure considerations especially the ADC and the red camera.
3.Perform a specific optical design
It works from optical point of view
Need further evaluation

16. Pre-assumed concept -- need to explore new possibilities
C-Option1 :
Multi-FOV concept
Very large optics
Crowded FOVs
Large space required

17. Explore Shared FOV concept
Does not lose sky coverage
Make available larger FOV for both guider & wavefront sensor
Other Merits
More FOV vacant
Share part of the optics between guider & wavefront sensor
Share patrolling scheme: might be a defect
Optics small & compact
*** Reduced FOV / Optics/ structure complexity

18. Structure Space near
MOBIE focal plane
150 mm
1850 mm
700 mm

19. Focal plane optics & mechanically available IFOV

20. Shared FOV concept with virtual patrolling schemes
C-Option 2: patrol 2-D together with small IFOV of the guider, optics small. Available FOV is the largest. Pupil distorsion is the least.
C-Option 3: patrol 1-D together + Wavefront sensor patrol 1-D , optics larger. Pupil distorsion >1%. Available FOV limited.

21. C-Option 3: 1-D patrolling of wavefront sensor
Remark: C-Option 3 still could be used if with special petrolling scheme .

22. Explore patrolling schemes -- Patrolling scheme 1
Remarks: A cocentric design is considered before to achieve rotation symetry. It is abandoned to make optics simple.

23. FOV strategy(off-axis distance) --Rotation angle issue
Larger perpendicular range / far off-axis configuration
--Compromise the restrictions of mechanical space available

24. Patrolling scheme 2 Field mirror -> telecentric design
1-D movement of M1 + Refocus of CCD instead of rotating of the detection unit
---for“perpendicular patrolling”
Remarks: 2-D patrolling of M1 is possible (No other movements)

25. Patrolling scheme 3
Rotate mirror-pairs instead of rotating of the detection unit:
---for“perpendicular patrolling”
2019/4/6
04/06/2011

26. Possible image quality (Scheme 1 without refocus)
Colimator : Triplet
Camera: CANNON
Lens array: AOA(only for guider)
Patrolling scheme 1
Box size 0.1”
IFOV: 22”

27. Patrol position near axis (Without Retune-focus)

28. Patrol position near edge of FOV (Without Retune-focus)

29. Realistic Optics : layout

30. Re-designed Colimator
-- All spherical lens surface

31. guider parameters F: 107396 ; F/#: 3.61 IFOV: 21” 0.52mm/1”
Remark: it is know that : FOV 3’ or larger could be achieved by patrolling with small IFOV

32. guider image quality

33. Wavefront sensor parameters
F: ; F/#: ; 0.63mm/”
IFOV: 7.2” ; field stop 3”;; efficiency 42%
Lenslet array: 7x7SUSS , 1015μm ,100mm
Remark: With required 3” IFOV, these design
makes available patrolling +/- 2.1” (field stop move)

34. Wavefront sensor full aperture image quality

35. Better image quality with smaller IFOV (3.6”)

36. Pre-selected CCD

37. Mechanical Concept
2019/4/6
04/06/2011

38. Comments AGWFS is a small complex system
It has interfaces with spectrograph and has feedback to telescope control.
It can help to achieve ultimate performance
It should be well designed and evaluated to meet the top level requirements(There are difficulties: off-axis WFS, Large Science FOV, magnitude & accuracy )
Cost effective
2019/4/6
04/06/2011

39. Thanks ! Thanks : Brent, Bruce, Rick, Rebecca
for Efficient Collaborative work
Jerry, Chuck, Mark, George, Luc,…
for video/ discussion
Mitch
for his previous work on requirements & his previous options idea
TMT office
UCSC
HIA