BigBoss Optical Corrector Intro to UCL M. Sholl 16 July 2010

Outline Corrector Design Tolerance Study Contact with SESO Contact with Corning Contact with Schott Liens on corrector design F/8 M2 Fiber verification camera Questions on slides 16 & 17 of CD1 presentation

Telescope Requirements RequirementValueDescription CompatibilityN/AUse existing telescope mount and M1 of Mayall and Blanco 4m telescopes. Include mount for existing f/8 M2. f/#4.53.8m aperture, 17.1m focal length Geometric blur<0.8as FWHM Maximum geometric FWHM (2.35*radius), entire field field (28µm radius) Zenith Angle0-60°Will require atmospheric dispersion corrector (ADC) to meet blur requirements Field of View3°Full field of view Wavelength Range µm Simultaneous correction required across entire band Pointing Jitter<40mas1-sigma, corresponds to 3µm on focal plane Fibers Table X1: Telescope requirements

Layout of Corrector 4

University College London (UCL) Built similar (but smaller) corrector for DES Peter Doel, David Brooks, —Optical Sciences Laboratory, UCL LLF6 (used in ADC design) no longer available —LLF6 used on existing Mayall ADC Use LLF1 instead —Quick check of Zemax prescription reoptimization suggests it will work Schott can do 980mm LLF1 sizes Corning can do fused silica lenses from standard boules —Standard size: 1500mm, 180 & 250mm thick —Need a homogeneity specification. Coatings may be an issue on such large lenses

UCL Suggestions Bottom line: —“Cassegrain option would be significantly more expensive (due to the 2m convex aspheric mirror)” Suggest you look into smaller designs, especially first lens

Stray Light Instrument must be “Well Baffled” —Stars have no direct view of focal plane —Prime Focus No direct view of focal plane Only view is reflection off M1 (signal) —Cassegrain Focus Requires optimally sized M1 and M2 baffle Sweet spot: M1 and M2 baffles’ area fraction equal Source: Hales, 1992 —Prime focus throughput: 72% —Cassegrain throughput: 63% Prime focus design superior in throughput!

DES Corrector Eric Prieto, in conversation with SESO suggested we look further at DES corrector (f/2.8 into fiber) 2.2º FOV (BigBoss is 3º- 3.5º) Band limited by filters ADC: Not present on DES corrector (needed on BigBoss) —Add effects of Kitt Peak atmosphere to quantify degradation to spot sizes SurfaceRadius (mm)Thickness (mm)Glass ccon = ( ) reflect Primary Fused Silica C Air Fused Silica C Air Fused Silica C Air 80-15Fused Silica Filter 90-10Air a4 = -3.15e-10 a6 = -9.48e-16 a8 = -4.83e Fused Silica C Air Fused Silica C5 (Window) (-41.09)Air 1400Focal Plane

Spot Sizes Without Atmosphere

DES Corrector Spot Sizes With Kitt Peak Atmosphere

DES Corrector (cont.) Needs an ADC Focal plane size: Ø0.45m (~Ø1m needed on BigBoss) Should try to investigate design form of DES corrector, but aforementioned items limit direct applicability to BigBoss

ADC on BigBoss Two opposite-rotating Risley prisms Entrance and exit surfaces of prisms are curved, for aberration control Schott Materials —LLF1 (available) —N-PSK3 (will need to melt) ~1° tolerances on prism rotation 12

Spots at 60° from Zenith 13 Risley prism 0° rotation Risley prism 85° rotation

Corrector Tolerance Budget Entire corrector and focal plane moves as a unit (via hexapod). This is system compensator. Tolerances may be accommodated by compensator —Manufacturing errors Vertex offset Figure (measured as surface sag) Wedge (measured as distance at lens endge) Lens thickness Homogeneity (as-built) —Alignment errors Lateral offset Despace Tilt Thermal drift cannot be accommodated in real-time, so it is budgeted as a fixed and “uncomponsatible” error. 14

Tolerance Starting point (preliminary) Goal: atmospheric seeing and as-built geometric blur shall have a FWHH < 100 microns 120 micron fibers —5 micron positioner tolerances —5 micron fiber view camera accuracy —3 micron pointing jitter Peak geometric blur on “perfect” design: 18.3 microns RMS —Multiply by 2.35 to convert to FWHH (Gaussian approximation) —28 micron blur on as-built design, adds to atmosphere (32 microns RMS) to make a 100 micron FWHH blur — =450 “quadrature” points, divided 50 ways —For equal pain, each error source can enlarge the spot from 18.3 to 18.4 microns RMS, after hexapod compensation (where appropriate) 15

Manufacturing Errors (compensated) (preliminary) 16 Vertex lateral error (microns) Thickness error (microns) Wedge (microns at edge) Surface 1 Radius error (microns sag) Surface 1 Conic (%) Surface 1 A4 (%) Surface 1 A6 (%) Surface 2 radius (microns sag) Surface 2 Conic (%) Surface 2 A4 (%) Surface 2 A6 (%) Homogene ity (PPM) C C ADC1_ Flat5 ADC1_ Flat505 ADC2_ Flat5 ADC2_ Flat205 C C Focal Plane 10000

Alignment Errors (Compensated) (preliminary) Lateral Error (microns) Despace Error from previous surface (microns) Tilt Errors (microns at edge) C110Compensator10 C210 7 ADC ADC C C Focal Plane

Stability (no compensation) (preliminary) Lateral Sag (microns) Thermal Despace Error from previous surface (microns) Tilt (microradians) C C21 ADC13 ADC210 C350 C43 Focal Plane1 18 Invar 36 or GRCE Structure

Comment on Tolerances Preceding three slides came from a manual “equal pain” analysis. Have already received pushback on uncompensated errors from Eric Anderssen. More detailed tolerancing underway… Now is the time to push back on these numbers, if they are unreasonable 19

Hexapod Motion requirements —Step size: 5 microns (goal) 10 microns (requirement) —Despace: ±2mm —Lateral : ±1mm —Tilt: : ±1° PI (Germany) contacted ADS-Int. (Italy) to be contacted 20

Corrector Design Details 21

Contact with SESO Spoke with Denis Fappani at SPIE Astronomical Telescopes (June, 2010) —Lens manufacturing generally feasible —Recommend you thick (50% at center) three ADC elements to reduce gravity sag —~2 years to polish, coating extra —Need to provide figure errors 22

Contact with Corning Asked for quote on blanks 23 Item #MaterialDiameterThicknessHomogeneity C mm220mm5ppm or better C mm180mm5ppm or better C mm5ppm or better Please advise as to the availability, cost and lead time for these boules. Homogeneity improves the corrector performance, of course, so please let us know if higher homogeneity levels are possible, or if the requested levels are difficult to achieve. The corrector is required to work from 0.35 to 1.1 microns, so material suitability (low absorption) is required in that band.

Contact with Schott Please advise as to the availability, cost and lead time for these boules. Homogeneity improves the corrector performance, of course, so please let us know if higher H- levels are available in N-LLF1 and N-PSK3. Risk (verbal): Melt required for N-PSK3 24 Item #MaterialDiameterThicknessHomogeneity ADC1_1N-LLF1900mm80mmH2-H3 ADC1_2N-PSK3900mm160mmH2-H3 ADC2_1N-PSK3900mm110mmH2-H3 ADC2_2N-LLF1900mm120mmH2-H3 C3N-PSK31100mm210mmH2-H3

Additional Consideration Should accommodate existing f/8 M2 Existing M2 support appears to be 460mm thick M1-f/8 M2 distance: 7491mm M1-C1 BigBoss distance: 8800mm Removable fiber view camera assembly attaches to same (or similar) interface as f/8 M2 25

Liens on Corrector Design Thicken three ADC surfaces N-PSK3 lens diameter is larger than ADC elements, can it be reduced? Adopt sensible curvature (fewer decimal places) for focal plane (constrain during next redesign) Improve telecentricity (constrain during next redesign) N-PSK3 may be difficult to obtain (per discussion with Schott), and homogeneity may be “best effort.” Different material selection? 26

Fiber View Camera Mounted in front of corrector Back-illuminate fibers View fibertips with camera to verify location 27

28 Lens Mounting (Source: DES, P. Doel CD1 presentation, Slide 16) LBT lens mounting (from Diolaiti et al. SPIE 4841) Baseline athermal elastomeric (RTV rubber) bonding technique Looking at two cell options Invar lens cell + flexures + thin RTV layer (see figure) Steel cell + thick RTV layer

Questions on CD1 Slide 16 What is assembly sequence? Nonuniform gap an issue? What is vertical screw (seems to be touching glass) Are glass/invar gradients an issue? Large invar difficult to procure? What decides thick/thin RTV layers Is gravity sag of lens in RTV an issue? 29

30 Lens to Cell Alignment (Source: DES, P. Doel CD1 presentation, Slide 16) Lens to cell —Lens to cell alignment performed using rotary table and digital dial gauges. Translation Stage Rotary Table D.G.I. Cell Cell Adjustment Screws Lens RTV inserted into gap

Proposal Inputs Any advice on this presentation, and design Help/advice (if possible) with tolerance analysis? —Also homogeneity Drawings or photos of DES Optical mounts? Cost Schedule Risk Test sequence, similar to that of DES? (Brooks presentations) 31