Improved Annular-Field Three-Mirror Anastigmat?

M.Lampton UCB SSL • Previous AFTMAs have pri-sec separation 2.4m

• Can pri-sec separation be reduced?

– Support structure blocks & diffracts incoming light

– Mechanical stiffness, strength, resonances, stability

– Separation << baffle length helps reduce glare

• Overall length should be also be minimized

• Can folding mirror be brought closer to cassegrain quasifocus?

• Can internal clearances be maintained or improved?

• Explored these questions with TMA55

SNAP Optics Requirements• Photometric accuracy and speed: 2 meter aperture

• Discovery rate: one square degree sky coverage

• CCD sampling & pixel size: 10 microns = 0.1 arcsecond– EFL=20 meters, speed=f/10

• SNR: diffraction limited at one micron wavelength– First Airy dark ring = 24.4 microns diameter

• CCD Array Fabrication: flat focal surface

• SNR and operational simplicity: achromatic optics

• Photometry: 15 contiguous bandpasses, 0.4 to 1.7 um

• Vehicle constraints: overall optics length <5 meters

Adjustability Requirements

• TMAs have 13 adjustments– Example: 5 secy + 5 tert + 3 detector– Better: 5 secy + 5 tert + 3 folding flat

• Demands room around secy, tert, flat• Previous TMAs have had negative flat clearance

– consequence: slight internal vignetting– vignetting varies with focus adjustment

• Goal: >10mm clearance at flat

Annular Field• Outer radius: 0.690 degrees

– corresponds to 240 mm at detector

• Inner Radius: 0.400 degrees– corresponds to 139 mm at detector

• Sky coverage 1.00 square degree– corresponds to 1210 cm2 array area

• Possible blockages--– cassegrain spider for folding mirror?

– Other cassegrain complications for auxiliary pickoffs?

– Small inclined focusser CCDs around/within WFArray?

– Small fast guiding CCDs around/within WFArray?

12 surfaces TMA55.OPT annular field, .007 to .012 optim to .014 index X Z pitch Curvature shape Diam diam Mirr?------:--.---:--.--:-----:---.-------:---.-------:-----:----:--------: : 0 : 0.0 : : -0.1944460? 0.0239387? 2.05:0.33:mir pri : : 0 :-2.1 : : -0.8472308? -1.0247897? : :mir sec : : 0 : 0.0 : : : : : :iris : : 0 : 1.5 : : -0.6543174? 0.4436413? : :mir ter : : 0 : 0.53: 45 : 0 : : 0.21: :mirfold : : 0.25 : 0.53: 90 : 0 : : 0.26: : lens : 1.456: 0.255: 0.53: 90 : 0 : : 0.26: : lens : : 0.27 : 0.53: 90 : 0 : : 0.26: : lens : 1.000: 0.275: 0.53: 90 : 0 : : 0.26: : lens : : 0.29 : 0.53: 90 : 0 : : 0.26: : lens : 1.000: 0.295: 0.53: 90 : 0 : : 0.26: : lens : : 0.98 : 0.53: 90 : 0 : : : :CCDarray: : : : : : : : : :

TMA55 Clearances

• Positive clearance around 45deg flat• minimum flat: oval, 198 x 120mm• maximum flat: oval, 212mm x 150mm• allows for adjustment of flat mirror in piston, tilt,

pitch• Flat is 300mm behind cassegrain quasifocus

– reasonably stiff support using cassegrain spider– allows pickoffs of axial rays for guider, IR, etc

Footprint of rays at 45deg folding mirror

TMA55 Optics Details• Primary Mirror

– diameter= 2000 mm; hole= 330mm

– curvature=-0.194446, radius=5.142816m; shape=+0.023939, asphericity=-0.976061

• Secondary Mirror

– diameter= 424mm

– curvature=-0.847231, radius=1.180316m; shape=-1.02479, asphericity=-2.02479

• Tertiary Mirror

– diameter=642mm

– curvature=-0.654317, radius=1.52831m; shape=+0.443641, asphericity=-0.55636

• Folding Flat Mirror

– minimum size: oval, 120mm x 192mm; maximum size: circle, 210mm diameter

• Triple Filter Stack

– minimum diameter: 250mm, nominal thickness 5mm, fused silica

• Annular Detector Array

– inner diameter 278mm, outer diameter 480mm

Refractive index of fused silica (E.Palik, Handbook of Optical Constants of Solids, 1985 p.760)

Band Center,um n(SiO2)U 0.365 1.47453B 0.44 1.4666V 0.55 1.4601R 0.7 1.45515I 0.9 1.45185J 1.25 1.4475H 1.65 1.4427

Refraction shifts focusdeltaZ = Thickness*(1/n2 - 1/n1)

For 5mm fused silica, deltaZ=70um between U and H

Filter Chroma

Filter Thickness Tablefor zero chromatic error

assuming substrate is fused silica

Band Center n T for 5mmU 0.365 1.475 4.972B 0.440 1.467 4.983V 0.550 1.460 4.993R 0.700 1.455 5.000I 0.900 1.452 5.005J 1.250 1.448 5.012H 1.650 1.443 5.019

TMA55 -- Summary• Reduces overall optical length to 3.6m

• Has positive clearances everywhere

• Powered surfaces are parallel and coaxial

• Is 0.39m between primary front-surface edge and forward edge of CCD array enough clearance? – Mirror structure, support, cryostat details

• Is 25cm filter diameter OK?– detector heat load issue

• Overall: TMA55 meets more of our requirements than previous releases

• I recommend we adopt TMA55 as our baseline