# Presentation: 126583

## 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:
-       :      :     :     :           :           :     :    :        :

## Slide 6

## Slide 7

## Slide 8

## Slide 9

## 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

## Slide 12

## 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)

## Refraction shifts focusdeltaZ = Thickness*(1/n2 - 1/n1)For 5mm fused silica, deltaZ=70um between  U and H

## Filter Chroma

## Filter Thickness Tablefor zero chromatic errorassuming substrate is fused silica

## 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