# Presentation: 431280

## SNAP OTA Baseline TMA62

- M.Lampton
- Jan 2002
- UC Berkeley Space Sciences Lab

## SNAP Mission Plan

- Preselect ~20 study fields, both NEP and SEP
- Discoveries & photometric light curves from repeated deep images
- huge multiplex advantage with “batch” observations, 1E9 pixels
- Spectroscopy near maximum light from followup pointings

- Deep Surveys:

- Followup spectroscopy:

- N

- S

- N

- S

- ~4 day period

## SNAP

**Simple Observatory consists of :**
**1) 3 mirror telescope w/ separable kinematic mount**
**2) Baffled Sun Shade w/ body mounted solar panel and instrument radiator on opposing side**
**3) Instrument Suite**
**4) Spacecraft bus supporting telemetry (multiple antennae), propulsion, instrument electronics, etc**
**No moving parts (ex. filter wheels, shutters), rigid simple structure.  **

## Payload Layout*transverse rear axis*shortest length

## Annular Field Three Mirror Anastigmat

- Aperture: 2 meters
- Field of view: > 1 square degree
- 1.37 square degrees in TMA62
- Diffraction limited longward of one micron
- 2 microns RMS, 15microns FWZ geometric
- Flat field
- Folded to obtain short overall length
- 3.3 meters in TMA62

## Wide-Field Telescope: History

- Wide-field high-resolution telescopes are NOT new
- Schmidt cameras (1930 to present)
- Field-widened cassegrains, Gascoigne (1977-); SDSS
- Paul three-mirror telescopes (1935) and Baker-Paul
- Cook three-mirror anastigmats (1979)
- Williams  TMA variants (1979)
- Korsch family of TMAs (1972...)
- Angel-Woolf-Epps three-mirror design (1982)
- McGraw three-mirror system (1982)
- Willstrop “Mersenne Schmidt” family (1984)
- Dark Matter Telescope (1996+)
- New Planetary Telescope (1998)
- IKONOS Earth resources satellite (1999)
- FAME astrometric TMA
- Multispectral Thematic Imager (1999)

## Three-mirror anastigmat (TMA)

- Identified as best choice for SNAP
- Can deliver the required FOV
- Can deliver the required resolution
- Inherently achromatic, no correctors needed
- Inherently flat field
- Inherently elastic: 9 d.o.f. to meet 4 Seidel conditions plus focus & focal length
- Can meet packaging requirements

## Telescope: Downselection

- 1999-2001:  Suitability Assessments
- sought 1 sq deg with diffraction limited imaging (< 0.1 arcsec)
- low obscuration is highly desirable
- off-axis designs attractive but unpackagable; rejected
- four, five, and six-mirror variants explored; rejected
- eccentric pupil designs explored; rejected
- annular field TMA concept rediscovered & developed
- TMA43 (f/10):  satisfactory  performance but lacked margins for adjustment; lateral axis between tertiary & detector
- TMA55 (f/10): improved performance, margins positive, common axes for pri, sec, tertiary.
- TMA56 (f/10) like TMA55 but stretched
- TMA59 (f/15): same but with longer focal length
- TMA62 (f/10.5) lateral axis between tertiary & detector

## Baseline Telescope

- Baseline Optical  System: Annular Field TMA62
- prolate ellipsoid concave primary mirror
- hyperbolic convex secondary mirror
- flat annular folding mirror
- prolate ellipsoid concave tertiary mirror
- flat focal plane
- provides side-mounted detector location for best detector cooling
- EFL = 21.66m matches 10.5 micron SiCCD pixel to 0.1 arcsec angular scale
- plate scale is 105 microns per arcsecond
- delivers  annular field 1.37 sqdeg
- average geometrical blur 2.5umRMS  =  6umFWHM; 16um worst case FWZ
- compare: SiCCD pixel = 10.5 um; HgCdTE pixel 18.5um
- angular geometrical blur 0.023arcsecRMS =0.06arcsecFWHM
- compare: Airy disk, 1um wavelength:  FWHM=0.12arcsec=13um

## Slide 10

## Annular Field Dimensions

- Outer radius: 0.745 degrees
- corresponds to 283.56 mm at detector
- Inner Radius: 0.344 degrees
- corresponds to  129.1 mm at detector
- Sky coverage 1.37 square degree
- corresponds to 1957 cm2 detector area
- Field Blockages-- none
- Can go to larger radii but image quality degrades rapidly
- Can go to smaller radii but vignetting becomes severe

## TMA62 Optics Prescription

- Primary Mirror (concave prolate ellipsoid) located at origin:
- diameter= 2000 mm; hole= 450mm
- curvature= -0.2037586, radius=4.907768m;    shape=+0.0188309, asphericity= -0.981169
- Secondary Mirror (convex hyperboloid) located at Z=-2.000 meters:
- diameter= 450mm
- curvature= -0.9103479, radius=1.0984811m;    shape= -0.8471096, asphericity= -1.8471096
- Folding flat mirror located on axis, Z=+0.91 meters:
- oval, 700mm x 500mm; central hole 190 x 120mm
- Tertiary Mirror (concave prolate ellipsoid) located at Z=+0.91, X= -0.87meters:
- diameter=680mm
- curvature= -0.7116752,   radius=1.405135m;     shape=+0.40203,  asphericity= -0.59797
- Filter/Window located along beam toward detector
- nominal thickness 5mm, fused silica
- Annular Detector Array located at Z=+0.91, X=+0.90 meters:
- inner diameter 129mm, outer diameter 283.6mm

## TMA62 Prescription -- BEAM FOUR format

- 8 surfaces     TMA62.OPT  f/10.83, optim 6 to 14mrad,  use 6 to 13mrad
-  index   X          Z   pitch  Curvature   shape      Diam   diam Mirr?
- ------:--.-------:--.--:-----:---.-------:---.-------:------:----:----------:
-       :  0       : 0.0 :     : -0.2037586:  0.0188309: 2.01 :    :mir pri   :
-       :  0       :-2.0 :     : -0.9103479: -0.8471096:      :    :mir sec   :
-       :  0       : 0.1 :     :           :           :      :    :iris      :
-       :  0       : 0.91: 45  :           :           :      :    :mir fold  :
-       :-0.87     : 0.91:-90  : -0.7116752:  0.4020288:      :    :mir tert  :
-       : 0.25     : 0.91: 90  :    0      :           : 0.3  :    :lensFilter:
-  1.456: 0.255    : 0.91: 90  :    0      :           : 0.3  :    :lensFilter:
-       : 0.9      : 0.91: 90  :    0      :           : 0.65 :    :CCDarray  :
-       :          :     :     :           :           :      :    :          :
-       :          :     :     :           :           :      :    :          :
-       :          :     :     :           :           :      :    :          :
-       :      EFL=21.66meters :           :           :      :    :          :
-       :          :     :     :           :           :      :    :          :
-       :          :     :     :           :           :      :    :          :

## TMA62 spot diagrams

## TMA55 Vignetting?

## Ray Trace ResultsFive radii: +X, +XY, +Y, -XY, -XTransmission vs off-axis angle,milliradians

## TMA62 Vignetting and Image quality issues

- Nominal annulus 6 to 13mrad
- no vignetting, but little or no tolerance
- 2 um rms average image blur over this field
- At 5mrad: approx 50% of rays are lost at edge of hole in 45deg flat mirror
- At 14mrad: vignetting losses depend critically on element sizing; geometrical blur about 40um FWZ.

## TMA56 sensitivity coefficients-secondary mirror-

- TMA56 sensitivity coefficients-secondary mirror-

## TMA56 sensitivity coefficients-fold mirror & detector-

- TMA56 sensitivity coefficients-fold mirror & detector-

## Glare & Stray Light Sources

- Ecliptic Poles places Sun 70 to 110deg off axis
- sunshade design “straightforward”
- Earth, moon can be up to 15 deg off axis
- needs  careful baffle study, now in work
- Stars, Zodiacal dust, diffuse Galactic light
- concerns are optics scatter, dirt, structure
- Stray light specification: must be small compared to natural NIR foreground
- Thermal emission from optics must also be small

## Baffle treatment: outer tube, secondary cone, inner tube

## Stray Light Baffle Concept

## Diffuse NIR foreground

## Mirror emissivity

## Optical Mirror Technologies

- Open-back weight relieved Zerodur or silica
- offers 75% to 80% LW
- potentially quicker procurement cycle
- Ultralight core+face+back: 90-95%LW
- typically use Corning ULE
- requires ion milling
- requires in-chamber metrology
- SiC technologies
- evolving; under study

## Materialshttp://www.minerals.sk.ca/atm_design  and other sources

## Primary Mirror Substrate

- Key requirements and issues
- Dimensional stability over time
- Dimensional stability in thermal gradient
- High specific stiffness (1g sag, acoustic response)
- Stresses during launch
- Design of supports
- Prefer < 100kg/m2
- Variety of materials & technologies

- Initial design for primary mirror substrate: 334 kg

## Primary Mirror Substrate

- Stresses from pseudo-static launch loads
- 6.5g axial, 0.5g transverse
- 3-point supports
- Baseline
- Face sheets (12 mm)
- Locally thickened web walls (10 mm)
- Thicker outer ring (8 mm)
- Mass (330 kg)
- Fundamental mode 360 Hz
- Conclusions
- 80% lightweighted design is workable
- 3 pt support *may* be usable for launch
- Vertical axis airbag support required for figuring

- Deformations of mirror top face under pseudo-static launch loads: peak deflection = 20 m

- Design with locally thicker web plates
- Standard web thickness = 5 mm (orange)
- Thickened plates = 10 mm (red)

## Primary Mirror Substrate

- Free-free modes
- Sag during 1g figuring
- Sag is too large (>0.1m) on simple supports (3 pt vertical, strap horizontal)
- Will likely require vertical axis figuring on airbag supports

- 1g sag on 3pt support
- vertical axis
- P-P Z deflection = 2.3 m

- 1g sag in 180o strap support
- horizontal axis
- P-P Z deflection = 0.5 m

- 1g front face ripple on perfect back-side support
- P-P Z deflection = 0.018 m

## Secondary Metering Structure

- Key requirements:
- Minimize obscuration (<3.5%) & interference spikes
- Dimensional stability
- 35 Hz minimum fundamental frequency
- Baseline design: hexapod truss with fixed end
- Simple design with low obscuration (3.5%)
- 6-spiked diffraction pattern
- Ø 23 mm by 1 mm wall tubular composite (250 GPa material) struts with invar end-fittings.

## Secondary Metering Structure

## Tertiary Metering Structure

- Key requirements:
- Dimensional stability
- 35 Hz minimum fundamental frequency
- Easier design problem than secondary metering structure
- Overall dimensions much smaller than secondary metering truss
- No obscuration concerns
- Use strut design from secondary metering structure (cost effective)

## Telescope: Focussing

- 13  mechanical  adjustments  is  minimum set
- focussing
- collimation
- centering
- alignment
- on orbit, may only need secondary  to be articulated
- Least squares optimization for focussing and collimation
- Alternatives: Zernike defocus analysis

## GIGACAM1 billion pixel detector

- 132 large format silicon CCDs
- 25 2Kx2K HgCdTe NIR detectors
- Larger than SDSS array
- Smaller than BABAR silicon vertex detector
- Outside diameter 480mm
- Each chip has dedicated bandpass filter
- Located within 150K cryostat
- Accommodates guiding and spectroscopy feeds