# Presentation: 210363

## SNAP Telescope

**SNAP Telescope**

**http://snap.lbl.gov**
**http://xxx.lanl.gov/pdf/astro-ph/0405232**

## SNAP Telescope

**M. Sholl****9****, M. Lampton****9****, G.Aldering****1****, W. Althouse****2****, R. Amanullah****3****, J. Annis****4****, P. Astier****5****, C. Baltay****6****, E. Barrelet****5****, S. Basa****7****, C. Bebek****1****, L. Bergström****3****, G. Bernstein****8****, M. Bester****9****, R. Besuner, B. Bigelow****10****, R. Blandford****2****, R. Bohlin****11****, A. Bonissent****12****, C. Bower****13****, M. Brown****10****, M. Campbell****10****, W. Carithers****1****, E. Commins****9****, W. Craig****2****, C. Day****1****, F. DeJongh****4****, S. Deustua****14****, T. Diehl****4****, S. Dodelson****4****, A. Ealet****7****, R. Ellis****15****, W. Emmet****6****, D. Fouchez****12****, J. Frieman****4****, A. Fruchter****11****, D. Gerdes****10****, L. Gladney****8****, G. Goldhaber****9****, A. Goobar****3****, D. Groom****1****, H. Heetderks****9****, M. Hoff****1****, S. Holland****1****, M. Huffer****2****, L. Hui****4****, D. Huterer****16****, B. Jain****8****, P. Jelinsky****9****, A. Karcher****1****, S. Kahn****2****,** **S. Kent****4****, A. Kim****1****, W. Kolbe****1****, B. Krieger****1****, M. Krim, G. Kushner****1****, N. Kuznetsova****1****, R. Lafever****1****, J. Lamoureux****1****, O. Le Fèvre****7****, M. Levi****1****, P. Limon****4****, H. Lin****4****, E. Linder****1****, S. Loken****1****, W. Lorenzon****10****, R. Malina****7****, J. Marriner****4****, P. Marshall****2****, R. Massey****17****, A. Mazure****7****, T. McKay****10****, S. McKee****10****, R. Miquel****1****, N. Morgan****6****, E. Mörtsell****3****, N. Mostek****13****, S. Mufson****13****, J. Musser****13****, P. Nugent****1****, H. Oluşeyi****1****, R. Pain****5****, N. Palaio****1****, D. Pankow****9****, J. Peoples****4****, S. Perlmutter****1****, E. Prieto****7****, D. Rabinowitz****6****, A. Refregier****18****, J. Rhodes****15****, N. Roe****1****, D. Rusin****8****, V. Scarpine****4****, M. Schubnell****10****, G. Smadja****19****, R. M. Smith****15****, G. Smoot****9****, J. Snyder****6****, A. Spadafora****1****, A. Stebbins****4****, C. Stoughton****4****, A. Szymkowiak****6****, G. Tarlé****10****, K. Taylor****15****, A. Tilquin****12****, A. Tomasch****10****, D. Tucker****4****, D. Vincent****5****, H. von der Lippe****1****, J-P. Walder****1****, G. Wang****1****, W. Wester****4**
**  ****1****Lawrence Berkeley National Laboratory****	****11****Space Telescope Science Institute**
**  ****2****Stanford Linear Accelerator Center****	****	****12****CPPM, CNRS-IN2P3, Marseille, France**
**  ****3****University of Stockholm****	****	****13****Indiana University**
**  ****4****Fermi National Accelerator Laboratory****	****14****American Astronomical Society**
**  ****5****LPNHE, CNRS-IN2P3, Paris, France****	****	****15****California Institute of Technology**
**  ****6****Yale University****	****	****	****16****Case Western Reserve University**
**  ****7****LAM, CNRS-INSU, Marseille, France****	****	****17****Cambridge University**
**  ****8****University of Pennsylvania****	****	****18****CEA, Saclay, France**
**  ****9****University of California at Berkeley****	****	****19****IPNL, CNRS-IN2P3, Villeurbanne, France**
**  ****10****University of Michigan**

## SNAP Mission Goals

**Supernova Acceleration Probe (SNAP)**
**Proposed mission to study dark energy and acceleration of the universe**
**Goals**
**16(x2) month supernova program (Type Ia supernova (SNe) Hubble diagram) 7.5 square degrees, North and South ecliptic poles**
**1-year, 300-1000 square degree Wide-field weak gravitational lensing survey**
**Optional 3-year 7000-10,000 square degree panoramic survey**

**Scientific requirements**
**0.35-1.7****μm band**
**20mas pointing control**
**Measure SNe 4 magnitudes fainter than 26 magnitude peak, out to z=1.7**
**S/N= 30:1 at peak brightness**
**Stray light <10% foreground Zodiacal light level at dark ecliptic poles**
**Stable PSF (<1%) for weak lensing shear measurements**
**FOV –vs- critical sampling ******** Dithering strategy and focal length currently being optimized via simulation**

## SNAP Reference Mission

**Spacecraft**
**L2 orbit**
**20mas pointing control ******** focal plane guiders (1/5 ccd pixel)**
**Ka band antenna (2.35Mbps, 3.4dB link margin)**
**Telescope**
**Three Mirror Anastigmat (Korsch, Lampton)**
**2m aperture, f/11 (130mas imaging at 1****μm)**
**Telescope design accommodates EFL in 15-30m range (currently 21.66)**
**1.4 square degree optical FOV (0.7 square degree instrumented)**
**0.9 Strehl at 1****μ****m**
**Room temperature optics (test and fly warm)**
**Low CTE structure (carbon fiber)**
**Instruments**
**36 LBNL CCDs (4kx4k, 10.5****μ****m pixels, 0.35-1****μ****m band)**
**36 HgCdTe (2kx2k, 18****μ****m pixels, 0.9-1.7****μ****m band)**
**3”x6” integral field unit (IFU) spectrograph (Ealet, et. al.)**
**Significant engineering work has gone into the design of the SNAP observatory**

## Observatory configuration

## Integration & Test

**Integration & Test**

**The SNAP observatory was designed to be simple to test, calibrate, and verify**
**Interface ring between payload and spacecraft (spacecraft bolts on)**
**Direct insertion and removal of science package from telescope without telescope deintegration**
**Semi-kinematic mount for focal plane assembly and spectrograph**
**Comprehensive test plan developed**

## Focal Plane

**140K operating temperature**
**Four fine guidance trackers**
**36 CCDs (6.5 arcmin)**
**36 HgCdTe detectors (6.5 arcmin)**
**Six filter panels for photo-Z measurement**

**Spectrograph light passes through fixed entrance port on focal plane**
**Built in test equipment (BITE) illuminates mirrors on secondary spider for stability check.**

**C. Bebek**

## Optical DesignMike Lampton

**Korsch AF-TMA configuration**
**Prolate ellipsoid concave primary mirror (PM)**
**Hyperbolic convex secondary mirror (SM)**
**Flat folding mirror (FM) with central hole**
**Prolate ellipsoid concave tertiary mirror**
**1****μm = 10mas = 1/10 pixel (ccd)**
**Telephoto advantage=1/7**
**Side-mounted detector (place near radiator for passive cooling)**
**Huge diffraction-limited field (****Ω****fov****/****Ω****diffraction blur****=3x10****9****)**
**Near-Hubble angular resolution at visible**

## Wavefront Error Budget

**Flowdown from science requirements**
**WFE tracked in OPD in the pupil plane**
**Incoherence between aberrations assumed**
**Error categories**
**Figure: manufacturing errors**
**Mount: mirror distortion caused by clamping glass**
**Positioning: residual alignment errors remaining after secondary correction**
**Stability**
**Thermal**
**Longterm (dryout, creep)**
**1-G-release**
**Effects of 1-G testing not eliminated by offloaders**
**Metrology errors: budgeted individually**
**Marechal approximation used to cast pupil-plane errors on focal plane (or vice versa)**
**All elements of this optical system fall well within existing state of the art**

## Mechanical Tolerances

**Initial alignment to surveying levels**
**Secondary mirror used for final alignment**
**Interferometer (terrestrial)**
**Starfield (on orbit)**
**Allowable mechanical tolerances are determined as follows:**
**Generate WFE budget line items corresponding to misalignment**
**Misalign/SM correct until corrected WFE approaches budget allocation**
**Initial mechanical misalignment becomes tolerance**
**Tolerances are well within capability of surveying equipment**
**Leica ****TM5100A theodolite (****≥3 units)****: 10****μ****m linear, 2.5****μ****rad angular**
**Leica Laser tracker: 25****μ****m linear (10ppm), ****μ****rad angular**

## Reference design mirrorsR. Besuner

|  | Primary | Secondary | Folding Flat | Tertiary |
| --- | --- | --- | --- | --- |
| Material | ULETM | ULETM | SiC | ULETM |
| Outside diameter (mm) | 2050 | 500 | 720 x 480  | 730 |
| Inside diameter (mm) | 686 | N/A | 190 x 120  | N/A |
| Radius of curvature of optical surface (mm) | 4908 | 1099 | N/A | 1406 |
| Shape of front (optic) | Concave | Convex | Flat | Concave |
| Shape of back | Convex | Flat | Flat, beveled | Flat |
| Thickness (mm) | 200, uniform | 100 at center | 100, uniform | 100 at edge |
| Construction | Closed-back | Closed-back | Open-back | Closed-back |
| Cell shape | Honeycomb or triangular | Honeycomb or triangular | Triangular | Honeycomb or triangular |
| Faceplate bonding | Frit or fusion | Frit or fusion | N/A | Frit or fusion |
| Mass of mirror (kg) | 205 | 8.9 | 13.2 | 18.3 |
| Bipod supports* | 3 ‘normal’ | 3 ‘inverted’ | 3 ‘inverted’ | 3 ‘normal’ |
| First natural frequency on mounts (Hz) | 111 | 85 | 218 | 123 |
| Minimum mirror material safety factor | 16 | 40 | 23 | 22 |
| Minimum bipod material safety factor | 2.7 | 8 | 14 | 4 |
| *’Normal’ bipods converge at the mirror; ‘inverted’ bipods converge at telescope mount end. |  |  |  |  |

## 1-G testing

**1-G testing**

**Full end to end test planned (avoid surprises)**
**Vertical testing preferred**
**Due to 1-g sag, testing to full Strehl requirements is not practical**
**Reduced figure requirements during 1-g test**
**96 Offloaders on PM**
**Prefer to not offload SM, FM, TM**

| Mirror | Figure error (nm RMS) | Figure error, 1-g testing (nm RMS) |
| --- | --- | --- |
| Primary (PM) | 10 | 10 |
| Secondary (SM) | 7.5 | 9 |
| Fold (FM) | 6 | 10 |
| Tertiary (TM) | 7.5 | 10 |

- PM sag without offloaders: 770nm RMS

- PM sag with 96 offloaders: 8.8nm RMS

## SiC fold mirror

**Optical design of TMA requires tight packaging between FM and passive cold stop**
**Because of central hole, FM experiences excessive 1-g sag when constructed from ULE or Zerodur**
**Silicon carbide (SiC), in an open-back configuration meets 1-g test figure requirements**
**Closed back ULE designs meet gravity sag requirements for SM and TM, without offloaders**

## Thermal effects on mirrors

**Bulk temperature changes considered**
**Radius of curvature of powered mirrors changes with temperature**
**Separate budgets developed for bulk temperature changes which may be corrected by motion of the SM, and instantaneous control requirements for the mirrors**

|  | Corning ULETM | Schott ZerodurTM glass | Silicon Carbide |
| --- | --- | --- | --- |
| Coefficient of thermal expansion (CTE) (10-6/K) | 0.010-0.015 (various grades) | 0.02-0.05 
 (various grades) | 2.4 |
| Conductivity (k) (W/m-K) | 1.31 | 1.46 | 156 |
| CTE/k (10-9 m/W) | 7.6-11.5 | 13.6-34.2 | 15.4 |

**CTE/k ratios for ULE, Zerodur and SiC suggest comparable gradient performance**

## Passive Cold Stop

**Reverse raytrace shown**
**Passive Cold Stop (PCS) extended to waist**
**Pixels see PCS mainly (see Table)**
**PCS at 140K**
**Zodi ~0.3 counts/s**

**P. Jelinsky**

## Flat-field lamps

**Desired:**
**Dome flat (diffuse light source) for interpixel variation quantification**
**Source should be in pupil plane (no features imaged on focal plane)**
**Source should be removed from view during observations**
**Constraints:**
**Passive Cold Stop (PCS) extends to internal pupil, and is at 140K (cold mechanisms are to be avoided)**
**Fold mirror blocks access from front of internal pupil**
**Solution:**
**Illuminate ring of diffuse material within PCS (in pupil plane, external to pupil)**
**S. Mufson (Indiana) characterizing sources**
**Angular input histogram, integrated through multilayer filters show minimal throughput variation between starlight and ring of light**

## Cassegrain Baffle & Shutter

**Cassegrain Baffle & Shutter**

**Cassegrain stray light baffle effectively shadows inactive areas of focal plane.**
**Shutter moved from internal pupil to Cassegrain focus**
**Allows PCS to extend without interruption to internal iris**
**Baffle is warm, but not visible to detector pixels**

**Redundant limited angle torque motor drives**
**Magnetic resolvers and feedback loop in design**
**Shutters open/close in <100ms**

## Stray light

**L2 orbit removes bright Earth (main stray light source)**
**Developing stray light budget**
**ASAP model of observatory developed**
**Visible and Critical surfaces & PST analysis**
**Mirror roughness **
**Mirror particulate contamination**
**Ghosting (filters)**
**Baffle BRDF**

## Conclusions

**The SNAP mission is designed to study the dark energy and alternate explanations for the accelerating universe**
**Significant engineering effort has gone into design of the observatory in areas of:**
**Ease of alignment, calibration and test**
**Strength, dynamics, thermal and optomechanical design**
**Stray light analysis**
**Comprehensive test plan for the SNAP telescope has been developed**
**The SNAP observatory design is:**
**Mature**
**low risk**
**straightforward to calibrate**
**The SNAP observatory can be built using today’s technology**