# Presentation: 517478

## SNAP: The Supernova Acceleration Probe

- Mission Design and Core Survey Parameters
- Tim McKay, University of Michigan

- Details at http://snap.lbl.gov

**Notes:**
To obtain the data required for the third generation supernova search Saul describes, the SNAP collaboration has developed a plan for an optical/IR space telescope. While the design of this telescope is closely driven by the requirements for supernova cosmology, the SNAP system parameters closely mirror those of a generic wide field space telescope. This mission will take full advantage of the two great benefits of a space telescope: high resolution imaging of a wide field, and sensitive imaging in the near infrared.

The work I will describe has been done by the entire SNAP team, especially the core design group at LBL. In five minutes I can only sketch the outlines of the SNAP concept. You can read about SNAP in detail at the web site noted here.

## 2m aperture, 3 mirror anastigmat
Diffraction limited in I across the 1 square degree fixed focal plane
Dedicated survey system

- 2m aperture, 3 mirror anastigmat
- Diffraction limited in I across the 1 square degree fixed focal plane
- Dedicated survey system

- The SNAP Telescope

- Optimized to repeatedly scan sky in 9 filters
- 3 year planned life: passive cooling, few moving parts or expendables
- May operate much longer

**Notes:**
Let’s begin with the telescope: it’s a 2m, three mirror anastigmat which is diffraction limited in I band across a one square degree field of view. The telescope feeds a single, fixed focal plane.

The SNAP system is designed to be a dedicated survey telescope. It is optimized to repeatedly scan the sky in 9 filters. The planned mission lifetime is three years, but since it is passively cooled, and has few moving parts or expendables, it is possible it would operate much longer.

## Two SNAP instruments

- CCDs

- HgCdTe

- Fixed filters: 6 optical and 3 near IR

**Notes:**
The SNAP telescope feeds two instruments. The primary instrument, filling most of the focal plane, is the Gigacam imager. The working Gigacam design combines 144 optical CCDs with 36 near-infrared HgCdTe arrays. The design calls for fixed filters, six spanning the optical range from 400 to 1000 nm, and three in the near-infrared from 0.9 to 1.7m.

The second instrument, crucial for the supernova project but less relevant for surveys, is a 2”x2” low resolution integral field spectrograph.

## SNAP survey scanning

- Mostly pure survey mode
- Step across the sky, each region in all nine filters
- Dithered exposures at each location
- Basic SNe search region covered every four days for two years

**Notes:**
The SNAP telescope will spend much of its time in a pure survey mode. To do this it will step across the sky, placing each region of sky on each of the nine filters. Dithered exposures will be obtained at each image location to remove cosmic rays. The basic supernova survey area will be revisited every four days during the two year baseline supernova search.

## SNAP optical detectors

- Fully depleted CCDs developed at LBL
- Very radiation hard, excellent red sensitivity to ~1m
- Now in production and on telescopes

- “Red-hot” LBNL CCDs at NOAO

**Notes:**
The optical part of the imaging array will be paved with the new fully depleted CCDs developed at LBL. These devices are very radiation hard, and have excellent quantum efficiency all the way to the Silicon band gap cutoff at ~1.1m, a huge improvement over traditional CCDs in this important range. 

These CCDs have come a long way in the last few years, are now been produced by an outside vendor, and are in use on telescopes at NOAO, where they are described in the NOAO newletter as “red-hot”.

## SNAP HgCdTe detectors

- Imager: 36 HgCdTe devices doped for a 1.7m cutoff
- Developed by Rockwell for WFC3
- Allows passive cooling: cuts cost, enables long mission

- Key element for SNAP science
- NIR observations much easier from space
- NIR allows:
- SNe to z=1.7
- Galaxies in the window z=1-2.5
- Quasars to z~10
- Cool or dusty objects

**Notes:**
The remainder of the focal plane will be instrumented HgCdTe arrays doped for a 1.7m cutoff. These devices have been developed by Rockwell for the WFC3 project. This cutoff is important because it allows the entire focal plane to be passively cooled, cutting the cost enormously and extending the possible mission lifetime.

The near-infrared imager is a crucial element of SNAP. Observations in the near IR are much easier from space, as Gary will tell you in the following talk. They are required by SNAP to follow supernovae out to z=1.7, but deep imaging in this range will be tremendously important for many reasons, as I expect all the speakers today will emphasize.

## SNAP orbit

- High Earth orbit
- Allows continuous contact with a few ground stations
- Every pixel from every exposure will be downloaded
- Annual eclipse provides very stable PSF
- High orbit allows passive cooling

**Notes:**
Another important feature of SNAP is its high Earth orbit. This orbit provides three major benefits. First, it allows continuous contact with the satellite from just a few ground stations. As a result, every pixel from every exposure can be downloaded raw. 

High Earth orbit also places the telescope in a very stable thermal environment, allowing an extremely stable PSF. Finally, moving the telescope far from the Earth allows passive cooling. 

## SNAP main survey

- Base SNAP survey: 20 square degrees at ecliptic poles
- This region, ~6000x as large as ACS deep field, to mAB=32 in nine optical and IR bands

**Notes:**
The base SNAP survey will cover about 20 square degrees at the ecliptic poles. By the end of base mission this region, 6300 times as large as the planned ACS deep survey, will be imaged to an AB magnitude of 32 in nine filters.

## SNAP main survey

_**Ultra-deep 11 band imaging survey**_
-   Galaxy populations and morphology to co-added *m *= 32
-   Low surface brightness galaxies in H’ band
-   Quasars to redshift 10
-   Epoch of reionization through Gunn-Peterson effect
-   Galaxy evolution studies, merger rate
-   Evolution of stellar populations
-   Ultraluminous infrared galaxies
-   Globular clusters around galaxies
-   Extragalactic stars (in clusters or otherwise)
-   Intracluster objects (globulars, dwarf galaxies, etc.)
-   Lensing projects:
-     Mass selected cluster catalogs
-     Evolution of galaxy-mass correlations
-     Maps of mass in filaments

- Biggest HST deep survey will be ACS survey:
- 	6300x smaller than SNAP *main* survey
- 	About as deep
- Wide field surveys have always complemented powerful narrow-field instruments
- 	1950s+1960s: Palomar 48” feeds 200”
- 	2000: SDSS feeds 8 and 10 meter telescopes
- 	2010: SNAP feeds NGST, CELT, etc.

**Notes:**
I won’t try to list all the things we know we could learn from such a survey. I’ll just comment that wide field surveys have always complemented the most powerful new narrow field instruments. Schmidt telescopes fed the 4m’s. The SDSS is providing targets for the new 8 and 10m telescopes. And by 2010, perhaps SNAP will feed the NGST, CELT, etc.

## SNAP as a survey telescope

- After primary mission:
- Example 1 year surveys

- Gary Bernstein: Michigan

**Notes:**
Once the primary SNAP mission is complete, the SNAP instrument can be used to conduct other surveys. This table shows the rough depths to which SNAP can survey 300, 3000, or 30000 square degrees in about a year. 

## Conclusions

- SNe provide focus for design and operation
- Optimal SNAP is a generic WF space telescope
- Will provide first wide field, high resolution, optical/NIR survey for the community
- Excellent complement to NGST, CELT, etc.

**Notes:**
To conclude; the SNe project provides an excellent focus for the SNAP design, but the requirements of such a mission lead to a pretty generic wide field space telescope. The SNAP telescope can provide the first wide field, high resolution, optical and especially near IR survey to the community. Such a survey, very powerful on its own, will also complement new narrow field instruments like NGST.