# Presentation: 349354

## The SNAP Instrument SuiteSession 126.04

**Chris Bebek**
**(for Mike Lampton)**
**Lawrence Berkeley National Laboratory**
**9 January 2003**

## Outline

**Outline**

**What drives the instrument implementation concept**
**Requirements**
**Constraints**
**What does the instrument implementation concept look like**
**How is the instrument operated**

## How Science-Driven Requirements map onto Instrument Concept

**Instrument**
- A large  FOV  (0.7 sq. deg. ).
- Observation cadence commensurate with SNe evolution (every 4 days).
- Allocation of time for photometry and follow up spectroscopy (60/40).
- Imager
- Wavelength coverage from 400 nm to 1700 nm.
- Use two plate scales to cover the wavelength range to obtain time efficient photometry.
- 9 filters.
- Required S/N(epoch) versus magnitude achieved with appropriate duration and number of exposures.
- Zodiacal light - limited measurements
- Spectrograph
- Wavelength coverage from 350 nm to 1700 nm.
- S/N = 20
- Resolution ~100 ()

**Measurement Program**
- ~50 Type Ia SNe per 0.03 in z from z=0.3 to 1.7 (2500 total).
- Follow-up spectroscopy near peak luminosity.

## How Science-Driven Requirements map onto Instrument Concept

**Measurement Program**
**Photometry**
- R.F. U, B, V, (R)-band light curves.
- R.F. B-band measurement to 2% at peak.
- K-correction
- R.F. B–V color evolution.
- Malmquist bias.
- Rise time.
- Peak to tail luminosity ratio.
- Spectroscopy
- UV metalicity features – strength and location.
- S and Si features
- SII 5350Å line, w = 200Å
- SII “W” shape, w = 75Å
- SiII 6150Å line, w= 200Å
- Ejecta velocity, 15Å
- Calibration

**Instrument**
**Imager**
- Wavelength coverage from 400 nm to 1700 nm.
- Use two plate scales to cover the wavelength range to obtain time efficient photometry.
- 9 filters.
- Required S/N(epoch) versus magnitude achieved with appropriate duration and number of exposures.
- Zodiacal light - limited measurements
- Spectrograph
- Wavelength coverage from 350 nm to 1700 nm.
- S/N = 20
- Resolution ~100 ()

## Photometry illustration

**Photometry illustration**

## Science-driven requirements on the Instrument Concept

**Measurement Program**
**Photometry**
**Spectroscopy**
- UV metalicity features – strength and location.
- S and Si features
- SII 5350Å line, w = 200Å
- SII “W” shape, w = 75Å
- SiII 6150Å line, w= 200Å
- Ejecta velocity, 15Å
- Host galaxy z.

**Instrument**
**Imager**
**Spectrograph**
- Wavelength coverage from 350 nm to 1700 nm.
- S/N = 20
- Resolution ~100 ()

## Spectroscopy illustration

**Spectroscopy illustration**

**SII “W”**

**SiII**

**Metallicity**

## Space operation impacts on the Instrument Concept

- Reliability
- Avoid moving parts
- No coolers
- No gimbaled solar panels
- No filter wheel
- Allow a shutter
- Avoid multiple focal planes
- Eliminate multiple adjuster sets
- Coalesce visible, NIR, and spectrograph into one focal plane
- Satellite
- Body mounted radiator and solar panels provide a stable platform for long exposures,
- Passive, radiative cooling,
- Folded TMA telescope,
- But, quantizes satellite orientation relative to Sun and hence orientation of the focal plane relative to observation fields.

## Other inputs into the Instrument Concept

- Cosmic rays
- Proton rate is ~4 /s/cm2, after shielding.
- CCD impact is about 1% of pixels are contaminated per 100 s of exposure time.
- Long integrations  need to be broken into a sequence of short exposures (say 300 s for photometry and 1000 s for spectroscopy).
- Dithering
- This is a procedure to increase photometric accuracy in undersampled images.
- Also necessary to average out sub-pixel size response variations.
- Long integrations need to be broken in several exposures with well known spatial offsets.

## Telescope

- Three-mirror Anastigmat:
- Annular field maximizes sky coverage
- Wide flat field available
- All-reflector design, no refractors
- Folded for compactness
- Convenient focal surface location for passive cooling
- Manufactured and operated warm

- SNAP Requirements
- Aperture approx 2.0 meters
- Field of view 1.4 sq degree
- Diffraction limited longward of 1.0 um
- Span wavelengths 0.35 to >1.7 um
- Flat focal surface with > 100um/arcsec
- Stray light << Zodiacal
- Design Features:
- Lightweight mirrors of ULE or Zerodur
- Structure of CFRP with low CTE
- Tripod secondary support structure
- Rigid aft structure for folding mirror, tertiary, and detector support
- MIrrors & structure run at 290K

## Instrument working concept

**Instrument working concept**

- Shutter

- Particle/
- Thermal/
- Light
- shield

- CCDs/
- HgCdTe

- Thermal
-  links

- Spectrograph

- Cables/
- FE elec

- Near
- electronics

- Radiator

- Guiders

- Cold plate

- Filters

## Focal plane - imager

**Focal plane - imager**

- Coalesce all sensors at one focal plane.
- 36 2k x 2k HgCdTe NIR sensors covering 0.9-1.7 μm.
- 36 3.5k x 3.5k CCDs covering 0.4-1.0 μm.
- 4 1k x 1k star guider CCDs.
- Two channel spectrograph on the back with access port on the front.
- Common 140K operating temperature.
- Guide off the focal plane during exposures.

- rin=6.0 mrad;  rout=13.0 mrad
- 	rin=129.120 mm;  rout=283.564 mm

- CCDs
- Guider
- HgCdTe

- Spectr. port

- Spectrograph

## Focal plane - imager

- Fixed filter mosaic on top of the imager sensors.
- 3 NIR bandpass filter types.
- 6 visible bandpass filter types.
- Note the symmetry – a star can be swept l-r, r-l, t-b, or b-t and still be measured in all filters. More on this later.

## Focal plane - spectrograph

- Spectr. port

- Integral field unit based on an imager slicer.
- Input aperture is 3” x 6” – reduces pointing accuracy req.
- Simultaneous SNe and host galaxy spectra.
- Internal beam split to visible and NIR.
- Separate prism disperser and detector for each leg.

- Input port

- Slicer

- Prism
- BK7

- Prism
- CaF2

- NIR
- detector

- Vis
- Detector

- Spectrograph

- Slicer

- Pupil mirrors

## 
Step the focal plane through the observation field.
N steps in each CCD filter; 2N steps in each HgCdTe filter (N will be 4).
Fixed length exposures determined by a shutter (Texp will be 300 s).
The multiple exposures per filter are used
to implement dithering/drizzle;
to eliminate cosmic ray pollution.
NIR filters have twice the area of visible filters; this combined with time dilation will achieve the desired S/N in CCDs and HgCdTe.
All stars see all filters (modulo scan field edge effects).
Fields revisited with fixed cadence. SNe evolution can be followed for 100’s of days.

- Step the focal plane through the observation field.
- N steps in each CCD filter; 2N steps in each HgCdTe filter (N will be 4).
- Fixed length exposures determined by a shutter (Texp will be 300 s).
- The multiple exposures per filter are used
- to implement dithering/drizzle;
- to eliminate cosmic ray pollution.
- NIR filters have twice the area of visible filters; this combined with time dilation will achieve the desired S/N in CCDs and HgCdTe.
- All stars see all filters (modulo scan field edge effects).
- Fields revisited with fixed cadence. SNe evolution can be followed for 100’s of days.

**Obs. Concept - repetitive program**

- Actual dithering would be at the sub or near pixel level.

- Note the longer integrated exposure time in the larger filters.

## Why 2D Symmetric

- Toy satellite demo.
- Solar cells must be kept within 90o of Sun.
- Radiator must be kept pointing to dark space.

## Why 2D Symmetric

## Why 2D Symmetric

- During each 4-day period, the survey filed is scanned.

## Why 2D Symmetric

## Why 2D Symmetric

- Must rotate the satellite 90o relative to survey field every 3 months.

## Why 2D Symmetric

## Why 2D Symmetric

- Note the scan is now along an orthogonal satellite axis to the prior scan.

## Why 2D Symmetric

## Obs. Concept – targeted program

**Obs. Concept – targeted program**

- SNe candidates are scheduled for spectrographic measurement near peak luminosity.
- Light curve and color analysis done on ground to identify Type Ia and roughly determine *z*.
- Note peak luminosity is 14 days to 40 days after discovery for *z *= 0.3 and 1.7 respectively.
- Star is steered into spectrograph port.

## What needs to be done

- First and foremost is the development of detectors
- CCDs - pursuing LBNL technology
- Enhanced radiation tolerance
- Good spatial response commensurate with small pixel size
- Extended QE in the red
- NIR
- Have been relying on WFC3 funding for developing of 1.7 μm material
- Exploring multiple vendors
- Establishing characterization sites within the collaboration

## Imager Sensors Specs

|  | Visible | NIR | Units |
| --- | --- | --- | --- |
| FOV | 0.34 | 0.34 | deg2 |
| Plate scale (nominal) | 0.10 | 0.17 | asec |
| Wavelength | 350-1000 | 900-1700 | nm |
| <Quantum efficiency> | 80 | 60 | % |
| Read noise
(multiple reads) | 4 | 5 | e |
| Dark current | 0.002 | 0.02 | e/s/pixel |
| Filters 
(1+z spaced B-band) | 6 | 3 |  |
| Read time time. | 20 | 20 | s |

## Spectrograph Sensors Specs

|  | Visible | NIR | Units |
| --- | --- | --- | --- |
| Wavelength  coverage | 350-980 | 980-1700 | nm |
| Plate scale | 0.15 | 0.15 |  asec |
| Spatial resolution | 0.15 | 0.15 |  asec |
| Field-of-View | 3 x 6 | 3 x 6 | asec2 |
| Resolution | 100 | 100 |  |
| <Quantum  Efficiency> | 80 | 60 | % |
| Read Noise | 2 | 5 | e- |
| Dark  Current | 0.001 | 0.02 | e-/s/pixel |

- With these specs, total integration time (w/ 1000 s exposures) is