The SNAP Instrument SuiteThe SNAP Instrument SuiteSession 126.04Session 126.04

Chris Bebek(for Mike Lampton)

Lawrence Berkeley National Laboratory9 January 2003

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OutlineOutline• What drives the instrument implementation concept

—Requirements—Constraints

• What does the instrument implementation concept look like

• How is the instrument operated

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How Science-Driven Requirements map How Science-Driven Requirements map onto Instrument Conceptonto 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.

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How Science-Driven Requirements map How Science-Driven Requirements map onto Instrument Conceptonto 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 ()

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Photometry illustrationPhotometry illustration

Wavelength

Flux

U B V R

Color:• K correction• Photo z• Classification

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Science-driven requirements on the Science-driven requirements on the Instrument ConceptInstrument 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 ()

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Spectroscopy illustrationSpectroscopy illustration

SII “W”

SiII

Metallicity

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Space operation impacts on the Space operation impacts on the Instrument ConceptInstrument 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.

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Other inputs into the Other inputs into the Instrument ConceptInstrument 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.

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TelescopeTelescopeThree-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

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Instrument working conceptInstrument working concept

Shutter

Particle/Thermal/

Light shield

CCDs/HgCdTe

Thermal links

Spectrograph

Cables/FE elec

Nearelectronics

Radiator

Guiders

Cold plate

Filters

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Focal plane - imagerFocal 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 mradrin=129.120 mm; rout=283.564 mm

CCDs

Guider

HgCdTe

Spectr. port

Spectrograph

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Focal plane - imagerFocal 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.

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Focal plane - spectrographFocal 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

SlicerPrismBK7 Prism

CaF2

NIRdetector

VisDetector

Spectrograph

Slicer Pupil mirrors

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• 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 programObs. Concept - repetitive program

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

Note the longer integrated exposure time in the larger filters.

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Why 2D SymmetricWhy 2D Symmetric

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

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Why 2D SymmetricWhy 2D Symmetric

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Why 2D SymmetricWhy 2D Symmetric

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

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Why 2D SymmetricWhy 2D Symmetric

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Why 2D SymmetricWhy 2D Symmetric

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

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Why 2D SymmetricWhy 2D Symmetric

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Why 2D SymmetricWhy 2D Symmetric

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

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Why 2D SymmetricWhy 2D Symmetric

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Obs. Concept – targeted programObs. 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.

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What needs to be doneWhat needs to be doneFirst 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

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Imager Sensors SpecsImager Sensors Specs

Visible NIR Units

FOV 0.34 0.34 deg2

Plate scale (nominal) 0.10 0.17 asecWavelength 350-1000 900-1700 nm<Quantum efficiency> 80 60 %Read noise(multiple reads)

4 5 e

Dark current 0.002 0.02 e/s/pixelFilters (1+z spaced B-band)

6 3

Read time time. 20 20 s

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Spectrograph Sensors SpecsSpectrograph Sensors Specs

Visible NIR Units

Wavelength coverage 350-980 980-1700 nmPlate scale 0.15 0.15 asecSpatial resolution 0.15 0.15 asecField-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

hrszzt6

7.21*8)(