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The SNAP Instrument Suite Session 126.04. Chris Bebek (for Mike Lampton) Lawrence Berkeley National Laboratory 9 January 2003. Outline. What drives the instrument implementation concept Requirements Constraints What does the instrument implementation concept look like - PowerPoint PPT Presentation
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The SNAP Instrument SuiteThe SNAP Instrument SuiteSession 126.04Session 126.04
Chris Bebek(for Mike Lampton)
Lawrence Berkeley National Laboratory9 January 2003
2
OutlineOutline
• What drives the instrument implementation concept—Requirements
—Constraints
• What does the instrument implementation concept look like
• How is the instrument operated
3
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 ()
5
Photometry illustrationPhotometry illustration
Wavelength
Flu
x
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 ()
7
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
11
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.
14
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
Slicer
PrismBK7 Prism
CaF2
NIRdetector
VisDetector
Spectrograph
Slicer Pupil mirrors
15
• 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.
16
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.
17
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.
23
Why 2D SymmetricWhy 2D Symmetric
24
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.
25
What needs to be doneWhat 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
26
Imager Sensors SpecsImager 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
27
Spectrograph Sensors SpecsSpectrograph 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
hrsz
zt6
7.2
1*8)(