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New calibration methods
N. Regnault
LPNHE
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Outline
● Why calibration ?
● Calibration requirements
○ For a 4th generation survey (e.g. LSST)
○ For a third generation survey (e.g. Subaru)
● Primary flux references
○ CALSPEC & NIST
● Survey flux metrology chain
○ Survey uniformity, GAIA
● Characterization of the survey telescope
○ Bench & in-situ throughput measurements
● Conclusion
2
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Outline
● Why calibration ?
● Calibration requirements
○ For a third generation survey (e.g. Subaru)
○ For a 4th generation survey (e.g. LSST)
● Primary flux references
○ CALSPEC & NIST
● Survey flux metrology chain
○ Survey uniformity, GAIA
● Characterization of the survey telescope
○ Bench & in-situ throughput measurements
● Conclusion
3
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Why calibration ?
● Cosmological signal from comparison of
○ Nearby SNe (g,r)
○ Distant SNe (i,z)
● SNe measured at different locations of the sky
○ Far away from each other
○ Far away from primary flux references
○ In different observing conditions (IQ, atm. transparency)
● Indirect measurement
○ Need instrument model to interpret fluxes
○ Need SN model + instrument model to infer rest frame fluxes
4
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● Distances to low-z SNe rely on
blue bands (g,r)
● Distances to high-z SNe rely on
red band (i,z)
● Critical calibration ingredients:
○ relative (band-to-band) flux
calibration
○ positions of filter
cut-on/cut-off
● Calibration errors affect
○ SN magnitudes
○ SN empirical model
○ -> SN distances (x2)
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● Relative flux calibration
○ Primary flux reference
○ Flux metrology chain
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Survey uniformity
7
WFD
DDF
CALSPEC starsCALSPECprimary
standards
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Instrument model & SN model
SN fluxes interpreted as:
8
● Spectrophotometric model
(SALT2, SALT3, SNEMO…)
● Trained from real data
● Carries calibration uncertainties !
● Instrument transmission
● Shape measured (bench / in-situ)
● Throughput at a given time btained
from star observations
Constraints on SN models: ● Need a way to propagate calibration uncertainties● Should be trained primarily on the cosmo sample (to
benefit from its calibration)
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● Filter metrology
○ On bench
○ In situ (e.g. with
stable monochromatic
light source)
● Relative flux calibration
○ Primary flux reference
○ Flux metrology chain
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Calibration ingredients
10
Survey flux metrology chain
Instrument model
SN distances
SN model
Primary flux
reference
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Outline
● Why calibration ?
● Calibration requirements
○ For a 4th generation survey (e.g. LSST)
○ For a third generation survey (e.g. Subaru)
● Primary flux references
○ CALSPEC & NIST
● Survey flux metrology chain
○ Survey uniformity, GAIA
● Characterization of the survey telescope
○ Bench & in-situ throughput measurements
● Conclusion
11
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Calibration requirements
● For a given dataset, what is the impact of calibration on the
cosmological parameters (e.g. the DETF FoM)
● To answer this question, need to build a fast emulator of the SN
analysis, including the training
12
(See also DESC SRD (Mandelbaum, Scolnic, Hlozek)
Work led by F. Hazenberg (LPNHE)
measurementscosmological parameters “SALT2”
1D surfacesSN parameters SN standardization
Calibration parameters
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Calibration requirements
● Model
○ Compress light curves into amplitudes
○ Reduce SALT2 to 2 polynomials (mean “surface” + color law)
○ Keep color standardization (large penalty associated to beta)
○ Fisher analysis taking all all parameters taken into account
simultaneously
○ Marginalize over nuisance parameters and yield a FoM
● ~ 20 minutes to simulate light curves,
● ~ 1 second to obtain the FoM from light curve amplitudes
13Work led by F. Hazenberg (LPNHE)
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Calibration requirements for LSST
20000 SNe from WFD + 15000 from DDF 14F. Hazenberg
State of the art
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Calibration requirements for LSST
1520000 SNe from WFD + 15000 from DDF F. Hazenberg
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Calibration requirements for LSST
1620000 SNe from WFD + 15000 from DDF F. Hazenberg
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Calibration requirements for LSST
17
JLA : 15
5 mmag
σ(δλb)~1 nm
20000 SNe from WFD + 15000 from DDF F. Hazenberg
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Calibration requirements for LSST
18
5 mmag
σ(δλb)~1 nmLSST : 55
JLA : 15
20000 SNe from WFD + 15000 from DDF F. Hazenberg
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Calibration requirements for LSST
19
5 mmag
σ(δλb)~1 nmLSST : 55
JLA : 15
LSST : 189
20000 SNe from WFD + 15000 from DDF F. Hazenberg
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Calibration requirements for LSST
20
20
5 mmag
σ(δλb)~1 nmLSST : 55
JLA : 15
LSST : 188
LSST : 266
20000 SNe from WFD + 15000 from DDF F. Hazenberg
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Calibration requirements in ~ 2020 ?
JLA + SSP(2 years) + 800 additional nearby SNe21
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Impact of training
22
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Outline
● Why calibration ?
● Calibration requirements
○ For a third generation survey (e.g. Subaru)
○ For a 4th generation survey (e.g. LSST)
● Primary flux references
○ CALSPEC & NIST
● Survey flux metrology chain
○ Survey uniformity, GAIA
● Characterization of the survey telescope
○ Bench & in-situ throughput measurements
● Conclusion
23
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CALSPEC● Primary flux references based on
DA white dwarf models
● Bohlin, Gordon, Tremblay, 2014
○ 3 DA WD (G191B2B, GD153, GD71)
○ NLTE atmosphere models
from (Rauch et al, 2013)
● The average defines the
HST/STIS flux calibration
○ Residuals ~ 1% (visible)
● Extended to a larger network
with HST/STIS
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CALSPEC : model uncertainties ?
● 2 models, same physics
○ ~ 4 mmag 400 < λ < 1000 nm
● What about physics unaccounted for ?
○ Metal lines found in
high-resolution spectra of
G191B2B
○ Lyman / Balmer line problem
○ Convection / turbulence
○ …
● Need for alternate primary references
with different systematics
25(Bohlin, 2014)
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Comparing CALSPEC and NIST
● Uncertainties on WD flux scale (~0.5%, Bohlin 2014)
● However,
○ It is easier, for large survey telescopes, to keep stars as flux
references (real point sources, same calibration beam)
○ Producing a good calibration source for a large survey telescope
(same beam as science beam) is very difficult
● Strategy
○ Keep using stars as calibration transfer tools
○ Keep CALSPEC as flux references (high quality
spectrophotometric standards)
○ Use a NIST-calibrated artificial source and a dedicated
telescope to intercalibrate NIST and CALSPEC26
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Projects
● First attempts by Stubbs & Tonry
○ e.g. Stubbs et al, 2010 (2010ApJS..191..376S) ~ 5% precision
● NIST stars
○ https://www.nist.gov/programs-projects/nist-stars
○ Use NIST calibrated spectrometer + additional hardware to monitor line of sight
○ Target bright standards (Vega)
● SCALA
○ See talk by G. Aldering (~ 1%)
○ Target all SNfactory calibration stars (include CALSPEC)
● starDICE
○ Use stable LED-based, NIST calibrated artificial source
○ dedicated, small focal length telescope
○ Target mainly CALSPEC stars
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New metrology chain
28
NIST Photodiodes
Narrow spectrum LEDs with stable drive elec
Small aperture telescope
Spectrophotometric standard stars
10-19 W/cm²/nm
NIST SCF
NIST POWR
10-17 W/cm²/nm
10-13 W/cm²/nm
10-3 W
10-6 W
LSST
sources detectors
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29
Telescope
small aperture
Calibrated light source
few 100’s m
CALSPEC stars (artist’s view)
variations controlled at 0.1%
variations controlled at 0.1%
F. Hazenberg
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30
Telesc
ope
small a
perture
CALSPEC stars (artist’s view)
variations controlled at 0.1%
Calibrated light source
variations controlled at 0.1%
F. Hazenberg
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starDICE
33
++
++
++
+
++
+
Star vs WD model measurements
Star vs LEDs measurements
● Output of starDICE○ a smooth recalibration function κ(λ)○ Based on photometric measurements of LEDs & CALSPEC
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starDICE
● Observations at Observatoire de Haute-Provence (south of France)
● Phase I completed
○ Bench measurements
■ Spectrophotometric characterization of the source
■ Assessment of source stability
■ Characterization of telescope & camera throughput
○ Measurements on site
■ ~ O(20) nights
■ O(4000) stellar observations (6 different CALSPEC stars)
■ O(3000) observations of LEDs
● Phase II starting
○ New telescope, new fainter source, monitoring hardware
○ O(100) nights 34
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Bench measurements
LED spectrophotometric characterization
35
LED flux vs. temperature
Telescope & detector throughput
F. Hazenberg
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Accuracy of bench measurements
● 12 LEDs with S(λ,T)
● Bench noise and LED noise
lower than 0.1%
● Model uncertainty < 0.5%
● Still ongoing
○ LED spectra at low flux
36F. Hazenberg
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Data taking
37
G191B2B (CALSPEC)
F. Hazenberg
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LED imaging
38F. Hazenberg
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Uncertainties
● Low current LED spectra (5.7%)
● Gray atmospheric variations (1.7%)
● Atmosphere models (1.5%)
● LED temperature dependent flux
(0.4%)
● Flux solid angle (0.4%)
39
● New spectroscopic bench
● Cloud monitoring
● OHP monitoring + models
● New standardization
technique (LED rev voltage
monitoring)
● New LED maps + laser
alignment
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Outline
● Why calibration ?
● Calibration requirements
○ For a third generation survey (e.g. Subaru)
○ For a 4th generation survey (e.g. LSST)
● Primary flux references
○ CALSPEC & NIST
● Survey flux metrology chain
○ Survey uniformity, GAIA
● Characterization of the survey telescope
○ Bench & in-situ throughput measurements
● Conclusion
40
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Uniformity: DES and Gaia DR2
● Test uniformity of DES r-band with Gaia DR2 G-band● 2.2 mmag uniformity at low Galactic latitude
○ MW Galaxy contamination because Gaia G-band is very broad, making comparison strongly dependent on SED
41[Rykoff]
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Uniformity: Gaia as a Reference
● Testing internal Gaia uniformity (Blue channel “BP”; red channel “RP”; broad “G” band)
● Map Gaia “flux excess” == (f_BP + f_RP) / f_G● To be able to use Gaia for mmag precision will require more work● Additional challenge of using spectrophotometry
42[Rykoff]
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Outline
● Why calibration ?
● Calibration requirements
○ For a third generation survey (e.g. Subaru)
○ For a 4th generation survey (e.g. LSST)
● Primary flux references
○ CALSPEC & NIST
● Survey flux metrology chain
○ Survey uniformity, GAIA
● Characterization of the survey telescope
○ Bench & in-situ throughput measurements
● Conclusion
43
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Re-characterization of the MegaCam filters
● MegaCam filters
decommissioned in Feb 2016
● Sent to LMA (Lyon) in Dec
2016 for a complete
re-characterization
● Two setups
○ Fast spectroscopic scan
(1cm beam, normal
incidence)
○ Slow spectrophotometric
scan (variable incidence0
44
● Measurements described in (sassolas et al, 2018, 2018SPIE10706E..4ES)
● Filters sent back to CFHT in
June 2018
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Filter measurements
● Model fitted simultaneously on data from both setups● Stray light within the bench taken into account (normal incidence)
45(Betoule et al, in prep)
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Consistency
● Filter model fitted
simultaneously on both
sets of measurements
● The model describes
measurements with a
precision better than
0.5-nm
46(Betoule et al, in prep)
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Filter uniformity maps
47(Betoule et al, in prep)
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Radially averaged residuals
● Comparing relative color
terms
○ Synthetic (B13, LMA)
○ Measured
● Re-measurement solves
spatial issues observed in B13
● Agreement better than
4mmag/mag everywhere
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In-situ measurements of filters
● LSST plans to use a Collimated Beam
Projector (CoBP)
● No bench re-measurements of filters
planned (so far)
● Tests of CBP precursors (aka
“Harvard CBP) have been done on
PanSTARRS and at CTIO
● CBP currently used to characterize
and monitor starDICE telescopes
○ In the lab
○ In situ (at OHP)
● Will allow to assess accuracy and to
gain experience 49
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Conclusion
● Calibration requirements for 4th generation surveys will be very
hard to meet
● Challenging goal
○ Relative flux calibration @ 0.1%
○ Characterization of filter wavelength positions @ 1A
○ Flux metrology chain controlled at better than 0.1%
● State of the art
○ Relative flux calibration :
■ ~0.5% (CALSPEC)
■ ~ 1% (NIST-CALSPEC)
○ Filter metrology : ~ 5A
○ Flux metrology chain < 0.5% (FGCM)
● Lots of instrumental work ahead
○ (starDICE, SCALA, filter metrology) 50