SNAP Calibration Program

Steps to Spectrophotometric Calibration

http://snap.lbl.gov

The SNAP (Supernova / Acceleration Probe) mission’s primary science goal is the determination of the properties of the dark energy. Specifically, observations of distant Type Ia supernovae will be used to measure the dark energy equation of state constant parameter, w0, and time varying parameter, w‘ or wa, to a fractional uncertainty of 0.05 and 0.3 respectively. This places stringent requirements on the control of systematics and on the absolute color calibration of these supernovae. The overall calibration for the SNAP CCD and NIR imagers and spectrograph will be conducted through several routes. We envision employing a variety of well-studied stars, certainly including the HST spectrophotometric standard stars and performing indirect transfer calibrations that permit comparison with NIST irradiance standards to close the loop with fundamental MKS quantities. We present the basic issues and possible strategies in order to achieve 2% uncertainties in color calibration over the wavelength range from 350 to 1700 nm.

The science requires observations of nearby and distant SNe Ia. • At redshift z=0,

• SNe Ia restframe B-band flux is measured in a B filter (Figure 1a) and• SNe Ia are bright at peak, m~19 mag

• At high redshifts, to z=1.7• SNe Ia restframe B-band flux shifted to longer wavelengths (Figure

1b)• SNe Ia are faint at peak, m~25 mag

• To standardize SNe Ia, we only need to know • the flux RATIO in any two filters: e.g. fB/fJ

• equivalent to the COLOR: e.g. B-J = -2.5 log fB/fJ

• Just need to know the SLOPE of the SED.• The problem to solve: how to accurately measure this ratio for faint

objects, and in the near infrared (NIR).What effect does calibration error have on the uncertainty in w0 and w?

Assuming uncertainty solely in the temperature of the calibrator:

A) A single calibrator gives rise to a negligible increase in systematics: the extinction correction effectively corrects for a one-parameter miscalibration (Figure 3a).

B) Two calibrators: at T=35000 and at T=6000, calibrating different wavelength regimes. The uncertainty in w0 and w increases by approximately 20% for uncertainties in the temperature of ~ 3%. If there is miscalibration of the spectra (i.e. mismatch in the overlap region) this is inferior to a single star spanning the entire spectral range (Figure 3b).

1. Wide range of target and standard star brightness (Figure 2a)

2. 180 filter + detector combinations (Figure 2b)– 4 3x3 HgCdTe arrays– NIR filters: 12 each of J-, H-, K-like– 4 3x3 CCD arrays each with 4 filters– Optical filters: 24 each U, B, V, R, I, Z

3. 2-channel integral field spectrograph

– 2 optical and 2 NIR detectors

1. Identify good calibration standards

2.Establish standard star network

3.Construct calibration error budget

– (iterative process with simulation and instrumentation teams)

4. Investigate optimal telescope platforms for fundamental calibration, tying standard star network to NIST standards: e.g. balloons, space & ground

5.Develop pre-launch and in-orbit calibration strategies

6.Develop on-board calibration observing program

– Study impact of on-board calibration on

• observatory management

• observing strategy, hence SN program

• observatory design

7.Design spectrophotometric calibration experiment

8.Develop realistic time, cost and manpower estimates for

– pre-launch calibration program

– in-orbit when calibration included in observatory management

• How well are fundamental flux standards calibrated?

• Uncertainties in calibration transfer (“error transfer”)

– from fundamental flux standards to spectrophotometric primaries

– from primaries to secondary standards

– from secondary standards to science targets

• Requirements on flat fields

• Uncertainties introduced by other effects, for example

– filter properties

– position on focal plane

– numbers/types of standards

Color calibration

Zero point (sets scale)

Secondary star field

calibration

Flatfields

Primary standards calibration

Aperture corrections

Scattered light

Detector

Telescope

linearityintra-pixel

OpticsFilter

Instrument: Imager &

Spectrograph

band pass shifts

uniformity

Error budget accounts for e.g. :How do calibration transfer errors propagate down the network? How well do filters+detectors, integral field spectrograph need to be calibrated?Flat field requirements?

Figure 2b

System throughput & bandpass of each of the SNAP filters shall be known to sufficient precision to predict the relative count rate in each of the bands to 1.5% rms and therefore the ratio of any two bands (i.e. the color) to 2% given a flux spectrum of a SN Ia with redshift 0.3 < z < 1.7. Systematic uncertainties are the limiting factors. Implies 2% maximum in systematic uncertainties in the calibration error budget.

HgCdTe

Spectrograph Spectrograph port

CCDs

Guider

~50 mags

Figure 2a

WD flux standard

KIII flux standard

NIST Lamp

SNe IaZ=1.7

Z=0

Figure 1a Figure 1b

SN Ia Restframe B-band

Peak Spectrum

SN Ia at z=1.4 redshifted B-band

Peak Spectrum

Figure 3a.

Figure 3b

Calibration Requirements

Abstract

The Challenge

Need to Check

Science Driver: w0 andwa

Error Budget Framework