1. 1 SNAP Calibration Hardware Contributors: Chuck Bower Nick Mostek Stuart Mufson Mike Sholl

2. 2 Fundamental Spectrophotometric Calibrators (white dwarfs, G2 stars) pre-launch measurements Fundamental Spectrophotometric Calibrators Primary Field Calibrators on same subpixel regions High Frequency Spatial Flats – LEDs, RoF Low Frequency Spatial Flats – star fields space-based measurements Focal Plane calibrated for photometry * * * * calibrate subpixel regions Ring of Fire Bare photodiodes monitor RoF irradiance SNAP throughput (transmission vs ) Transfers calibration from subpixel regions SNAP Focal Plane Photometric Calibration (2% color, 1.5% individual band) Spectrograph monitors LED spectral shape STARCal?

3. 3 SNAP Calibration Requirements The cosmological parameters measured by SNAP will be limited by the systematic errors of SNIa photometric measurements The SNAP calibration group has been working towards an error requirement of 1.5% in- band (2% color) photometric calibration IU has the responsibility for designing and testing the SNAP onboard photometric calibration system IU is also taking a lead role in the design and calibration of SNAP broadband filters Ring of Fire

4. 4 Monochromator LED Illuminator RoF Focal Plane Photodiode Silica Fiber Bundle Radiation Shield Calibration Light System Overview Fiber Bundle Connector Ring of Fire (RoF) illuminates the SNAP focal plane LED illuminator is placed away from the focal plane and operated warm Fiber bundle connectors are included to help with system integration Monochromator could still be incorporated in the light system (LEDs retained for pulsing capability and stray light reduction) Filters Light from Telescope

5. 5 Design and engineering done by Mike Sholl at LBNL Fiber bundle illuminates an annular mirror and projects an elliptical spot on a diffuse Spectralon surface Initial design uses 7 fiber bundle ports to achieve uniform illumination 300 micron core diameter fiber with wavelength coverage from 400nm-1700nm IU involved in designing and testing optical fiber bundle Ring of Fire Design

6. 6 LED Illuminator Testing Illuminator must support thermally controlled, redundant LEDs and uniformly illuminate a fiber bundle Fiber port variation causes azimuthal non-uniformity on the focal plane LED illuminator mock up will test the efficiency and uniformity of the conceptual design Initial calculations show that 30k e - signal can be achieved in 300s integration 4inch diameter hemisphere should hold at least ~150 LEDs (depending on how they are packed) Thermal Stage Fiber Bundle Spectralon Focal Plane Uniformity from RoF  2.7% variations Fiber Port Variation = 10% azimuth

7. 7 Ongoing LED Testing IU continues to catalog a wide range of LEDs with undergraduate help Multiple LEDs of the same type have also been cataloged Total LED output changes by 1% per 1°C Testing will include radiation testing at the IUCF this fall

8. 8  System to help determine tolerances on SNAP calibration hardware  Designed to be a versatile, fully automated illumination and measurement system  Immediate Goals: 1.Transfers the irradiance calibration from a NIST photodiode to a photodiode operated at SNAP temperature (PITS) Calibrated photodiodes will be used in QE measurement systems at Michigan, CalTech, and LBNL 2.Test interference filters transmission at SNAP temperature with an f/10 beam at a SNAP range of angles  Most parts are in hand and assembly is now in progress Dewar Photomax Imaging lens & baffle Integrating Sphere Linear Stage Monochromator Blocking Filter Wheel Monochromatic Illumination & Cryogenic Calibration System NIST Photodiode Monochromator Front Back 6

9. 9 NIST TE-cooled InGaAs photodiode Transfer candidate no TE cooler NIST photodiode has a precision-bored, black anodized aperture with near-knife edge thickness 1  error is 1.5% at 0.7  m and 2% at 1.7  m, a large improvement over previous 5% calibration Effective aperture area measured independently by NIST, contributes largest portion of error in irradiance (1%) Photodiode uniformity was calibrated from NIST cryogenic radiometer beam which was raster scanned over the diode surface NIST Calibrated InGaAs Irradiance Standard

10. 10 PITS Setup Mounting Rails Dewar Dewar Mounting Bracket Electronics Connector

11. 11 Linear Stage Transfer Diode Housing NIST Diode Bracket Cold stage Insulating Halon PITS Setup

12. 12 What tolerances are required to maintain calibration of filter throughput? Interference filter throughput will change from room temperature to SNAP focal plane temperature Interference filter throughput is also affected by the incident angle of light. Most filters are referenced and tested at normal incidence, but SNAP photon incidence is significantly off- normal SNAP filters require a cryogenic testing facility that mimics the SNAP observing conditions Incidence angle Photon Count Cold Interference Filter Testing

13. 13 Laboratory setup to simulate filter conditions onboard SNAP Filter will be held at SNAP focal plane temperature SNAP secondary and spider shadow can be modeled and attached to the exit port of an integrating sphere NIST diode will be placed a working distance from the sphere port to simulate the 5° solid angle of incident light expected on the SNAP focal plane (apodization) Filter rotation stage will place filter at an incident angle of 10 °, 15 °, and 20 ° (SNAP off-axis incidence angle) Cold Interference Filter Testing SNAP secondary shadow Cold filter Rotation stage NIST photodiode Dewar Integrating sphere Monochromatic light

14. 14 Integrating Sphere Focusing Optics Monochromator Photodiode 3-D Stage LED Thermal Controller Integrating Circuit Filter on Rotating Stage Procedure 1.Measure filter transmission with QTH lamp at normal incidence and 10  filter angle 2.Measure LED spectra w/o filter and then with normal and 10  filter angle 3.Measure entire LED emission (mirror replaces grating) at normal and 10  filter angle Grating LED Filter Tracking Method QTH Lamp Mirror

15. 15 LEDs work as well as a QTH lamp source when combined with a monochromator LED 1 LED 3 LED 2  Overlapping LED transmissions combined and weighted by S/N of monochromator measurement Filter Transmission with LEDs and a Monochromator

16. 16 Can LEDs be used without a monochromator? —Simplifies calibration system design —Increases number of photons to the focal plane Must compare “Broadband” LED Signals —For each LED and filter incident angle, integrate to get the total spectral power —Compare transmission of integrated LED spectral power to measured transmission from broadband LED power —Back out filter transmission change using initial LED / Filter throughput and the change in the LED broadband measurements LED 1 LED 3 LED 2 Broadband LED Measurements

17. 17 Prescription for using Broadband LEDs Create spline functions from LED spectral power P( ) and original filter transmission  ( ) Expected broadband transmissions for i=3 LEDs come from Perturbed broadband transmissions used in  2 where  ΄ is allowed to vary through three parameters of eff, ,  and A=  eff  *  where  LED is the measured broadband LED transmission Minimize  2 to get parameters for final  ΄( )  ’’  ( ’ eff ) eff ’ eff  ( eff )

18. 18 Monochromatic Light vs Broadband LED Points = Monochromatic transmission measurements Lines = Transmission fits from Broadband LED measurements LED Fit Method differs from monochromatic measurements by at most 3% Input this transmission error into a SNAP simulation to get requirements on calibration method

19. 19 Effect of Filter Calibration on Cosmology Using the SNAP simulation framework to test filter calibration Introduce a filter set based on the shape of our measured filter Compare cosmologies between a perfectly known filter set and a filter set with imperfect calibration (such as our measured filter throughput via broadband LEDs) Work in progress as SNAP simulation software is being debugged

20. 20 Summary IU has a principal role in the design and testing of SNAP calibration hardware Ring of Fire design is progressing. Now includes fiber interface and LED illuminator concept LED Illuminator testing continues in earnest Monochromatic Illumination and Cryogenic Calibration System being built at IU PITS characterization to begin this fall, cold filter transmission testing will follow thereafter Filter tracking can be done with monochromatic light or characterized LED light when coupled with a monitoring photodiode Using laboratory filter tracking measurements, a simulation of science-driven calibration requirements is in progress