Dark Energy and Large Synoptic Survey Telescope

Dark Energyand

Large Synoptic Survey Telescope

Precision Calibration Apparatus:Calibrating the Throughput and Response of

Astronomical Instrumentation

Department of Energy Site VisitAugust 21, 2009

Harvard UniversityDepartment of PhysicsLaboratory for Particle Physics and Cosmology

Peter DohertyHarvard University Laboratory for Particle Physics and Cosmology


2DOE Site Visit 8/21/2009 - Dark Energy and LSST– Precision Calibration Apparatus-Doherty

Precision Calibration: Introduction

We improve upon traditional celestial calibration sources by:• Measuring a source with a known spectrum, namely, a narrowband

tunable laser• Compare the system response (telescope throughput) to a known

detector (NIST-calibrated photodiode)• In a related (but separate) effort, continuously monitoring the

atmosphere during acquisition of astronomical data

Measurement of a source is a sum of integrals:



Current and Future Efforts:

Calibrating the Telescope and Detector:

• Pan-STARRS Calibration Screen

• LSST Calibration Screen

• Portable Calibration System

Monitoring Atmospheric Throughput:

• Real time water vapor monitor system (Leibler)

• Tunable-Lidar monitoring of atmospheric extinction (with UNM/GTRI)

Precision Calibration: Current and Future



Precision Calibration: Calibration Procedure

Wavelength Calibration Procedure

• Illuminate full telescope aperture with monochromatic light• Take a calibration “flat” while monitoring input light with calibrated

photodiode• Normalize flat to flux seen by photodiode• Move to the next wavelength and repeat, until entire visible

spectrum is spanned• Construct wavelength-dependent response for each pixel in the

telescope’s CCD camera

• End result: Measurement of relative system throughput, including telescope mirrors, corrector optics, filters, and detector



Pan-STARRS Calibration Screen: Overview

Accomplished:Installation of the world’s most advanced telescope instrument calibration system in the Pan-STARRS dome at Haleakala Hawaii

Features:

Rear projection screen mounted inside telescope dome

Multiple fixed and tunable light sources:Quartz tungsten halogen white lightNIST SIRCUS Laser (680 to 1100 nm)Supercontinuumlaser and monochromator (450 to 1100 nm)

NIST calibrated photodiode for flux calibration

Digital Micro-mirror Device for image projection

Integrated into telescope control system



Pan-STARRS Calibration Screen: Diagram



Pan-STARRS Calibration Screen: Assembly



Pan-STARRS Calibration Screen: Photograph



Pan-STARRS Calibration Screen: Installed



System Enhancements 2009/2010:

Installation of Ekspla Tunable laser for wavelength coverage from 350 nm to 1100 nm with a single source

Improved Photodiode measurement and control system for more precise flux measurements

Closing the loop with the CCD/DMD image flattener

Further integration of instrument into telescope system software

Incorporating calibration data into the Pan-STARRS Image Processing Pipeline

Pan-STARRS Calibration Screen: Future Plans



LSST Calibration Screen: Overview

Similar in concept to Pan-STARRS calibration screen,but much different in scale:

Similarities:

Ekspla (or similar) tunable laser source withwavelength coverage from 350 nm to 1100 nm

NIST calibrated photodiode(s) for flux measurements

Differences:

Cannot use single DMD projector: not enough focallength, and a large central obscuration

Much larger!Pan-STARRS = 1.8 meters, LSST = 8.5 meters

Multiple small projectors?



Comparison of Pan-STARRS Mirror Size to LSST Mirror Size

LSST Calibration Screen: LSST vs. Pan-STARRS



Filling the LSST Aperture with “Small” Projectors

LSST Calibration Screen: Filling the Aperture

12 Projectors - 1.7m Diameter 48 Projectors - 0.85m Diameter



LSST Calibration Screen: Small Projector

Concept: A fiber fed parabolic reflector

Parabolic Mirror

Optical Fiber

Diffuse emitter and parabolic collimator. Light from a fiber is collimated and illuminatesa diffuser. The size of the diffuse emitter determines the range of angles into which lightis emitted. Uniform surface brightness on the emitter ensures uniform intensity into all angles.

Diffuser

Lens

LPPC is currently constructing a prototype in our optics lab



LSST Calibration Screen: “Leaky” Fiber Panel

Concept: A flat screen built of glowing optical fiber

Optical Fiber

Mirror

Collimator

Diffuser

A prototype screen assembled in LPPC LabWould a spiral work better?



Portable Calibration System: Overview

A transportable light source and photon flux measurement system for calibrating throughput of astronomical telescopes and instrumentation

Makes use of existing observatory dome screen

Flexible I/O and triggering modes to support a wide variety of operating modes allowing integration with diverse telescope systems

In 2009/2010 LPPC will design and construct the Integrating Photodiode Amplifier. Other parts are COTS.



Atmospheric Water Vapor: Overview

Variation in aerosols and water vapor are the main sources of temporal variation in atmospheric transmission, and the airmass dependence of extinction varies with wavelength.

To investigate these two effects (aerosols and water vapor) we are engaged in two lines of research:

1.Monitoring water vapor in the atmosphere via differential imaging

2.Measuring atmospheric aerosol content via lidar backscatter

We are engaged in experiments of both types.



Calibration: Undergraduate Projects

Atmospheric Water Vapor via Differential Imaging

Concept: Monitor bright stars simultaneously in two pass-bands:940nm (on absorption line) and 880nm (off absorption line).

1.Monitor cloud density and correlate with simultaneous imaging through the CTIO 4m Blanco telescope

2.Distribution of water vapor and its variability spatially andtemporally as well as airmass dependence.

Two undergraduates travelled to CTIO in Chile with Dr. Stubbs inearly July and are working to reduce their data and determine the results.



Monitoring Atmospheric Extinction

“Light from distant galaxies travels towards Earth-based telescopes for millions of years, and in the last millisecond of its trip about 20 percent of that valuable light, which carries information about the very structure of the universe, is lost as it traverses our atmosphere. For centuries, astronomers have looked through the atmosphere with their telescopes, but have seldom looked at the atmosphere in an effort to precisely correct for this lost light.”

-John McGraw, Professor of Physics and Astronomy, University of New Mexico

UNM/GTRI Astronomical Lidar for Extinction (ALE)

ALE has been developed by the University of New Mexico and the Georgia Institute of Technology Research Institute

Measures backscatter from the stratosphere to monitor minute-to-minute changes in extinction due to aerosols

Current system uses a fixed, 537 nm source

Harvard/LPPC will collaborate with ALE creators in testing the system using a tunable laser source with wavelength coverage from 410 to >1100 nm



Precision Calibration Apparatus: Credits

Harvard University/LPPC:Christopher Stubbs, Professor of Physics and AstronomyPeter Doherty, Senior Instrumentation EngineerSteve Sansone, Scientific Instrument BuilderGautham Narayan, Graduate StudentClaire Cramer, Post-Doctoral Research ScientistCamille Leibler, Undergraduate StudentKenneth Gottlieb, Undergraduate Student

National Institute of Standards and TechnologyKeith Lykke, Physicist, Laser Applications GroupSteven Brown, Physicist, Laser Applications GroupJohn Woodward, Physicist, Laser Applications GroupAllan Smith, Physicist, Laser Applications Group

University of Hawaii, Institute for AstronomyJohn Tonry, Astronomer Jeffrey Morgan, Pan-STARRS Sr. Telescope Supervising Engineer Robert Calder, PS1 Operations Manager

Large Synoptic Survey TelescopeKirk Gilmore, LSST Camera System Scientist, SLACDavid L. Burke, Kavli Institute for Particle Astrophysics and Cosmology, SLAC

Laser Applications Group

Dark Energy and Large Synoptic Survey Telescope

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