A Balloon-Borne Light Source for Precision Photometric Calibration
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This document summarizes a technique using a balloon-borne light source to enable precise photometric calibration for ground-based astronomy surveys. The method involves launching lasers on a balloon payload to backlight the atmosphere, and measuring the brightness of the light source from the ground to determine aerosol extinction and allow for sub-1% absolute photometry. Initial test flights in 2010 and 2012 demonstrated maintaining stable thermal and power conditions for over 2 hours of data collection. The next steps are to perform a zero-pressure "float" flight over an observatory to test the calibration technique.
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A Balloon-Borne Light Source for Precision Photometric Calibration
- 1. CalCON, Aug 2012 A Balloon-Borne Light Source for Precision Photometric Calibration Maxwell Fagin(1) , Yorke Brown(1) , Justin Albert(2) , Christopher Stubbs(3) (1) Dartmouth (2) University of Victoria, (3) Harvard
- 2. Background Current dominant source of error is atmospheric extinction, Due to: AerosolH20 Molecular <1% (absolute) optical photometry needed for ground based cosmology surveys (Stubbs and Tonry, The Astrophysical Journal, August 2006) Can be computed in real time with atmospheric models (MODTRAN) and ground based measurements. Goal: Provide a technique for real time measurement of aerosol extinction and enable <1% absolute photometry
- 3. 3) Use a ground based NIST photodiode to measure apparent mag at detector Technique 1) Backlight atmosphere with balloon borne polychromatic light source 2) Use an on board NIST photodiode to measure absolute mag of source 4) Account for extinction due to H20 and molecular scattering with MODTRAN 5) Remaining difference must be due to aerosol extinction
- 4. Integrating Sphere Balloon Payload Technique Well characterized Lambertian Profile 440 nm 532 nm 639 nm 808 nm 10 mW Lasers NIST Photodiode #1 NIST Photodiode #2 Telescope NIST defined spectral response Known View angle
- 5. First flight, Hanksville Utah at the Mars Desert Research Station (2010)
- 6. - Logistically simple - 4 people (2 launch, 2 recovery) - All equipment fits in a 4-door sedan - Multiple launches per night if required Why a balloon? - Long integrations times (>2 min) for a 2° FOV (SDSS) - Best possible SNR for fixed photometric output - All instrumentation is recovered after flight - Calibration can be re-checked in the lab
- 7. Optical Systems - Lasers (4) - Integrating Sphere Instrumentation/Sensing - NIST Photodiode - Attitude Support Systems - Power (~2 W) - Cutdown - Recovery Communication Systems - GPS - Telemetry - ELT radio beacon Total weight < 6 lbs Payload Design
- 8. Optical Systems Laser Diodes Photometric stability <.5% 100 g, <.5 W per laser Integrating Sphere
- 9. Communication Systems 900 MHz (UHF) DNT Transceiver Radio Robust line of sight communication
- 10. Instrumentation/Sensing Mag/Accelerometer forMag/Accelerometer for real time attitude datareal time attitude data
- 11. Challenge: Thermal Management Peak Cooling Ascent: airspeed 5 m/s through 30% atm at -55 °C Peak Heating Floating: airspeed 0 m/s in 2% atm at -45 °C
- 12. Flight Systems Test: ALTAIR 1 2011
- 13. Flight Systems Test: ALTAIR 2 April 13, 2012
- 14. Flight Systems Test: ALTAIR 3 & 4 Aug 16 & 24, 2012
- 15. Test Flight Results Can maintain lasers and NIST optics at viable temperatures
- 16. Test Flight Results Can measure attitude and source brightness in real time
- 17. Test Flight Results Can position light source at 30 km altitude, within 2° FOV
- 18. Test Flight Results Can track light source from incoming GPS telemetry
- 19. Long term? Calibrate an all sky catalog of White Dwarf standards - Stable <1%, ~1000 yrs - Near black body - Full sky coverage - magnitudes 8-20 Credit: Fossati, Bagnulo, ApJ 08/01/12
- 20. Goals Completed: - Maintain stable thermal conditions for flight - Sufficient power for >2 hours of data collection - Control, communication and data handling - Cutdown and separation from balloon - Telescope and radio tracking - Precision landing targeting (+/- 2 km) - Real-time Telescope tracking - Night launches Still to do: - Zero pressure “float” balloon - Flight over Mt. Hopkins - First astronomical flight in early 2013
- 21. Thanks to: Yorke Brown, Chris Stubbs, Kristina Lynch and Justin Albert Questions?
Editor's Notes
- Collaboration project between Harvard, Dartmouth and the University of Victoria Specifically our contribution to the project which is ALTAIR. The vehicle which provide a high altitude platform for conducting sub 1% photometry.
- Problem was outlined by Stubbs and Tonry in 2006 Do this this without any astronomical sources, complete NIST traceability
- Back light the atmosphere by lofting a light source on a balloon. The absolute magnitude of the light source is measured at the source by the NIST photodiode on board the payload An identical photodiode at the telescope aperture is used to measure apparent magnitude of the source Using atmospheric models like MODTRAN to determine the loss due to molecular scattering and water extinction Zimmer, McGraw and Zirzow’s LIDAR experiments on Wednesday morning Any unaccounted observed extinction must be due to aerosols
- Multiplexing
- Why not an orbiting payload like in Dr. Best’s on orbit black body? Satellite in LEO will cross the sky in a matter of minutes (slew rate 1°/second) Once you get the balloon above the jet stream, it is functionally geostationary. Close to telescope
- 12” wide and 6” tall weighs only 5.9 lbs. Our biggest challenge has been to keep the weight under 6 lbs. From a science perspective, we have the worst reason for this limit, and that is because of FAA regulations. 6 lbs is the threshold of a balloon borne payload before FAA regulations kick in, Want to fly several times in a single night.
- Lambertian profile. Apparent brightness will depend on horizontal angle that the sphere makes to the telescope. At float, balloon will stay within a degree of horizontal Need to confirm this with accel board
- Addressed via a series of CFD models in programs like Ansys FLUENT and COMSOL multiphysics. All thermal management is passive
- All our flights have been done in connection with the Dartmouth College GreenCube program, an undergraduate cubesat lab. Our first demonstration flight was in 2011. Fortunately Dartmouth is a very outdoor place, and if you send out an email saying does anyone have rock climbing equipment we could borrow, the answer is usually yes.
- Implemented automated tracking, GPS Signal slewed along with the payload.
- Night flights. Confirmed the payload could be seen from the ground, @ 20 km, it was about as bright as a 2nd magnitude star
- We’ve built the payload with enough insulation to punch through the thermopause. These are temperature profiles from our last flight, with the exterior thermal loading in black. Never got bellow freezing.
- We’ve proved we can keep the payload within horizontal. Pitch and roll angle of the payload with respect to the telescope over the flight profile. Turbulence at launch and due to cutdown. This is not at float. At float the payload should be even more stable.
- So we’ve made some very big steps towards implementing this scheme of moving away from astronomical sources as photometric standards
- McGraws LIDAR Kerber’s balloons