Artemis: A Stratospheric Planet Finder

The near-space environment of the stratosphere is far superior to terrestrial sites for optical and infiared observations. New balloon technologies will enable flights and safe recovery of 2-ton payloads at altitudes of 35 lan for 100 days and longer. The combination of long flights and superb observing conditions make it possible to undertake science programs that otherwise could only be done fiom orbit. We propose to fly an "Ultra-Hubble" Stratospheric Telescope (UHST) equipped with a coronagraphic camera and active optics at 35 km to search for planets around 200 of the nearest stars. This ULDB mission will establish the fiequency of solar-type planetary systems, and provide targets to search for earth-like planets. The Stratosphere: A Superb Observing Site At 35 km (1 15,000 fi) the stratosphere is cold (-50°C), very dry (< 0.3 pm precipitable water vapor), and rarified (5 mbars pressure). Table 1 compares the atmospheric parameters at a good site like Mauna Kea with atmospheric conditions at an altitude of 35 km. Astronomers seek out high and dry mountaintops around the world as sites for observatories. If there were a mountaintop with conditions like the stratosphere, astronomers would want to build telescopes there. Fortunately, ULDBs will make the stratosphere accessible for large telescopes (but not astronomers!). Fried's Parameter at MountahtoD Observatories Fried's parameter ro (the characteristic distance across the wavefiont for the phase to change by 4 h ) is commonly used as measure of image quality. The 3.8-m Canada-France-Hawaii Telescope (CFHT), which has a good thermal design and is at an excellent site, typically has an ro of 18 cm at 500 nm. The corresponding fullwidth at half-maximum (FWHM) of the point spread function (PSF) is 0.7". The field of view Bo for phase coherence is 3", and the timescale TO for phase coherence is 3.6 msec. On exceedingly rare occasions ro might be as large as 36 cm, and the corresponding FWHM as small as 0.35". Because image degradation is caused by atmospheric turbulence that has a continuous spectrum of sizes for phase coherence, the 0.35" CFHT PSF is very inferior to a 0.35" PSF produced by a diffraction limited 0.33-m telescope. The small scale, turbulence-induced fluctuations in the index of refiaction diffract light to large angles, producing a halo that extends to several arc seconds around the core of the image. The Strehl ratio, which is the ratio of the peak intensity in the observed image to the peak intensity in a perfect (1.e. diffraction limited) image, is S 1% for the best CFHT optical images. Consequently, atmospheric seeing limits both the resolution and the contrast. Fried's Parameter at 35 km https://ntrs.nasa.gov/search.jsp?R=20030105974 2017-09-13T21:35:29+00:00Z