Several classes of planetary science observations require high spatial resolution in UV and visible wavelengths. Key examples include (a) the detection of satellites and characterization of their orbits, (b) the discovery of faint and small objects among the NEO, asteroid, Kuiper belt or Sedna-like populations and (c) cloud or trace gas observations in planetary atmospheres. Hubble Space Telescope (HST) observations have been very productive in these areas: consider the recent discovery of Makemake's satellite (Parker et al., 2016), the discovery of 2014 MU69 (now the flyby target of the New Horizons spacecraft) or the OPAL (Outer Planet Atmospheres Legacy) program.
Like HST, large-aperture ground-based telescopes with adaptive optics can also achieve spatial resolutions of 50 mas, but normally at wavelengths longer than ~1 μm. Projects like MagAO-2K are working on improving image quality at visible wavelengths, but while the core PSF (Point Spread Function) width might be narrow (projected to be 15 mas at the Magellan telescope), the Strehl ratio drops steeply with wavelength (Males et al., 2016). Not all science goals suffer equally from low Strehl ratios, however: cloud tracking on Venus is more tolerant of a low Strehl ratio than searching for a close satellite of Makemake.
A telescope on a NASA super-pressure balloon would float above 99.3% of the atmosphere, where the inner Fried parameter is thought to be two meters or more. While atmospheric turbulence is not expected to impact image quality, there are other sources of wavefront error (WFE), such as mirror figuring, misalignment of the OTA (Optical Telescope Assembly) or asymmetric heating from the Sun or Earth. We reference recent work that estimates balloon telescope WFEs from different sources to generate a suite of plausible PSFs. We apply these PSFs to the UV and visible wavelength science cases outlined in the GHAPS/SIDT report (Gondola for High Altitude Planetary Science/Science Instrument Definition Team). We quantify the impact that WFE has on achieving the planetary observations outlined in the SIDT report.
The environment of a balloon based telescope puts opto-mechanical stability demands that wavefront sensing and focus control are important features to consider. The GHAPS telescope is designed to correct for rigid body motion of the secondary mirror based upon wavefront sensing from reference stars. In order to support the concept of operations of this approach, the precision of wavefront sensing with candidate reference stars needs to be addressed. Precision is expected to degrade with reduction in irradiance. To confirm this, a study was conducted to determine the relationship between broadband and narrow spectral irradiance and wavefront measurement precision. The results of the study demonstrate that a precision of 5 nm RMS can be achieved with a 20 x 20 sampling of the wavefront with an irradiance of a visual magnitude 5 star on 1 meter aperture with an exposure of 30 msec.
Balloon based telescopes represent an opportunity to observe science in an environment with almost no atmospheric effects. However, balloon based platforms include a wide range of thermal environments as well as pointing a lightweight telescope over a large elevation range. The Gondola for High Altitude Planetary Science (GHAPS) was designed to provide nearly diffraction limited performance observations over the visible and infrared spectrum with a 1- meter aperture. To achieve such performance, detailed Structural Thermal Optical Performance (STOP) was used to predict telescope performance. Software was built to automate the process of analysis, enabling thermal, structural and optical analyses to be executed quickly with less effort. The end result was the capability to analyze both generic operating conditions and Design Reference Mission conditions, producing predictions that could be used to evaluate the quality of science return.
There has been a recent surge in interest in hosted and rideshare payloads that would launch aboard commercial
communications satellites. Much of this interest originates with the satellite customers themselves as a way to sell
excess mass and power margins that exist at launch. In 2008, NASA selected GOLD (Global-scale Observations of
the Limb and Disk) as a mission of opportunity to fly as its first hosted payload experiment on a geosynchronous
commercial communications satellite, a STAR-2 bus satellite built by Orbital Sciences. CHIRP (Commercially
Hosted Infrared Payload), a hosted payload to test infrared sensors for the Air Force, is also being developed for a
STAR-2 bus communications satellite.
The mass limitation on a STAR-2 bus hosted payload is roughly 50 - 60 kg and the volume is roughly constrained to
a 25" x 30" x 28" box on the nadir deck. Telescope apertures are therefore limited is size to about 50 cm in diameter.
The diffraction limit for visible (much less IR) imaging missions barely improves upon ground-based image performance,
but UV missions can achieve better than 0.1" resolution. There is at least one family of optical designs
that (a) provide the necessary focal length and (b) are light and compact enough to fit within the STAR-2 bus mass
and volume constraints. These designs also afford opportunities to maintain 0.05" pointing accuracy through a
combination of a fine steering mirror and an orthogonal transfer CCD.
Astronomical balloon-borne telescopes have the potential to improve seeing over ground-based telescopes, but are
compromised by their instability. A one-meter telescope in the Earth's stratosphere could achieve diffraction-limited
seeing superior to the performance of any ground-based telescope in optical or UV wavelengths. If the stability issues
could be addressed, such a telescope could be used for a variety of scientific purposes, including high- resolution optical
imaging, or infrared imaging of targets that are usually precluded from ground-based systems, such as Jupiter, Saturn or
Venus. An image stabilization system was developed with the goal of maintaining the position of the image to within 0.1
arcseconds on the focal plane during image acquisition. This effort requires both deriving an error signal and applying
that error signal to a corrective element. Using a control loop with an optical reference provides a greater bandwidth than
an inertial reference and improves control of high frequency vibrations. The control feedback signal was generated by
monitoring the position of an image using a lateral effect cell. A fine steering mirror was used as a diagonal flat to
control the position of the image and correct for small disturbances in pointing. To evaluate the system, vibration was
induced in a synthetic image and the resultant motion of the image measured. The system was implemented and tested
on a 14-inch f/10 Schmidt-Cassegrain telescope. Large disturbances were attenuated by a factor of 10-100, with a noise
level of less than 2 arcseconds on the test telescope.
A terrestrial stratospheric telescope is ideally suited for making infrared observations of Venus' night hemisphere during
inferior conjunctions. The near-space environment at 35 km altitude has low daytime sky backgrounds and lack of
atmospheric turbulence, both of which are necessary for observing Venus' night side at the diffraction limit when Venus
is close to the Sun. In addition, the duration of the observing campaign will be around 3 weeks, a time period that is
achievable by current long duration flights. The most important advantage, however, will be the ability of a balloonborne
telescope to clearly image Venus' night side continuously throughout a 12-hr period (more for certain launch site
latitudes), a capability that cannot be matched from the ground or from the Venus Express spacecraft currently in orbit
around Venus. Future missions, such as the Japanese Venus Climate Orbiter will also not be able to achieve this level of
synoptic coverage. This capability will provide a detailed, continuous look at evolving cloud distributions in Venus'
middle and lower cloud decks through atmospheric windows at 1.74 and 2.3 μm, which in turn will provide
observational constraints on models of Venus' circulation.
The science requirements propagate to several aspects of the telescope: a 1.4-m aperture to provide a diffraction limit of
0.3" at 1.74 μm (to improve upon non-AO ground-based resolution by a factor of 2); a plate scale of 0.1" per pixel,
which in turn requires an f/15 telescope for 13 μm pixels; pointing and stability at the 0.05" level; stray light baffling; a
field of view of 2 arc minutes; ability to acquire images at 1.26, 1.74 and 2.3 μm; and ability to operate aloft for three
weeks at a time. The specific implementations of these requirements are outlined in this paper. Briefly, a 1.4-m
Gregorian telescope is proposed, with stray light baffling at the intermediate focus. A three-stage pointing system is
described, consisting of a coarse azimuthal rotator, a moderate pointing system based on a star tracker and ALT/AZ
gimbals, and a fine pointing system based on analog photodiodes and a fine steering mirror. The science detectors are
not discussed here, except to specify the requirement for moderate resolution (R > 1000) spectroscopy.