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.
The Gondola for High-Altitude Planetary Science (GHAPS) project is a balloon-borne astronomical observatory designed operate in the UV, Visible, and near-mid IR spectral region. The GHAPS Optical Telescope Assembly (OTA) is designed around a one meter aperture narrow field-of-view telescope with near-diffractionlimited performance. GHAPS will utilize Wallops Arc-Second Pointing System (WASP) for pointing the OTA with an accuracy of 1 arc second or better. WASP relies heavily on a self-contained star tracker assembly to determine the OTA line of sight. Preliminary structural analysis indicates that potential misalignments could be present between the OTA line of sight and the star tracker FOV center during the expected flight conditions that could compromise GHAPS pointing accuracy. In response the GHAPS project undertook a trade study to resolve the following issues: (1) estimate the worst case long-term (or bias) pointing misalignments for the GHAPS opto-mechanical configuration, (2) examine the need for additional hardware to correct pointing errors, and (3) determine the best hardware and software implementation to do so. Quantitative comparisons of performance and qualitative estimates of other factors such as mass, volume, power consumption, and cost are combined into an overall assessment of potential solutions. Results are discussed and a recommended implementation is given that is optimized to best achieve pointing performance goals, while minimizing impact to the design, cost, and resources of the GHAPS project.
Fiber Bragg gratings (FBG) have become preferred sensory structures in fiber optic sensing system. High sensitivity, embedability, and multiplexing capabilities make FBGs superior to other sensor configurations. The main feature of FBGs is that they respond in the wavelength domain with the wavelength of the returned signal as the indicator of the measured parameter. The wavelength is then converted to optical intensity by a photodetector to detect corresponding changes in intensity. This wavelength-to-intensity conversion is a crucial part in any FBG-based sensing system. Among the various types of wavelength-to-intensity converters, unbalanced interferometers are especially attractive be-cause of their small weight and volume, lack of moving parts, easy integration, and good stability.
In this paper we investigate the applicability of unbalanced interferometers to analyze signals reflected from Bragg gratings. Analytical and experimental data are presented.
The development of integrated fiber optic sensors for smart propulsion systems demands that the sensors be able to perform in extreme environments. In order to use fiber optic sensors effectively in an extreme environment one must have a thorough understanding of the sensor’s limits and how it responds under various environmental conditions. The sensor evaluation currently involves examining the performance of fiber Bragg gratings at elevated temperatures.
Fiber Bragg gratings (FBG) are periodic variations of the refractive index of an optical fiber. These periodic variations allow the FBG to act as an embedded optical filter passing the majority of light propagating through a fiber while reflecting back a narrow band of the incident light. The peak reflected wavelength of the FBG is known as the Bragg wavelength. Since the period and width of the refractive index variation in the fiber determines the wavelengths that are transmitted and reflected by the grating, any force acting on the fiber that alters the physical structure of the grating will change what wavelengths are transmitted and what wavelengths are reflected by the grating. Both thermal and mechanical forces acting on the grating will alter its physical characteristics allowing the FBG sensor to detect both temperature variations and physical stresses, strain, placed upon it. This ability to sense multiple physical forces makes the FBG a versatile sensor.
This paper reports on test results of the performance of FBGs at elevated temperatures. The gratings looked at thus far have been either embedded in polymer matrix materials or freestanding with the primary focus of this paper being on the freestanding FBGs. Throughout the evaluation process, various parameters of the FBGs performance were monitored and recorded. These parameters include the peak Bragg wavelength, the power of the Bragg wavelength, and total power returned by the FBG. Several test samples were subjected to identical test conditions to allow for statistical analysis of the data. Test procedures, calibrations, and referencing techniques are presented in the paper along with directions for future research.