Ted A. Reed, US Air Force Academy; Markus D. Parrish, US Air Force Academy; Joshua A. Key, US Air Force Academy; Charles J. Wetterer, KBR; Philip J. Castro, Applied Optimization, Inc.; David M. Strong, Strong EO Imaging, Inc.; Casey P. Schuetz-Christy, US Air Force Academy; Francis K. Chun, US Air Force Academy
Keywords: Falcon Telescope Network, GEO Satellites, Space Situational Awareness, Space Domain Awareness
Abstract:
The importance of space domain awareness has increased significantly with the growing reliance of satellites in our technological infrastructure. The United States Air Force Academy (USAFA) has been collecting spectral signatures to characterize satellites since 2014 using a 16-inch f/8.2 telescope (USAFA-16) on campus as well as its off-campus telescopes that comprise the Falcon Telescope Network (FTN). The FTN consists of eleven identical 20-inch, f/8 Officina Stellare ProRC-500 telescopes located at Sterling, CO, La Junta, CO, Durango, CO, Yoder, CO, Woodland Park, CO, Grand Junction, CO, State College, PA, Canberra, Australia, Gingin, Australia, Vicuna, Chile, and Braunschweig, Germany. The filter wheel contains B, V, R band Johnson-Cousins filters and a 100 lines-per-millimeter diffraction grating for low-resolution slitless spectroscopy.
Before using the FTN for satellite spectral observations, the diffraction grating used in each FTN telescope was spectrally calibrated by determining the pixel-to-wavelength conversion for each site. Known emission and absorption features from calibration stars are collected and these features are used to build a relationship between pixels and wavelength; thereby, observations of satellite spectra can be displayed as a function of wavelength. Approximately half of the telescopes in the FTN have had their spectrum calibrated and are compared in this paper.
We automated, optimized, and standardized the manual steps within the processing pipeline to ensure the repeatability and improve the accuracy of the resulting spectra. First, we implemented a method to determine spectral features on a sub-pixel level using the Kwee van Woerden method, increasing the precision of our pixel-to-wavelength calibration. Second, we are investigating a new method for generating spectroscopic flat-fields to further improve its accuracy and the resulting accuracy of the satellite spectra. A method for spectroscopic flat-field correction was already established, but this paper describes a new and novel method for obtaining and applying a flat-field. Data were collected on an A-type star so that its zero and first-order spectra were allowed to drift across the entire length of the image. This process was repeated for different vertical positions to ensure that all pixels received a spectral band, and all positions were then repeated multiple times to average out atmospheric scintillation. All of the images were then compiled and averaged to build the spectroscopic flat-field. Third, the atmospheric extinction theory has been developed and implemented. Calculating the atmospheric extinction involves observing a standard star throughout the night as its airmass varies. Fourth, we implemented solar analog normalization as a final calibration step. This involves observing solar analogs (e.g., G2V type stars) and normalizing the satellite spectra by the average solar analog spectra. This process removes the solar component of the reflectance spectra, leaving only the satellite component. These calibration enhancements are integrated into a new spectroscopic pipeline to process spectral signatures of geosynchronous satellites.
Additionally, a solar analog analysis was performed to assess the spectral type of stars used for solar analog normalization and attempt to constrain what spectral types are valid for this type of calibration. The spectra of solar-type stars (spectral types near G2V) were collected, extinction corrected, used to normalize a GEO, and the resulting solar analog normalized spectra were compared.
DISTRIBUTION STATEMENT A: Approved for public release: distribution unlimited.
DISCLAIMER: The views expressed in this article, book, or presentation are those of the author and do not necessarily reflect the official policy or position of the United States Air Force Academy, the Air Force, the Department of Defense, or the U.S. Government.
PA#: USAFA-DF-2022-125
Date of Conference: September 27-20, 2022
Track: Non-Resolved Object Characterization