Timothy Giblin, i2 Strategic Services LLC; Chia-Hsiang Shen, US Air Force Academy; Benjamin Roth, US Air Force Academy; Luke King, United States Air Force Academy; David Strong, Strong EO Imaging, Inc.; Francis Chun, USAF Academy
Keywords: limiting magnitude, telescope specifications, telescope capability
Abstract:
The United States Air Force Academy (USAFA) suite of telescopes consists of fourteen total telescopes: the Falcon Telescope Network (FTN), and the USAFA one-meter telescope. The limiting magnitude of these telescope systems determines their capability to support specific astronomical and space domain awareness (SDA) research objectives. The limiting magnitude characterizes the brightness of the faintest object detected in an image for a given signal-to-noise ratio (SNR). In this paper we describe the process to determine the systems limiting magnitude and report results for the USAFA 1-m ASA telescope, the decommissioned USAFA 0.4-m DFM telescope, and the Falcon Telescope Network.
The USAFA FTN consists of twelve 0.5-m f/8 RitcheyChrétien telescopes positioned strategically around the globe in Colorado, Pennsylvania, Australia, Chile, and Germany. The purpose of the FTN is to support cadet space research and education at USAFA in the areas of space domain awareness and astronomy. For multi-color digital imaging capability, the FTN telescopes are equipped with 1024 × 1024 Apogee CCD cameras and BVRI broad-band Johnson-Cousin filters. The USAFA observatory houses a one-meter f/6 ASA RitcheyChrétien telescope system, installed in late 2019. The system became operational in early 2022 and is still in the commissioning phase. This system is equipped with a large format 9216 × 9232 Spectral Instruments multi-sector CCD camera, UBVRI broad-band Bessell- Kron Cousin filters, polarization filters, and transmission grating filters. The smaller USAFA 0.4-m f/8.2 DFM RitcheyChrétien telescope, equipped with a 1024 × 1024 Apogee CCD camera and BVR broad-band Johnson-Cousin filters, was decommissioned in early 2023 and replaced with a 0.5-m Falcon Telescope, currently in the commissioning phase.
A limiting magnitude study was performed by USAFA cadets on the DFM 0.4-m telescope during the spring 2022 semester by observing multiple Landolt Standard Fields (SA98 SF1, SA98 SF2, and SA26). A limiting magnitude of R = 16.1 was determined for a SNR of 10 with an exposure time of 250 seconds. This result serves as a quantitative comparison for the USAFA 1-m and Falcon 0.5-m results presented in this paper because, to first order, the limiting magnitude depends on the diameter of the telescopes primary mirror.
To determine the limiting magnitude of the USAFA 1-m and 0.5-m Falcon, we observed multiple Landolt Standard Star Fields (SA98, SA26, and GD363) close to the zenith (to minimize air mass) through BVR broad-band filters with a series of increasing exposure times. Standard image reduction techniques were applied to produce the reduced science images for analysis. Source extraction was performed by using the DAOPHOT algorithm on each image for a range of user-specified SNR values. Instrumental magnitudes were determined via standard aperture photometry and calibrated using the known Landolt standard stars magnitudes in each field. Performance curves for each field were generated to determine the apparent magnitude for a given photometric uncertainty. The apparent limiting magnitudes with 10% uncertainty for the USAFA 1-m telescope were determined as follows: B = 18.6, V = 18.4, and R = 18.2 at a SNR = 5 and an exposure time of 30 seconds. A stacked image with an effective exposure time of 300 seconds produced a limiting magnitude of R = 19.4. These results are consistent with prior results from the USAFA DFM 0.4-m telescope. At the time of this writing, limiting magnitude analysis for the 0.5-m Falcon telescope is still in progress.
Although each of the twelve FTN telescopes consists of the same hardware, the site conditions at the various locations can affect the limiting magnitudes of each telescope. The Falcon hardware is very similar to that of the USAFA 0.4-m hardware. Thus, we expect the limiting magnitude of the FTN telescopes to be slightly fainter magnitude than ~16.1 since their diameter is 1.25× larger. Each Falcon telescope will have slightly different limiting magnitudes due to site characteristics.
The limiting magnitude of a telescope drives research goals for that telescope. If a research target is dimmer than a telescopes limiting magnitude, the target will not be detected for a prescribed statistical significance. For example, the optical afterglow of a Gamma Ray Burst (GRB) is known to exhibit a light curve that generally decays with time as a power-law. Therefore, there will come a point (typically on the timescale of hours) when the GRB afterglow brightness will fall below the detection limit for a specific telescope. Knowing the limiting magnitude allows observations to be scheduled with a specific cadence before the source drops below the detection limit. In space domain awareness, cislunar satellites can be difficult to observe due to their proximity to the Moon. Thus, it would be a waste of precious telescope time to observe a cislunar satellite if its magnitude is fainter than the telescopes limiting magnitude. Knowledge of the limiting magnitude of USAFAs ground-based telescope assets therefore will maximize research productivity and operations. Note that the global network of Falcons telescopes may help to relax this constraint. The determination of the limiting magnitudes of the USAFA 1-m and 0.5-m FTN telescopes will undoubtedly help to maximize research productivity and operations at USAFA.
Date of Conference: September 19-22, 2023
Track: SDA Systems & Instrumentation