INTRODUCTION

Optical Telescope Assemblies (OTAs) implemented in silicon carbide (SiC) provide performance advantages for space applications but have been predominately implemented in the government sector. A new generation of lightweight and thermally-stable designs is commercially available, expanding the application of SiC to small satellites.

One of the major challenges in satellite telescope design is the ability to maintain performance in the thermal environment of low-earth orbits (LEO). Thermal stability analyses of two similar OTA’s designed by AOS, one of Silicon Carbide and one of Aluminum with Glass mirrors, are compared in this application note. The effect of temperature changes under soak conditions on ground resolved distance (GRD) are explored using image analysis.

SiC possesses the highest combination of specific stiffness (E/ρ) and thermal stability (k/α) of any optical grade material. These properties make SiC ideal for maintaining optical and mechanical performance throughout launch and in the dynamic thermal environment of low-earth orbit (LEO).

Figure 1: Material Properties & Calculated Specific Stiffness vs. Thermal Distortion Coefficient for commonly used mirror materials. Room temperature properties of POCO Graphite SuperSiC-Si, 6061-T6 Aluminum and Corning HPFS Fused Silica

SiC vs. TRADITIONAL AL-GLASS TELESCOPES

Small satellite OTAs are typically required to meet optical performance specifications across a temperature range of approximately -30° C to +40°C for LEO applications.  The analysis performed explores the impact of focus and GRD as a function of temperature. The telescope design used in the analysis is a two mirror, reflective system with a clear aperture of 125-mm designed to enable GRD ≤ 7.5-m for NIR wavelengths at an altitude of 500-km.

Figure 2 and 3: Cross-sectional view of AOS 125-mm telescope design

Point spread function (PSF) plots are shown at -30, -20, +20 and +40°C for SiC and Aluminum-Glass telescopes. A narrow PSF corresponds to less image blur at the detector. The dominant impact to the telescope resulting from change in temperature is displacement in the optics which causes defocus (and thus image blur at the detector). Figure 12 illustrates the comparative focal shift in the SiC and Aluminum-Glass systems. Image blur is illustrated by the PSF resulting from the various shifts in focal length (Figure 4 – 11). The impact on GRD is then calculated. (Figure 13). This difference in thermal soak sensitivity is also an indicator of the relative sensitivity to thermal gradients, which are much more challenging to correct for in low-earth orbits.

Figure 13: Ground resolved distance (GRD) versus temperature for SiC and Glass-Aluminum telescopes from -30 to + 40°C.

CONCLUSIONS:

• SiC system maintains the designed GRD over the typical required temperature range  (± 35°C).

• Aluminum-Glass system performance rapidly decays, even after ± 2°C, from its optimal GRD.

Thermal properties of SiC allow for telescope systems that outperform Aluminum-Glass telescope. In soak conditions, SiC focus shift is nominally zero. Furthermore, SiC  shows up to 37x better performance across the temperature range for ground resolved distances over Aluminum-Glass telescopes in thermal soak conditions. Real-world scenarios present more complex challenges that reveal even more extensive benefits of optimized material selections.

AUTHORS:

Dave Aikens; Savvy Optics Corp.,  Chester, CT 06412

Kevin Dahlberg, Chip Ragan, Flemming Tinker; Aperture Optical Sciences Inc., Meriden, CT 06450

REFERENCES:

1. K. J. Kasunic, D Aikens, D. Swabowski, C. Ragan, F. Tinker, “Technical and Cost Advantages of Silicon Carbide Telescopes for Small-Satellite Imaging Applications”, SPIE Optical Engineering and Applications, San Diego 2017, paper #10402-11.

2. Tinker, F., Xin, K., “Aspheric Finishing of Glass and SiC Optics”, Optical Fabrication and Testing, Monterey, California United States, June 24-28, 2012, Figuring and Finishing Science (OM4D),

3. “SAGE Handbook of Remote Sensing”, T. A. Warner, M.D. Nellis, G.M. Foody, (SAGE Publications Ltd., London, 2009), 101-102.

4. F.P. Incropera, D.P. DeWitt, T.L. Bergman, A.S. Lavine, “Introduction to Heat Transfer”, (Wiley Publishing,  New Jersey, 2006) Fifth Edition.

5. SuperSiC Material Properties [Online], POCO Graphite, http://poco.com/MaterialsandServices/tabid/124/Default.aspx [22 June 2017].