A.

Aperture Size vs. Mass

In an effort to optimize the configuration of a flight transceiver, we undertook an extensive trade study, considering the size, form and materials for such a transceiver. We began with an estimation of the largest flight transceiver aperture that might reasonably meet our 5 kg mass limitation. We analyzed a proprietary database of flight optical systems maintained by Optical Research Associates, and based on particular thermal, wavefront quality, alignment, a material considerations, derived the ‘Top Down’ parametric curves shown in Fig. 1. Two curves are shown: one for systems for which standard lightweighting practices were applied, and a second on which more aggressive lightweighting was used. The mass of our system was assumed to be bounded by these two curves, but was expected to come closer to the aggressive lightweighting curve shown. As a check of this ‘top down’ approach, we developed a component-based model, in which the various components of an optical communications transceiver were identified, and the size of each component was parametrically scaled to the instrument’s aperture size. The mass of the component was then estimated based on a database of material parameters (including material density and allowable thickness-to diameter aspect ratios for flight systems). The calculated masses of all of the components were then summed to arrive at the particular masses shown at discrete points in Fig. 1. The envelope of these points closely matches the standard lightweighting curve, both in parametric form and absolute magnitude, which is not surprising since it was based on standard aspect-ratio rules-of-thumb for flight materials. Note that these data are specific for the application, constraints, and expected operational environment of a deep-space optical communications transceiver, and may not be appropriate for other types of optical instrumentation. The 5 kg mass allocation can be expected to limit the flight system’s aperture to about 22 cm, given the use of aggressive lightweighting techniques.

Fig. 1

Comparison of top-down parametric study of flight system mass as a function of aperture (continuous curves), ‘bottom-up’ component-based model (discrete points)

B.

On-axis vs. Off-axis configuration

We next set out to determine whether the transceiver should employ an on-axis or off-axis system. Telescopes usually make use of on-axis optical systems; the small areal obscuration of the secondary mirror and support structure is generally a small price to pay for the reduction in volume and concomitant reduction in mass afforded by having light traverse the same volume multiple times. A deep-space optical communications system is different from most telescopes though, in that the performance of the system is not simply related to the collection area of the aperture, as it is with most telescopes, but is more related to the ability of the optical system to efficiently concentrate the downlink signal on the target receiver. A central obscuration results in a diffraction pattern with a narrower central lobe [1], exacerbating the already daunting tasks of accurate pointing and platform jitter rejection, and throws more of the energy into the outer rings of the diffraction pattern, where it is not useful. Finally, the central obscuration removes the 'sweet spot’ of the Gaussian downlink beam [2], completely removing it from the transmitted beam, and turning it into a potential liability by reflecting it back toward the laser system.

If the central obscuration were a very small fraction of the aperture, the on-axis advantages of mass and volume might still dominate the aforementioned effects; however, an optical communications transceiver is also different from most telescopes in that it must maintain operation while pointing near the Sun. This requires extra shielding and baffling to protect the transceiver and prevent significant amounts of stray light from overwhelming the faint pointing beacon transmitted from Earth. This extra baffling generally results in a relatively high linear obscuration ratio (D_secondary/D_primary) of 30-35%. Given this, and the antennal gain analysis for a Gaussian beam laid out in [3], the trade can easily be performed, as shown in the nomogram of Fig. 2.

Fig. 2

Nomogram for computing mass effects of on-axis system’s obscuration. The obscuration generates a system loss, which requires an aperture increase to compensate, resulting in an overall mass gain of 70%.

This analysis uses the effective far-field gain and mass analysis summarized in Fig. 1 to demonstrate that a near-Sun-pointing optical communications transceiver would have to grow in aperture by over 22% to compensate for the large obscuration, resulting in a total mass increase of almost 70%. This mass increase is roughly 3 times larger than the estimated mass savings afforded by an on-axis system. Inverting the analysis, we estimate that for the trade to favor the on-axis system, the linear obscuration ratio would have to be less than 0.23, a difficult task for a near-Sun operational system.

C.

Cassegrain vs. Gregorian form

Cassegrain-type telescopes in which the secondary mirror is a negative element placed before the telescope’s prime focus are more common than Gregorian systems where the positive secondary mirror is placed beyond prime focus because they provide a significantly more compact system. This compact system results in significant cost savings for ground-based telescopes because the telescope dome can be much smaller and the telescope’s superstructure can be much smaller and lighter. These same advantages generally apply to a space-based flight transceiver architecture as well.

However, once again the requirement to point close to the Sun alters the trade considerably. The accessible focus of the telescope’s primary mirror provides an ideal location for an early field stop, the advantages of which are difficult to overstate. This field stop completely prevents illumination of the secondary mirror by direct sunlight, down to angles far below the minimum operational angle of the system. While the primary mirror is still subjected to full-Sun exposure, this exposure is not nearly as critical as exposure of the secondary mirror to sunlight is.

The secondary mirror of a telescope pointing near the Sun is subjected to significantly higher irradiances; while the primary mirror may be exposed to approximately 0.014 W/cm² from direct Sun at 1AU, the focusing power of the primary mirror will result in irradiances as much as 25-100 times higher on the secondary mirror. This level of irradiance can result in significant thermal distortion to the secondary surface, rapidly resulting in significant aberration to the observed uplink beacon and the finely-tuned downlink. Given the tight wavefront error requirements, it is imperative to prevent Sunlight focused by the primary mirror from depositing significant amounts of heat in the secondary mirror.

A second reason to avoid direct illumination of the secondary mirror with light focused by the primary mirror is that this light will result in significantly more scattering at the uplink sensor detector than that produced directly by the primary mirror. Though the primary mirror is generally considerably larger than the secondary mirror, the concentration of the light by the primary mirror generates roughly the same amount of total scattering from both surfaces. However, the secondary mirror is optically much closer to the detector than the primary mirror, resulting in a significantly larger effective solid angle presented by the detector. We estimate that for a typical optical transceiver system, scattering from the secondary mirror generates approximately 25 times the amount of scattered light at the detector as the same total amount of light scattered from the primary mirror.

Introducing a field stop prior to the secondary mirror almost completely eliminates secondary mirror scattering, thus enabling near-Sun operations. It also acts as a first line of defense against damage produced by accidental direct-Sun pointing. A deep-space optical communications terminal can operate with a required field of as little as 1 mrad, approximately 1/10^th the angular subtense of the disk of the Sun when observed from 1 AU. Implementing a Gregorian collecting telescope with a 1 mrad field stop in a transceiver can therefore limit the total amount of sunlight transmitted on to the remaining optical channels to about 1% of the power collected by the primary mirror, even during direct-Sun pointing.

D.

Telescope Materials

Achieving light weight optical designs which can operate over the wide range of thermal loading conditions to which a deep-space optical transceiver will be subjected requires careful consideration and selection of candidate materials. To that end, we considered several different candidate materials for inclusion in the system.

The choice of material was driven largely by the two overriding concerns repeated throughout this paper: system mass and near-Sun operation. Reducing system mass (while maintaining optical alignment and optical wavefront requirements) requires material with very high specific stiffness (ratio of Young’s modulus to density). Maintaining operations under different solar loading conditions requires low transient thermal distortion. These parameters for various optical materials are shown in Fig. 3. Ideal candidate materials would fall within the triangular region shown at the upper-left portion of the graph. While ULE is slightly better in transient distortion, and Beryllium is better in specific stiffness, SiC is the best material for meeting both requirements simultaneously.

Fig. 3

Specific stiffness and transient distortion plotted for various candidate materials. The system requirements drive the choice of materials within the magenta triangular region.