The scope of this project enabled the generation of a multitude of possible architectures and the downselection to and maturing of two reference designs, which were explored in detail, although many important facets of each design were either left for future work or estimated. In some cases, insufficient data was available for detailed analysis; in other cases the reference designs required a higher level of maturity to define completely an aspect of the design.
Continued work on these two reference designs is necessary for further maturity, and this section discusses certain areas which have been identified as meriting particular attention.
10.1 Further Design Trades
10.1.1 LIRA
The LIRA telescope has the potential for breakthrough astronomy, and its compact and lightweight design would allow it to be transported in the planned Constellation vehicle architecture. The concept, however, requires further analysis in three key areas.
Deployment
Autonomous deployment of 215 clusters of radio dipoles would require a sophisticated system of rovers, communications, and planning. Deployment by human power alone would likely strain the abilities and usefulness of astronauts on the Moon, and even automated deployment represents a stretch for existing capabilities. The design of a capable deployment rover, and the tradeoffs of complexity and cost between the rover and other elements of the telescope concept are important issues.
Optical communications
A second major consideration is the transmission of high data rates using laser relays on the far side of the Moon. The main objective is to avoid emitting radio waves, which may interfere with the astronomical measurements being done; it was deemed desirable to keep the far side of the Moon radio-quiet. This drove the design to include an optical relay system, the cost of which was found to be less than that of an orbiting satellite. However, the Technology Readiness Level of a laser-relay communications system such as this is still low, and further investigation into this problem is suggested to justify the choice or generate alternatives for the communications system.
Location
A significant driver in cost will be the determination of where to put the LIRA telescope.
First, the attenuation of radio noise from Earth needs further study to determine the distance behind the limb necessary to achieve the sensitivity required for the primary science goal, observing the Epoch of Reionization. This choice impacts areas such as communications, power, and deployment, with the effects of location selection spiraling into the resultant cost.
10.1.2 LIMIT
The LIMIT array presents strengths in its modularity and evolutionary features. Building on the proven capabilities of the Spitzer optics, the telescope elements would be well suited for phased transportation to the facility site on the lunar surface, where growing array sizes allow increased performance with marginal costs. Some areas of remaining concern are:
Beam combination
Whereas other subsystems have had adequate development in other contexts, the system of beam combining for the Golay array would require further research and study. The need for an optical delay line on the same order as the array baseline implies the need for a system
consisting of a moving trolley and/or an optical switchyard. If fiber optics are suitable for this use, a series of paths with different lengths and the ability to switch between them would allow the physical distance of trolley movement to be reduced to something on the order of a meter instead of tens of meters. This central beam combining unit is a primary area for additional work.
Final array configuration
The array configuration should be reassessed primarily in establishing its size and layout.
The cost of building the array larger will limit the ultimate performance, but it is expected that effective diameters as large or larger than planned concepts can be achieved by the LIMIT observatory at a much lower cost. The flexibility in deployment allows for the incremental build up of performance over time. With this flexibility, it may be desirable to consider not a Fizeau interferometer design, but a long-baseline nulling interferometer design. While there are limitations to being on the lunar surface for something aimed at finding extrasolar planets (a space-based TPF mission would have several advantages), the stable surface allows precise metrology without the need for active station-keeping. The LIMIT concept as presented has focused on achieving numerous science goals with a Fizeau imaging array, but other
configurations are possible, and this would depend heavily on the progress of the TPF and SAFIR missions. The development (or lack thereof) of TPF and SAFIR would then clarify the niche of the LIMIT concept.
Thermal and dust issues
The operation temperatures for particular wavelengths of interest should be considered more closely, particularly in determining the maximum allowable temperature for the telescope optics. It is desirable to allow for passive cooling of the mirrors, since that reduces weight, cost, complexity and risk. The permanently shadowed regions of the Shackleton Crater reach very cold temperatures, but the work done in this study has not been able to confirm that the design includes viable passive cooling to temperatures around 10 K. An unavoidable issue for Moon-related designs which is particularly relevant to a lunar telescope is dust, which will complicate the design of many components. Further means for mitigating the detrimental effects of moon dust should be considered in future work.
Other issues
Other work to be done would include optimizing the positioning of telescope elements to achieve the best performance over different viewing angles and for various targets (also with consideration of using the Moon's rotation to improve non-optimal configurations), improving the power transmission over about a 10 km distance to reduce the weight of the system, assessing future detector development for increased sensitivity, redesigning a more compact telescope support structure, and outlining the logistics of deployment with consideration of available human and robotic resources.
10.2 Other Future Development
10.2.1 Data gathering on the lunar environment
In order to conduct the detailed trade studies described in Section 10.1, more data on physical conditions prevalent on the lunar surface will be necessary. The manned lunar exploration program and the Lunar Reconnaissance Orbiter mission may provide key data that will be used to select exact locations for the proposed telescope reference design concepts, as well as to inform the proposed trade studies and allow for a better assessment of the detailed technical issues relevant to telescope design, such as dust mitigation.
10.2.2 Integration into planned science programs
Although there currently exist plans to carry scientific equipment to the Moon as part of the returned human presence there, the payloads are distributed between missions and do not include allowances for the placement of a major telescope on the lunar surface. However, the ability to place, maintain, and upgrade such a telescope, which will leverage the unique advantages of the lunar surface and deliver valuable information in previously-unexplored realms of astronomy, is itself a significant justification for the return to the Moon.
Evaluation of the astronomical targets which can be observed from given locations on the Moon, as well as assessment of the expected flows through the stakeholder value delivery network model developed in Section 3, will provide further weight to the case currently presented for returning to the Moon. Further detailed comparisons with other existing or planned space telescope programs may also be informative. If the case can be presented convincingly, the proposed telescope designs can be integrated into the overall plan and launch manifests for a lunar campaign.