Appendix C: Far Side Astronomical Observatory

Appendix C. The Eye of the Beholder

C1. Advantages of Lunar Telescopes 

With telescopes, big is beautiful. Giant eyes on the sky reveal the romance of the cosmos. 

Yerkes Observatory in Williams Bay, Wisconsin, has the world's largest refracting telescope. Built in the 1890s, this giant spyglass is a direct descendent of Galileo's first telescope, with a 40-inch diameter lens in a 60-foot-long tube. 

My father worked both at Yerkes ("the birthplace of modern astrophysics") and at McDonald Observatory located in the Davis Mountains of Texas, where he helped install the 2.1-meter (82-inch) Otto Struve Telescope in 1939. [At the time, it was the second largest in the world.] Currently, the Gran Telescopio Canarias (GTC, at 10.4 meters) in the Canary Islands is the world's largest single-aperture optical telescope, but several larger telescopes, each with unique advantages, are in development. 

There are several advantages to locating a large observatory on the Far Side of the Moon. 1) Instrument weight is reduced (due to the Moon's weaker gravity) allowing increased size. 2) Near vacuum conditions provide undistorted viewing at all wavelengths, without atmosphere or weather. 3) The Moon's slow rotation to reveals the entire sky. 4) The Moon shields the telescope from light pollution and radio interference from Earth (and, during half of each month, from the sun). 5) The lunar surface is stable for long-baseline interferometric instruments (although there are occasional Moonquakes and regular Moon tides). 

Space telescopes (Hubble, Kepler, James Webb, etc.) offer some of the same advantages, with the exception of the last three. In addition, assuming a long-term Moon colony, lunar telescopes can be serviced and upgraded (difficult for Hubble, impossible for James Webb). 

C2. Larson Optical Telescope at Far Side 

The design of the lunar optical telescopes in the novel was inspired by the Keck Telescope near the summit of Mauna Kea in Hawaii. Keck has twin 10-meter telescopes, separated by 85 meters, which can operate together to perform high-resolution imaging. An astronomical interferometer is housed in the building between the two domes. This enables the two telescopes to operate as a single 85-meter diameter telescope. 

The fictional 100-meter Larson twin telescopes on Far Side are separated by 2,800 kilometers (the diameter of the Moon is 3,474 kilometers). The goal is to combine images from the two telescopes giving an effective aperture of 2,800 kilometers. Using the Dawes limit formulae, this very long baseline allows a theoretical optical acuity of .33 millimeters at L1 (which is about 1.2 million kilometers from closest approach of the Moon), 30 centimeters on Mars (about the size of a footprint, depending on the relative position of Mars), and about 20 kilometers on 16 Psyche in the asteroid belt. 

This extreme resolution is a technological challenge. The long baseline separation depends on a fiber optic interferometer between the two telescopes. The OHANA Project (Optical Hawaiian Array for Nanoradian Astronomy) on Mauna Kea is pioneering this technology. Baseline length compensation for Moon tides (squeezing and stretching of the Moon) is also necessary. 

Other details of the Far Side telescopes are not described in the novel. In addition to adaptive optics (A0 — micropositioning of primary mirror segments to compensate for lunar gravity), a proper astronomical dome over each optical telescope makes sense. Why, when there is no atmosphere? "Space weathering" from the solar wind and human-made particles can corrode mirrors and sensors. Shielding from the sun will improve imaging in the near infrared (as well as limit thermal expansion). Also, Moon dust lifted by electrostatic levitation (ionized by solar bombardment) may interfere with viewing. 

Adding more telescopes will increase light gathering power and sensitivity. However, the optical telescopes at Far Side represent in principle the best that can be achieved, based on current technology. Multiple orbiting space telescopes with even longer baseline separations may improve on lunar observatories. 

As a final note, a large infrared telescope could be located in Shackleton Crater at the lunar South Pole, where it could be kept cold. It would not see the Solar System or the Milky Way, since the field of view would be perpendicular to the ecliptic. For these reasons, it was not included in the novel. 

C3. Gordon Radio Telescope at Far Side 

Radio astronomy provides the highest resolution images in all astronomy. The Very Long Baseline Array (VLBA) is the world's largest dedicated, full-time astronomical instrument using the technique of long baseline interferometry. The combined telescope consists of ten 25-meter-diameter antennas stretching from Mauna Kea, Hawaii to St. Croix, Virgin Islands. The VLBA has a total collecting area of .02 sq km and the longest baseline is 8,611 km. Sensitivity is related to collecting area and image resolution is related to aperture width (baseline length). 

In contrast, the world's largest operational radio telescope is the venerable Arecibo Observatory (Puerto Rico), with a single dish that is .305 kilometers in diameter with a collecting area of .292 square kilometers. In principle, Arecibo can be 15 times more sensitive than the VBLA, but makes images with far less resolution. Thus, Arecibo will be better at picking up very weak alien transmissions but may not be able to locate them precisely. 

The Chinese have begun construction of the Five-hundred-meter Aperture Spherical Telescope (FAST), which will have a collecting area more than twice the size of Arecibo. FAST is designed to change its shape from a sphere to a paraboloid, which will allow the telescope to cover 40 degrees from the zenith, compared to the 20-degree-wide strip covered by Arecibo. However, FAST will only be sensitive to 5 GHz, whereas Arecibo’s bandwidth stretches up to 10 GHz. 

The most advanced radio telescope design is the Square Kilometer Array (SKA), planned for both South Africa and Australia. The SKA will have receiving stations extending out to a distance of 3,000 km from a concentrated central core (6,000-km-diameter baseline). The SKA will provide continuous frequency coverage from 70 MHz to 30 GHz with a wide field of view; it will also allow multiple users to observe different pieces of the sky simultaneously. With a capture area of one square kilometer, the SKA will be, in principle, about 3.5 times more sensitive than Arecibo. However, with advanced signal processing on multiple frequencies, the SKA specification claims 50 times more sensitivity than any other radio instrument (although with less angular resolution than the VLBA). 

In this novel, the fictional Gordon Radio Telescope is named after William E. Gordon, the engineer who conceived, designed, built and operated Arecibo. The concept behind both Arecibo and the Gordon is to build the biggest dish (capture area) to achieve the greatest sensitivity. 

At the equator of the Far Side of the Moon is Daedalus Crater, with a diameter of 93 kilometers. If this dish-shaped depression were lined with a reflective metallic screen, the capture area would be 27,157 square kilometers. This gives Daedalus Gordon Telescope a basic sensitivity of more than twenty-seven thousand times the SKA. Being on the Far Side of the Moon, it is shielded from radio noise from Earth (and for half of each month it is shielded from radio interference from the sun). 

Of course, this monster dish needs to position the radio receivers in the focal point of the dish, which is 49 kilometers above the crater (modeled on Arecibo). [N.B. Both Arecibo and the Gordon are not parabolic dishes; they are sections of the inner surface of a sphere. Both require a movable secondary dish aloft to focus and steer signals.] 

The best plan to suspend receivers above the main Gordon dish is to use a "Moonstalk," a space elevator ribbon suspended from a satellite anchored beyond the Earth-Moon Lagrangian L2 point above Far Side. In the novel, the alternative to a Far Side space elevator is a Rube Goldberg-esque contraption called the Juggler, which uses a cascade of iron cannonballs to transfer momentum to the receivers, keeping them aloft. Both schemes are possible but difficult. 

© G.B. Immega 2014