The development of telescope technology has been a long process. In recent years, most large single mirrors for telescopes have been based on the traditional design of a glass base, shaped into a concave parabola, coated with a reflecting surface (typically aluminum) deposited in a evacuated vaporization chamber. The parabolic shape is often obtained by beginning with a liquid glass which is placed into a mold and spun as the glass is allowed to cool. The balance of the centrifugal and gravitational forces creates a parabola, the depth of which (and hence the focal length of the telescope) can be controlled through the rotation speed.
The major disadvantage to this design is that the glass mirror becomes excessively heavy and is unable to support its own weight well above a radius of eight to ten meters. As such, new techniques have been developed that use segmented mirrors. Another novel technique that is currently beginning to see use is that of using liquid mirrors that do not solidify. This practice has already been put into use with the Large Zenith Telescope which boasts a 6m diameter liquid mercury mirror.
Unfortunately, mercury and most other highly reflective liquids are prohibitive due to their toxic and/or unstable natures. Additionally, with the current drive to establish new locations for possible observatories, such mirrors would be required to function in new environments. One of these proposed environments would be a possible moon base. In such a case, traditional glass mirrors would be impossible, due to the high cost of transportation. Thus, a liquid mirror becomes much more practical, if an appropriate liquid can be found.
The requirements for such a moon based mirror would be that they have extremely low freezing temperatures and retain their liquid state in vacuum as well as be non-toxic and stable. Although a number of possible candidates exist, few have an intrinsically reflective nature, although Burns (2008) reports that Lithium Ammonia may be suitable. This requires that such a material be coated with a reflective surface.
Finding suitable materials that fit these criteria and attempting to coat them was the subject of a 2007 Nature paper by Borra et al. In addition to the restrictions placed on a possible telescope by the inhospitable lunar conditions, the coating process also adds additional constraints in that the coating must be smooth enough, and reflective enough to serve as a functional mirror in the desired wavelength range (infrared in this case).
Since coating of liquid surfaces had not yet been attempted, the researchers attempted to coat several different materials using a vaporization deposition resulting in films only a few nm thick. Most of these attempted coatings were unsuccessful. From this, they further refined their liquid criteria to require the liquid to have nearly zero vapor pressure and high viscosity. The ideal class of compounds they determined to be ionic liquids which are salts in liquid form at low temperatures (defined as below 373 K). The challenge was then to apply a silver coating in order to make a suitably reflective surface.
To do this, Borra et al. applied a thin film of silver nanoparticles to the surface with diameters of only a few tens of nanometers. However, the nanoparticles tended to diffuse into the liquid substrate forming a colloid, reducing reflectivity with respect to metallic silver. In order to mitigate this problem, the group first applied a chromium layer. This layer tended to bond easier and form a better surface to which the silver could then be applied. With this additional coating, Borra et al. were able to increase the reflectivity as shown in Figure 1.
Although reflectivity is as high as 80% for some wavelengths, Borra notes that this is still low for standard astronomical mirrors which often have reflectivity over 95%. In an earlier paper (Borra, 2004), it was noted that these surfaces would often begin with a higher reflectivity, which would decrease over the course of a week by ~8% and remain relatively constant thereafter. They surmise that greater stability and reflectivity may be reached with different metallic nano-coatings. Borra did not note whether or not the chromium backed silver coating had the same degradation as previously mentioned.
Aside from just being able to reflect incoming light, mirrors would also be required to have a suitably smooth surface that they would produce high quality images. To analyze this, the group mapped the topographical distribution via electron microscopy, of the produced surface (with a 5nm chromium layer and a 30 nm silver coating) and found it had a peak to valley depth of 0.0373 µm which gives an excellent optical surface (see Figure 2).
In the 2004 paper, Borra also describes the possibility that mirrors could be further shaped with the application of magnetic fields, if the mirror’s substrate was composed for ferromagnetic liquid. Although it would not be necessary in the lunar vacuum, controlled deformations have been used on telescopes to correct for atmospheric aberrations. This technique is known as adaptive optics.
However, the liquid mirror technique is not entirely without drawbacks. One of the most obvious is that the mirror must remain horizontal. Tilting the mirror’s axis would cause deformation in the topography, rendering the surface unusable for observations. In other words, the mirror must be permanently fixed on the observer’s zenith. However, telescopes such as the famous Arecibo, are also zenith based and function quite well for their particular sorts of observations. Namely, this liquid mirror telescope would be quite well suited for deep field imaging since, at large distances, a greater volume of space is shown. But to do imaging of such distant objects, exposures must be taken for a longer time. Since the telescope would be turning with the moon, this would limit the total exposure time as the object swept over the field of view. As such, several images would have to be taken and subsequently added to produce a suitably deep image. Again, on the moon, this would be less of a problem since the moon has a slow rotation rate (~28 days), allowing objects to stay in the field of view for longer times.
Although not discussed, another problem I could foresee would be a possible interruption of power. If power to the rotator would be lost, the centrifugal force would vanish, and the liquid substrate would settle back into a flattened shape. Most likely, this would result in a tearing of the nano-coating, causing the mirror to need to be entirely recoated. If the liquid were viscous enough, short power interruptions may be mitigated. Regardless, this may not be as large of a problem as it may otherwise seem, since telescope mirrors are frequently resurfaced as it is, in order to retain a fresh and dust-free surface.
Overall, the work of Borra et al. has clearly shown that there is a great deal of promise in the formation of mirrors using nano-coated ionic liquids. Even in these initial tests, they have been able to achieve surfaces which are smooth enough to make excellent surfaces. Although the surfaces are not yet reflective enough to meet the standards for astronomical instrumentation, it is likely that with further experimentation with different coatings as well as additional intermediate substrates, it is likely that this can be greatly improved. Yet while the concept has been well established, the question remains whether or not this is the most economic or feasible option for a liquid mirror. As Burns suggest, it is likely that there exist other materials that have similarly high reflectivity and would not suffer from the drawback of requiring an additional coating. For example, the liquid lithium ammonia mirror they recommend has reflectivity curves similar in quality to that of these more complicated, coated mirrors of Borra (see Figure 3).
As such, although the concept is sound, it would seem that further development and testing will be necessary before it can be determined which sort of mirror would be best for use in a lunar environment.
Borra, E.F., Seddiki, O., Angel, R., Eisenstein, D., Hickson, P., Seddon, K.R., Worden, S.P. (2007). Deposition of metal films on an ionic liquid as a basis for a lunar telescope. Nature, 447(7147), 979-981. DOI: 10.1038/nature05909
Borra, E.F., Ritcey, A.M., Bergamasco, R., Laird, P., Gingras, J., Dallaire, M., Da Silva, L., Yockell-Lelievre, H. (2004). Nanoengineered astronomical optics. Astronomy and Astrophysics, 419(2), 777-782. DOI: 10.1051/0004-6361:20034474
Burns, C.A. (2008). Is Lithium Ammonia Suitable for a Liquid Lunar Telescope?. Publications of the Astronomical Society of the Pacific, 120(864), 188-190. DOI: 10.1086/526539
McKnight, W. H., & Thompson, J. C. 1975, J. Phys. Chem., 79, 2984