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Astronomy

New architecture for space telescopes uses Fresnel lenses

A lightweight primary optic can be repeatably manufactured at a low cost for space applications.
9 August 2006, SPIE Newsroom. DOI: 10.1117/2.1200608.0333
NASA image

Progress in astronomy and astrophysics demands larger and more precise telescopes so astronomers can see details at greater distances. The rewards of placing these telescopes in orbit are evident in the extensive body of work that has been enabled by the Hubble Space Telescope (HST), which is providing access to previously inaccessible wavelengths (such as x-rays).

The HST's success suggests that an even larger space telescope would be of tremendous value. However, the cost to launch the HST was approximately $1.5 billion,1 and funding another mission is challenging. In fact, the HST ‘replacement’ is not scheduled to launch for another seven years, making it roughly 30 years between generations. Precision manufacturing and the need to lift the large, heavy primary optic contributes to most of the cost.

In order to greatly reduce the cost of a space telescope, we have developed an architecture with a deployable, transmissive Fresnel lens as the primary optic. A Fresnel lens, as shown in Figure 1, is a diffractive optic that is capable of forming a focus. It is produced by removing the ‘excess’ portions of a conventional lens and retaining the focusing profile. Face-on, the lens is made of concentric grooves whose depth and width are matched to the incident wavelength. The resulting optic has a ribbed appearance.


Figure 1. A Fresnel lens is made by removing the non-refractive part of a conventional lens, leaving the lens with a thin profile.
 

By etching or turning the Fresnel grooves on a transmissive substrate, such as flexible plastics or vinyl, a very lightweight optic can be repeatedly manufactured at very low recurring costs. This is a major advantage of this new architecture. For example, with DuPont™ Kapton® as the lens substrate, a 0.5mm thick primary optic will have an areal density of 0.71kg/m2. Compare this to the James Webb Space Telescope primary at ∼10kg/m2 for the mirror only,2 and our new architecture is an order of magnitude more mass efficient. By using a flexible plastic material, we can fold the etched primary lens into a small volume, which can be launched with a smaller launch vehicle.

Fresnel lenses are highly chromatically aberrant. Each groove is specifically tuned to one wavelength, so shorter and longer wavelengths are proportionally focused at different locations along the optical axis. Traditionally, designers corrected for chromatic abberation by adding highly precise optics, which are expensive and alignment critical.3 We have, however, developed a method to mitigate the chromatic aberration. Instead of focusing multiple wavelengths to a single location, we place the detector for a desired wavelength at its appropriate location along the optical axis (dictated by the telescope optics). In effect, we use the optical properties of a Fresnel lens to spatially separate the incident wavelengths (see Figure 2).

The trade-off for simplicity is reduced throughput. In Figure 2, for example, each detector is only seeing 25% of the total intensity. The projection of the cross-section of the detectors will also reduce the effective aperture. In our research, we compensated for reduced throughput by using a much larger aperture. In addition, we developed a method of extracting sub-band information from the composite image.4


Figure 2. In the new architecture, a deployable Fresnel lens forms the primary optical element, and detectors are placed along the optical axis to intercept the appropriate wavelength.
 

We have developed a simple space telescope architecture that is inexpensive and lightweight. By using a Fresnel lens primary etched on plastic, and by spacing out the detectors along the optical axis, we have created a system that is capable of multi-wavelength observation at a small fraction of the cost of current space telescopes. This architecture is scalable to a specific mission's performance requirements and budget. Our next steps are to manufacture and test a transmissive, foldable Fresnel lens. If we prove the architecture is feasible, it may enable the next generation of astronomers to use affordable telescopes that are orders of magnitudes larger than current ones.


Authors
Amy Lo, Jonathan Arenberg
Northrop Grumman Space Technology
Redondo Beach, CA

References:
2. JWST Science Working Group Report, January 2004. http://www.jwst.nasa.gov
3. L. Shupmann, Die Medial-Fernrohre, Eine neue Konstruktion für grosse astronomische Instrumente, 1899.
4. A. Lo, J. Arenberg, Architectures for space astronomical telescopes using Fresnel optics, Proc SPIE, Vol: 6265, pp. 626522, 2006.