Centauri Dreams – An inventor named Tom Ditto has been casting a serious look at diffraction gratings as large primary collectors for telescopes, work that has been getting a bit of a buzz on the Internet.
The new approach could enable kilometer length telescope membranes to be launched with current rockets for radically improved space telescopes.
In a presentation to the NASA Institute for Advanced Concepts back in 2007, Ditto showed that his Dittoscope could be conceptualized as a standard telescope capable of spectroscopy aimed at a flat grating. The standard telescope’s spectrometer has its own grating and slit. The use of the second spectrometer eliminates the overlapping spectra from the flat primary grating. Each object is thus imaged at a single wavelength at any unique angle of incidence. There are no moving parts other than the rotating Earth, with the instrument oriented east to west.
The ground based Dittoscope, then, takes advantage of the Earth’s rotation, as described in the presentation for Ditto’s Phase I study: “The precession of objects in the night sky causes their incident angles to rotate. For any incident angle there is a corresponding wavelength, so an entire spectrogram can be assembled over the course of a night.” With this enormous field of view — a 40 degree arc — millions of stars are placed within view simultaneously.
The roof is coming off the observatory. Gone are the domes, the sliding hatch doors and the rotating walls. A Dittoscope can lay flat to the ground. Its roof may be the primary objective. Wind resistance is negligible. The secondary optics are buried in a trough, and the ray paths can be protected within a pacified atmosphere, even a vacuum. Credit: Tom Ditto.
The other advantages that accrue from Ditto’s implementation of the idea. Because the diffracting grating primary collector is flat, many of the size constraints imposed on standard telescopes are eased, especially the requirement to contain heavy mirrors. Ditto’s paper argues that tolerance specifications for flatness in the axis of diffraction may not be prohibitive — he even talks about plate glass as a possible medium. All this points to the ability to construct enormous collecting surfaces at relatively low cost. One possibility Ditto mentions is a lunar observatory at the Moon’s equator that has no moving parts, returns detailed spectra for all objects along the zenith, and operates with a service life of decades.
Materials used in grating fabrication have proliferated over the past century to the point that diffraction gratings of very high quality can be seen everywhere. Integrated circuits are fabricated with rulings that could meet astronomical specifications for regularity and blaze. Decorative embossed gratings are made in rolls as large as 50,000 foot length by five foot width costing approximately $1 sq/ft. These include the Holosheen grating made by Spectratek. In a world where large telescope primaries cost millions per square meter, such grating materials are virtually free. Even commonplace compact discs and DVD’s are diffraction optical elements that can take coarse absorption spectra. Every hologram of a 3D surface is a diffraction grating of extraordinary complexity. A hologram of two plane waves or a point source and plane wave, the types being discussed here, can be mastered on the bench where slow emulsions and vibrations are less of a challenge than with typical 3D subjects. Angular resolution of point source or plane wave holograms is limited by the angular resolution of the laser, and lasers achieve the diffraction limit at their wave length.
One type of holographic grating that merits study is the edge-lit hologram. Conceived in terms of a display medium and then further developed as a means of illumination for LCD displays and fingerprint stations 8, in the Dittoscope this type of hologram provides an extremely high angle of grazing exodus. The edge lit hologram has a substrate that serves as a light pipe. Diffracted light is tunneled into the substrate near the evanescent angle, and the trapped waves exodus at the distal end
(Figure 22). In such a device there is no secondary mirror. The edge illumination is sampled directly by a sensor. Identical modules of evanescent wave receivers would be in a collection “farm” for star light. In a configuration that studies one star at a time, the target is segregated from other stars using a blocking plate in the non-diffracted axis and a variable bandpass filter along the axis of diffraction. The sensors can be sensitive to photon events. While it is true that the photons captured must be at the wave length that conforms to the star’s angle of incidence, there may be enough photons arriving to allow exoplanet detection and spectrographic characterization when large arrays of evanescent receivers cover square kilometers. The modularity of such a device suggests economies of scale in mass production.
The economics of very large collectors favor diffraction. There are expenses associated with the mounts, but the mechanical complexity is reduced to a single axis which is static during observation runs; the support structures are load-limited thanks to segmentation; the substrates are flat, and tolerances for flatness are only sub wave length in the non-diffracted dimension. Experimentation can be ramped up from a demonstration to a practical observatory in discreet incremental steps, thereby limiting risk.
The Dittoscope is not a telescope for imaging in two dimensions, but it will provide exquisite spectra suitable for exo-planet discovery as well as for conventional velocity detection and chemical spectral analysis.