Atelier Bonryu(E)
zone plate photography
Laboratory: Zone Plate Photography
Applications of Zone Plate Photography
- Astronomical Observation -
Astronomical observation: Though there are a lot of papers on the application of zone plates to experimental physics such as nuclear physics, these are too specialistic and we don’t describe them in this page. However, as the application of the zone plate to a telescope for the astronomical observation is a topic which photographers may be interested in, we summarize this application briefly.
Sunspot observation: There are several books describing a lensless telescope such as “Unusual Telescope” by Peter L. Manly (1991), where there are descriptions on a solar observation by a pinhole and a zone plate telescope. In a book “Observation of sunspots (in Japanese)” by Ichiro Shimizu, et al. published in 1969 it is described how to make a pinhole telescope and a zone plate telescope for the sunspot observations, and photographs of sunspots by using these telescopes are also inserted. The focal lengths of the zone plate telescope as well as the pinhole telescope are 2 m, and the number of zones is 22. Photographs of sunspots taken by a single lens (6 cases with different apertures), a zone plate with a diameter of 10 mm, and a pinhole with a diameter of 1.8 mm were presented. The contrast of the zone plate photograph is weaker than that of the photograph by a single glass lens with an aperture of 6 mm, but it has higher resolution in comparison with the pinhole photograph and sunspots are clearly seen. It is consistent with the theoretical value for the zone number of 22, about 5 times that of the pinhole.
Invention of a zone plate: By the way, the zone plate was considered to be invented by Lord Rayleigh (John William Strutt; 1842.11.12 - 1919.6.30) as it was written in his datebook of 1871.4.11. But as he did not publish it the inventor of the zone plate is reputed officially as I.L. Solet who published a paper on a zone plate in 1875. It was referred to as Robert William Wood (1868.3.2 - 1955.8.11 #), a famous American physicist who first took a zone plate photograph in 1898 (Renner’s book, Tom Milster’s transcript of a lecture). Wood is the author of a famous book, “Physical Optics”, where a zone plate is described. The first edition of Physical Optics was published in 1905. In 1911 and 1934 the second and the third editions of the book were published. Though it became out-of-print, the book was reprinted in 1988 by the US Optics Society after 33 years of his death. In truth the zone plate which Wood used for taking a photograph was “a phase inverted zone plate” which was suggested by Rayleigh and realized by Wood. By using the phase inverted zone plate four times brighter image is obtained in comparison with the usual zone plate which we are using. The photograph of the zone plate itself is shown in Physical Optics but I have not seen yet the zone plate photographs taken by Wood.
(#) For reference’s sake, Wood is called as “the father of ultraviolet photography and infrared photography” and he is also famous of the person who put a period to the N-ray scandal in the scientific community at the beginning of the 20th century. Moreover, he is the author of long seller books “The man who rocked the earth” and “How to tell the birds from the flowers”, and in 1935 he assumed a president of American Physical Society.
Small zone plate telescopes: In the field of astronomical observations various lensless optical systems in addition to a usual zone plate are studied and used practically as it is necessary to observe an object by X-ray or gamma ray. As for applications of a zone plate to observations by visible lights there are two pint-sized cases reported as far as I know and there are also several serious projects considered to use a satellite telescope. One case is the previously described sunspot observation by a zone plate telescope (Ichiro Shimizu, Minoru Ono, Hisako Koyama, 1969). The other is an observation of Venus by a German skygazer Peter C. Slansky (2007). The zone plate telescope of Slansky （ZPPT: Zoneplate Planetary Telescope） is made of a Gabor-type (sinusoidal) zone plate with 29 zones attached to a paper pipe with length of 4 m and diameter of 15.8 mm. It seems considerably difficult to take a zone plate photograph of an astronomical object other than the sun unless a large-scale, high precision device is used. Slansky is supposed to be the first taking a zone plate photograph of Venus as an amateur skygazer, and in his web page he asks a question of if there is a forerunner of this attempt.
Large-scale projects of zone plate telescopes: There are several large-scale projects to construct a zone plate space telescope on a satellite aiming at detecting life signs on an exoplanet celestial body. For example, the group lead by Laurent Koechlin of the Observatoire Midi Pyrénée, Toulouse, France is actively contending with this theme. In US two projects, TPF (The Terrestrial Planet Finder) and NWO (The New World Observer) are being pursued. A super-large pinhole space telescope for detection of exoplanet life is also considered in the project NWO. The sizes of the “large” telescope may be difficult to imagine but in these cases the focal length of several 10 kilometers, sizes of a sensor and a zone plate of several meters are considered. If a glass lens is used for such a large telescope it will be too heavy to construct. But if the lens is replaced by a thin zone plate the difficulty concerning the weight is at least cleared. However, it should be noted that extremely precise control technology is required for the alignment of the large system with a size of 10 kilometers within a precision of sub-millimeters.
Though a space-based zone plate telescope has not been realized yet, recently (2012) the above-mentioned research group of the Observatoire Midi Pyrénée, Toulouse, France reported that they have completed a ground-based prototype of the large zone plate telescope and successfully attained the aimed high performance of the zone plate telescope system (L. Koechlin, et al., First high dynamic range and high resolution images of the sky obtained with a diffractive Fresnel array telescope, Experimental Astronomy (2012) 33, pp.129-140). The 20 cm x 20 cm zone plate of this telescope is a Fresnel type zone plate with the focal length of about 18 m and the zone number of 696, where the concentric zone pattern superimposed on a gridiron supporting structure is carved on a metal foil. The important issues to be solved for realizing a practically usable zone plate telescope are to attain higher dynamic range and higher resolution by removing the background light and correcting the chromatic aberration. These problems are solved in this telescope and the resolution of several arc second and the very high contrast of about 400000 are successfully attained. To confirm the performance of this telescope photographs of the moon surface, the Mars and its planets (Phobos and Deimos), and the Sirius A-B couple have been taken and are presented in their paper.
