Zone plates for sunnier pictures
doi:10.1038/nindia.2009.11 Published online 27 January 2009
Sitting in a small building in the suburbs of Kolkata, a group of Indian scientists have put together the first ever 'X-ray imager with the zone plate concept' that will make its maiden flight into space on board a Russian satellite on January 29.
The imager, which overcomes the drawbacks of traditional Coded Aperture Masks (CAMs) while taking pictures of X-ray emitting sources in space, will be used in the solar mission to get high resolution images of solar flares in the 24th Solar cycle which begins shortly.
The imager, created by the Indian Centre for Space Physics (ICSP), Kolkata, is a part of three Indian Payloads (RT-2/S, RT-2/G and RT-2/CZT) to be launched by the Indo-Russian Coronas-Photon Satellite. ICSP is one of the Indian partners in the project along with Tata Institute of Fundamental Research (TIFR), Mumbai and Vikram Sarabhai Space Centre (VSSC), Thiruvanthapuram . The project is funded by the Indian Space Research Organization(ISRO), Bangalore.
Explaining the concept of the zone plate, Sandip Chakrabarti, a senior professor at the S.N. Bose National Centre for Basic Sciences and in-charge of ICSP says, "So far, all X-ray imagers were using the crude Coded Aperture Mask (CAM) that resolves a few tens of arc minute while taking pictures. Thus, if there is some intricate activity such as a large X-ray flare on the sun, we can't say exactly where the flare started and exactly where the energy was generated. This gave only vague inputs to theoreticians."
Here the zone plate concept comes in handy. "This is also a kind of CAM, but the thickness of its smallest element is a few micron. Two zone plates are needed instead of a single CAM. These two are aligned and pointed at the sun. The shadow of one falls on the other and their combined shadow falls on the detector," he says.
The combined shadow behaves like a Vernier scale. It makes reproduction of the image easier and the resolution is at least five to ten times better. "Since the field of view of our zone plate is 6x6 degree, we may also discover bright gamma-ray bursts, if we are lucky," Chakrabarti, who is the co-principal investigator of the Indian Payload RT-2 system, adds. The plates are made of one mm wide tungsten so that 100 KeV X-ray can be blocked by the opaque rings.
Faint black holes cannot be seen with the present technology of zone plates, but ICSP is developing 30cm zone plates to meet this challenge.
Coronas-Photon (Complex Orbital Observations Near-Earth of Activity of the Sun) is a Russian (Roskosmos) solar-terrestrial science mission for the study of the solar hard electromagnetic radiation in the very wide energy range from Extreme UV (EUV) up to high-energy gamma-ray radiation. The goal of the project is to investigate the energy accumulation and its transformation into energy of accelerated particle processes during solar flares; the study of the acceleration mechanisms, propagation and interaction of fast particles in the solar atmosphere; the study of the solar activity correlation with physical-chemical processes in the Earth upper atmosphere.