The Magdalena Ridge Observatory Interferometer can be considered as an improved version of the COAST. The ideas behind it are the same except that the MROI is constructed at a much more ambitious scale, aiming to produce images with resolution at least 10 times better than that of COAST, and a hundred times better than the Hubble Space Telescope.
How it works
The MROI makes use of optical interferometry, much like COAST.
With a maximum baseline of 340 meters (~6 times as wide as COAST), the MROI can produce image with the resolution of a single telescope of the same scale while being immune to the negative effects of the Earth's atmosphere to the quality of the image. It is also infinitely cheaper because actually building a telescope of this scale would be near impossible (a 30-meter telescope would cost around Â£1,000,000,000).
The construction of the MROI requires a lot more technical sophistication compared to COAST as extreme precision is required over a much bigger distance. As a major collaborator, the Cambridge Astrophysics Group is currently developing some of the mechanisms below, specifically the delay line and the fast tip-tilt system.
In the diagram, the overview of the optics for a single unit telescope in the MROI is shown. First, light from the sky of the observed object falls on the 1.4m unit telescope, which is housed in its own enclosure.
Then, a series of mirrors reflect the incoming light beam into the delay line, where path-compensation occurs to make sure all the different light beams arrive at the beam combiner in phase. A tip-tilt sensor controlling the secondary mirror of the unit telescope compensates for the majority of the distorted wavefront, meaning that higher resolution images can be obtained.
The light beams travel through a vacuum in aluminium tubes so that they are unaffected by the distorting effects of air. When the light beams reach the delay line, also under a vacuum, they are reflected off a â€œCat's-Eyeâ€ telescope, so-called because the mirror reflects the light back to its source with minimal scattering.
After the path-compensation has been carried out in the delay line, the light beam exits the vacuum aluminium tubes and enter the beam compressor where the original 9.5cm light beam is compressed into a 1.3 cm light beam.
This 1.3 cm light beam then travels to the beam combining area, where beams from all the unit telescopes are combined and the resulting fringe pattern used to collect data.
The delay line for the MROI has the same function as the trolleys used in COAST, but the design of the â€œtrolleysâ€ in the MROI is very different. They are situated in 190m long vacuum tubes rather than in open air which reduces the distorting effect caused by the atmosphere. In addition, the computerised control system for each â€œtrolleyâ€ is attached to the trolley itself, reducing the need for trailing wires through the vacuum tube. Each delay line trolley has wheels which run directly on the inside of the vacuum tube, and power is provided to the trolleys using an inductive pickup which draws around 100W from the power line running along the bottom of the tube.
A peek at the interiors
A view of the exterior of the trolley. The black shell is made of carbon-fibre
Tip Tilt System
The tip-tilt system is the simplest form of adaptive optics, and is used to correct the 2-dimensional distortions of the incoming wavefronts of light as shown in this diagram.
The system being designed at Cambridge adjusts the tip-tilt system 1000 times per second, which removes most of the major perturbations of the wavefronts. However, to remove the finer distortions, a higher-level adaptive optics system has been proposed. This system would use a wavefront sensor to calculate the distortions of the wavefronts, and accordingly adjust a deformable mirror so that the wavefronts are smoothed. The corrected wavefronts can then be used to obtain higher resolution images.
|The current fast tip-tilt system (being built in Cambridge)||The proposed system: notice the additional adaptive optics system in pink.|
Beam relay system
Whereas in COAST, the beam relay system consists of plastic tubes filled with air, the aluminium beam relay tubes in MROI are under a vacuum to reduce the distorting effect of the atmosphere on the light beams.
The MROI has been designed to reduce the number of reflections between the light from the sky and the detector in order to minimise distortions, reflection losses and wavefront distortions.
In the MROI, separate beam combiners are used for the fringe tracking equipment and the science cameras. On the spectrographs used for fringe tracking, light is combined from the nearest telescope pairs, and the resulting signal is used to control the positions of the delay lines. For the science cameras, the beams from several telescopes are combined so that data can be collected about the fringe patterns, and used to reconstruct a final image. In the MROI,there will be two science beam combiners directing light onto a visible light camera and an infrared wavelength camera, along with a fringe tracking beam combiner and space for a fourth visiting instrument.
The diagram above shows one beam combiner used in conjunction with two spectrographs in order to carry out fringe tracking. Fringe tracking is used to control the positions of the delay line trolleys to make sure the light beams from the different telescopes in the MROI arrive at the science camera beam combiners in phase so that an interference pattern can be produced. In the MROI, the fringe tracker enables measurements to be taken which have a similar resolution to that of a telescope of a 340m diameter.
The satellite view of the MROI site
This webpage was created by Oscar Chung and Sudhir Balaji as a work-experience project from 20/8/2012 to 24/8/2012