Nov 11, 2005

ObservatoryScope has been selected to build and install two custom designed and manufactured 16" telescopes for a college in the continental United States. State law governing the bidding process prohibits ObservatoryScope from mentioning either the name of the college or even the state within which the college resides in. Each telescope features simple yet robust construction, large conventional worm gear drive systems, and a simple optical layout consisting of a paraboloidal primary mirror coupled to a CCD camera at the focal plane. The instruments will be spaced several hundred feet apart and used in tandem to search for extra-solar planets by employing the Hanbury-Brown and Twiss effect (HBT). The paraboloidal mirrors will have Strehl ratios of 0.95 or better in order to increase the resolution and statistical accuracy of the HBT when searching for extra-solar planets. A CAD rendering of one of the 16" telescopes is shown below.

The Hanbury-Brown and Twiss Effect

Definition:

The Hanbury-Brown and Twiss effect (HBT) is any of a variety of correlation and anti-correlation effects in the intensities received by two detectors from a beam of particles. HBT effects can generally be attributed to the dual wave-particle nature of the beam, and the results of a given experiment depend on whether the beam is composed of fermions or bosons. Devices which utilize the effect are commonly called intensity interferometers and were originally used in astronomy, although they are also heavily used in the field of quantum optics.

History:

In 1956, Robert Hanbury Brown and Richard Q. Twiss published A Test of a New Type of Stellar Interferometer on Sirius, in which two photomultiplier tubes (PMTs), separated by about 6 meters, were aimed at the star Sirius. Light was collected into the PMTs using mirrors from searchlights. A correlation was observed between the two intensities, despite the fact they were aimed at different locations on the star. Hanbury-Brown and Twiss used that signal to determine the apparent angular size of Sirius, claiming excellent resolution.

An example of an intensity interferometer that would observe anti-correlation.

This result met with much skepticism in the physics community. Although intensity interferometry had been widely used in radio astronomy where Maxwell's equations are valid, at optical wavelengths the light would be quantized into a relatively small number of photons. Many physicists worried that the correlation was inconsistent with the laws of thermodynamics. Some even claimed that the effect violated the uncertainty principle. Hanbury Brown and Twiss resolved the dispute in a neat series of papers which demonstrated first that wave transmission in quantum optics had exactly the same mathematical form as Maxwell's equations albeit with an additional noise term due to quantization at the detector, and secondly that intensity interferometry should work according to Maxwell's equations. Others, such as Edward Mills Purcell immediately supported the technique, pointing out that the clumping of bosons was simply a manifestation of an effect already known in statistical mechanics. After a number of experiments, the whole physics community agreed that the observed effect was real.

The original experiment used the fact that two bosons tend to arrive at two separate detectors at the same time. Morgan and Mandel used a thermal photon source to create a dim beam of photons and observed the tendency of the photons to arrive at the same time on a single detector. Both of these effects used the wave nature of light to create a correlation in arrival time - if a single photon beam is split into two beams, then the particle nature of light requires that each photon is only observed at a single detector, and so an anti-correlation was observed in 1986. Finally, bosons have a tendency to clump together, but due to the Pauli exclusion principle, fermions tend to spread apart, and so when the Morgan and Mandel experiment is performed on electrons, an anti-correlation in arrival times was observed for the first time in 1999. All of these are considered HBT like effects.

Finding Extra-Solar Planets

Using anti-correlation effects to measure the diameter of a star is rather straightforward and doesn't require good optics since we are working with plenty of light intensity. As noted, crude search light mirrors and photo multiplier tubes were used in 1956 to measure the diameter of Sirius! Searching for extra-solar planets, however, requires measuring anti-correlation effects from much dimmer light sources (planets) which are in orbit about their sun. Obviously the anti-correlation effects for planets will be much harder to measure, especially with moderate aperture telescopes. ObservatoryScope has chosen to optimize the customer's potential for success in measuring these anti-correlation effects by employing mirrors with very high Strehl ratios (better than 0.95). This will dramatically increase the spatial resolution of these measurements when measuring anti-correlation since the whole idea is to measure the anti-correlation of the light beams arriving at the two telescopes. As noted, the telescopes do not have to be pointed exactly at the star. Also note that the photons received from any extra-solar planets orbiting the star will be very few, and that these clumps of photons will strike the primary mirror at different locations upon the primary mirrors. This explains the need for extremely smooth and accurate primary mirrors so that the anti-correlation measurements for any extra-solar planets can be obtained in a timely fashion. After all, the goal is for the anti-correlation data for any extra-solar planet to readily "stand out" in the measurements! Thus the time required for measuring anti-correlation effects due to extra-solar planets will be limited by local atmospheric seeing conditions at the time when these measurements are made.