Technology

With the size of the universe, the Solar System could not be alone. Galaxies which have become known to man and visible to telescopes is approximated at 50 billion. The existence of other star orbiting planets in a system other than our own is the thrust of continuous space explorations. In this light, it has been a basic curiosity for the layman and a primary concern for astronomers to find out the existence of other planets outside our Solar System.

The study of the existence of Extrasolar planets or Exoplanets has established its own field in astronomy. Part of this discovery is to gain a substantial amount of knowledge about their attributes which would necessarily involve a great deal of successfully recording their images for study. With this as the principal objective for most astronomical research, the need for improving the technology for recording images with a high degree of accuracy is of utmost importance.

The first planetary detection was in 1988 with the observations of a planet orbiting the star called Gamma Cephei by astronomers Campbell, Walker and Yang. However, with the limited technology back then, most people were skeptic of the veracity of the observations.  The discovery was supported the next year and in 2002, with improved techniques in planetary detection, it was verified.
In 1992, planets PSR 125712 were discovered and were immediately confirmed. These pulsar planets discover by Wolszczan and Frail are believed to have been formed by a supernova are considered as the first definitive planetary discovery.

The first definitive discovery of a planet orbiting around a main-sequence star called was announced by astronomers Mayor and Queloz in 1995.

The early discoveries with the capabilities the technology had at the time expectedly had imperfections. Technological advancement, particularly of spectroscopy, marked the age of modern extrasolar planetary detection. It paved the way for a lot of extrasolar planetary discoveries. However, the main problem in imaging techniques for extrasolar planetary detection remained to be the presence of speckle or photon noise in the image produced. This has been the primary concern of developers of imaging techniques, thus, aiming to reduce speckle noise through adoptive optics and differential imaging.

Adoptive optics corrects the deterioration of image quality with the use of wave-font sensors sending signals to initiate corrections. Since astronomers focused on reconstructing degraded image, AO system, in reality, is a post-detection processing technique. The first system was used to correct the contrast of two-dimensional images. In 1982, a version was created for the AMOS in Haleakala. Thereafter, it was widely implemented for applications in military defense.

Planetary detection imaging involves dealing with light sources coming from the stars around which the planets orbit. The aberrations consisting of lightwaves produces light patterns and noticeable noise in the image. Adoptive optics has the capacity to lessen but not totally eliminate the light contribution.

Other modern imaging techniques consist of the use of the Angular Differential Imaging (ADI) and the Simultaneous Differential Imaging (SDI). The ADI, similar to the SDI addresses the problem of speckle noise in planetary imaging. It is a powerful technique in detecting extrasolar planets which have faint light sources particularly those closely separated from their stars. It matches the contrast produced by SDI without requiring specialized optics and is much simpler to use than the SDI.
SDI is an instrument which utilizes a quad filter for capturing astronomical images. By employing data reduction techniques, the SDI takes the difference of the images simultaneously recorded by its quad filter, then aligning these images taken with the result of a reduced photon noise in the data. Effectively suppressing speckle in the image, it is considered as one of the first cameras dedicated to discovery of new planets.

The SDI is employed at the VLT (Very Large Telescope) and the MMT (Muliti Mirror Telescope). The simultaneous taking of the difference in images is done with 1.62 mum methane bandhead and at 3 wavelengths.

Images are simultaneously taken with the use of the quad filter and are in the gas giants and cool brown dwarfs spectrum. The reduction or attenuation of the speckle noise is possible by performing a difference of images.

Extrasolar planetary detection is extra difficult due to the intrinsic faintness of the planets which are especially close to the parent star which are much brighter. The use  of VLT increases the resolution of the objects, however, the parent star gets blurred because of the interference of the image of the Earths atmosphere. The distortions are addressed with the use of AO systems using deformable mirrors fixing the star to become a point of light as it should be.

Upon removal of the stars halo, the image produced shows the speckles in the area surrounding the stars center and can sometimes look like a planet. The light sources and the reflections in the optics causes the speckles to show usually filling up the field closely surrounding the star. When the shape of the optics change during the night, one can find the speckles roam about the image from time to time in the course of observation. This problem, as caused by the device, is experienced even with the use of Hubble telescopes.

The two major problems which the SDI technique addresses are the following a) The difference in the contrast between the planet and the star. Old giant gas planets, or those which are 2 Gyr, are fainter than primary although within the primary. On the other hand, young planets are fainter ten times as compared to a primary but are self luminous b) the use of photon noise limited AO systems for an object 10 times fainter than and 1 from its primary can be detected with an hour of exposure.  Speckles remain in the image filling up the area within 1 of a star. This is true even after correction with the use of AO system.

Reduction of the speckles is achieved by subtracting PSF images using a multi-channel camera whereby the images taken are normally  found in narrow spectral bands.

The multi-channel camera converts the image with the help of a holographic diffuser in fixing the degraded illumination. Re-imaging takes place comparable to a convolution of the PSF image. The aberrations caused by the optics device get diverted to a convolution kernel in the channels. Better and more coherent images are then produced.

The efficiency in speckle reduction would dramatically increase the rapidness of  direct detection of extrasolar  planets.

Conquering the problem of speckles starts with understanding how they are produced. Speckles are scattered light coming from the brighter parent star. They naturally occur brought about by the interaction of wavelength, light sources and the optic devices creating blotches as the light beams are reflected and refracted passing through varying filters and mirrors. The light patterns create the resulting speckles which roam around the image due to the optics consistent change in shape. Since the speckles are sourced from the star, they will have the same color as the source, whereas, the planet will register a different color.

Astronomers have standard answers to issues that may come up in the use of the optics device. It is, however, impossible to conquer all aspects of the problem. The development of differential imaging technique, whether angular or simultaneous, seeks to address and dissolve all aspects of the problem of speckles together with the AO sytem.

The SDI imaging technique was pioneered by in the year 2000by Marois et al.  There is a strong CH CH4 (methane absorption) bandhead at 1.62 m in extrasolar giant planets (T  1200 K). The technique uses a subtraction routine whereny the star and speckles can be distinguished through the effective use of quad filters   any CH CH4 rich companion remains.

How, exactly does the technique work
SDI measures light in varying narrow-band filters. Since the images are taken simultaneously using the same optical path and differentiation is thereafter aligned, the resulting images would show identical speckles allowing them to be removed.  A double-Wollaston prism is utilized to split a light beam into two. Two prisms will produce four beams which passes through the quad filters taken at 3 different wavelength.

Easy detection of the planet from the speckles produced would require a proper choice of narrow-band filters. Carbon and hydrogen are always present  near the field of the parent star. On the other hand, Methane is present in the atmosphere of a planet which is much colder than the star around which the planet orbits. Cool objects would be much fainter. The heat of the star does not allow methane to form, thus, can be distinguished from the planet which is fainter.

Data reduction tasks are employed to align images taken in each of the filters with the use of a custom shift and a reduction routine. The routine formula for reduction of data does not at once eliminate the speckles but merely weakens them. The final touch is the rotation of the telescope. This would now reveal the planet because the image would rotate along with the rotation of the telescope, whereas, those which remained fixed in the image are mere speckles.

A survey of 54 nearby young stars at the VLT and MMT conducted by Biller, et. al.at the Steward Observatory of the University of Arizona, the researchers obtained H band contrasts 25000 (5  sigma  Delta  F1 (1.575 mum) 10.0 mag,   Delta  H  ,   , 11.5 mag for a T6 spectral type object) at a separation of 0.5 from the primary star. These SDI images have the highest image contrast, to date,  obtained from ground or space.

The pursuit for a technology advancement in the field of extrasolar planet direct detection, the use of differential imaging techniques implemented with ground-based telescopes proved to be a vital step towards perfection of the technology.  The Planet Finder project using VLT and MMT provided groundbreaking and critical specifications for future advancement in the field of imaging techniques. The use of ground telescopes with the application of adoptive optics systems and implementation of simultaneous differential imaging technique radically changed and improved the future of successfully discovering extrasolar Giant Planets.