Technology

With the size of the universe, the Solar System could not have been in existence singly. Galaxies, which have ever become known to man and visible by telescopes, are approximated at a distance of 50 billion miles away. The existence of other planets orbiting around the star in a system other than our own is the thrust of continuous space explorations. In this light, it has been a promising curiosity for the layman and 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 insight into 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 in the early days of technological birth, most people were sceptic about the validity of the observations.  The discovery was supported the following 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 discovered by Wolszczan and Frail are believed to have been formed by a supernova and considered as the first definitive planetary discovery. The first definitive discovery of a planet orbiting around a main-sequence star was announced by astronomers Mayor and Queloz in 1995.

Planetary Detection and SDI
The early discoveries with the capabilities of technology had at the time expectedly developed the need for perfections. 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 researchers on imaging techniques thus aiming at reducing speckle noise through adoptive optics and differential imaging.

Adoptive optics (AO) 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, is actually 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 defence (Biller, et al 389).

Planetary detection imaging involves dealing with light sources coming from the stars around which the planets orbit. The aberration consisting of light waves produces light patterns and noticeable noise in the image. Adoptive optics (AO) has the capacity to lessen but not totally eliminate the light contribution. The failure of AO is in the inability to resolve and overcome contaminating speckles. Eliminating speckles will therefore, requires nearly perfect understanding of their nature as being influenced by variation in appearance as a function of varying AO exposure time understanding the variability of wavelength with the degree of micron-wavelength (interfering speckles of 1.5 micron being more than 2.1 microns wavelength) and marking out differences in speckles formation as a degree of optical path imperfections.

Other modern imaging techniques developed to address the speckles obstruction consist of the use of the Angular Differential Imaging (ADI) and the Simultaneous Differential Imaging (SDI).

Simultaneous Differential Imaging is sometimes referred to as Simultaneous Spectral Differential Imaging (SSDI). The ADI though slightly advanced but constructed with close similarity to the SDI addresses the problem of speckle noise in planetary imaging. It is a powerful technique in detecting extrasolar planets even with faint light source 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. It entails a technique that uses a uniform optical path to determine lights of different narrow band filters at the same time. The multiple filters absorb unwanted speckles thus removing image fainting.  SDI technique is developed to resolve optical image resolution challenge. It utilizes the knowledge of accurate wavelength, optical path and timing to bring about a clear separation by eliminating obscurity. By employing data reduction techniques, the SDI takes the difference of the images simultaneously recorded by its quad filter, then aligning these taken images 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 the 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 and 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 make 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 centre and mimics the image of a planet. The light sources and the reflections in the optic path cause the speckles to show usually filling up the field closely surrounding the star. When the shape of the optics changes during the night, one can find the speckles roam on the image from time to time in the course of observation. This problem caused by the device is equally experienced with the use of Hubble telescopes.

The two major problems which the innovation of 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 and 1, 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 rapidity of direct detection of extrasolar planets.

Conquering the problem of speckles starts with understanding of how exactly they are produced. Speckles are scattered light coming from the brighter parent star. They are naturally brought by the interaction of wavelength, light sources and the optic channels 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 colour as the source, whereas, the planet will register a different colour.

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 system (McLean, 25).

The SDI imaging technique was pioneered in the year 2000 (Marois et al. 233). 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 whereby the star and speckles can be distinguished through the effective use of quad filters  any CH CH4 rich companion remains.

How exactly does the SDI 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 pass through the quad filters taken at 3 different wavelengths (Source Our SDI Techniques).

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 obtained from ground or space.

One of the breakthroughs recorded was the unravelling of new images using the European Southern Observatory (ESO) VLT on the surface of Titan. Titan happens to be the largest moon containing in the Saturnian System. The SDI, which is an optical device, uses its very high contrast camera resolution to produce distant sharp images in three colours at the same time. The SDI device can equally be utilized to observe objects within a very thick atmosphere in the solar system (methane filled). This made it very possible for the discovery of Titan. According to a press release on ESO-Reaching New Heights in Astronomy, a simultaneous examination through a narrow, unobstructed near-infrared spectral window in a highly dense atmospheric humidity with methane alongside the presence of adjacent translucent waveband, revealed or produced images that clearly eliminated the contamination of the atmospheric components. The viewed detail of the produced objects surface was such with an unprecedented clarity and outline. Regions with different refractive and reflective capacities were picked on view this includes those of high and low reflectivity.  According to the press release, one of the essential advantages of the discovery is the possibility of the delivery of Huygens probe. Though further report is yet to ascertain the outcome, it added that the Huygens probe was projected to approach the Saturn system through a Cassini spacecraft. The final destination was aimed at Titan surface. The successful descent of Huygens probe on Titan surface will allow getting a more detailed report of the Saturn system (Markus Hartung, et al. 1).

Conclusion
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 optic systems and implementation of simultaneous differential imaging technique radically changed and improved the future of successfully discovering extrasolar Giant Planets.

In few decades time, the vastness of the universe will be filled with a corresponding enormous knowledge that will help us understand the Solar Systems miniature existence among the billions of galaxies that are resident in the space. Extrasolar detection of planets will upgrade to an Exogalactic pursuit towards an infinite possibility that lies in space.