Michelson stellar interferometer
Principle
The Michelson interferometer Sterninterferometer is one of the earliest used in astronomy. It is because the light of the star is recorded on two separate paths which are made to interfere. The two paths are generated by two slot-shaped openings which are spaced apart. Means of two mirror reach the two light paths on the primary mirror of the telescope, and from there to the secondary mirror, and finally come together in focus.
Would be a star, a point source, the superposition of the two light paths would produce a double-slit experiment, the corresponding interference pattern. It would result in an array of stripes, which would have an angular distance (hereinafter, the wavelength of the incident light ). In fact, stars have a tiny indeed, but non-vanishing angular diameter in spite of their large distances. Each point of the star provides a surface separate interference pattern, so that a plurality of stripe patterns is formed. But they are shifted from each other, according to the angular distance between the corresponding points on the star surface. The superposition of the individual fringe systems with the result that no interference pattern is seen when the angular diameter of the star is the same. The measurement is carried out in that the distance between the two gap apertures will be varied until the interference pattern disappears.
Measurement accuracy
As a result of air turbulence, the classical method according to modern standards, only a moderate accuracy. According to Hale ( 1921), the accuracy was in the initial measurement of the angular diameter of Betelgeuse is about 0.005 ". Scheffler and Elsässer (1990 ) According to the measurement error can be up to 0.01 " are ( reach modern instruments, as will be shown below, an accuracy of up to 0.00002 ", that are up to 500 times more accurate than the classical interferometer ). The uncertainty corresponds to about the angular diameter at which the sun appears from the nearest star from. It is thus clear that the Michelson Sterninterferometer generally fails in main-sequence stars, and only relatively close in the field of giants and supergiants objects at distances up to about 100 parsecs can be reliably measured.
To derive from the angular diameter of the actual diameter of the star, other effects must be considered. One difficulty is in particular the so-called limb darkening, after already Hale ( 1921) has pointed out. The center of the stellar disk is brighter than the skirt, contributes more to the interference pattern at. There is a tendency to underestimate the diameter of the star.
Especially the interferometry still the most approachable giants and supergiants often have extended photosphere and thus represent, in contrast to the sun very diffuse objects dar. Now there is a tendency to overestimate the diameter, since the measurement of not only the actual star body, but also its shell involving. On the resulting problem, as the diameter of a star is to be defined at all, in the article stellar surface will be discussed in detail.
Of course, the removal of the star needs to be known in order to calculate the angle in the actual diameters. As the Michelson Sterninterferometer was used, and the knowledge of the distance just of giants and supergiants was very uncertain. Thus, by Pease ( 1921) distinguished compiled information on distances for the red giant Arcturus by more than 100% ( from 6.3 to 13.5 parsecs entspreched 21 to 44 light years. During the 1990s were by the Hipparcos satellite for more than 100 000 stars of almost all types of reliable distances can be determined., the extended photosphere of some stars make but also for today's instruments, a significant limitation of the measurement accuracy dar.
History
The Michelson Sterninterferometer was designed in 1890 by A. A. Michelson. About 20 years earlier Hippolyte Fizeau had the French Academy of Sciences shall submit a proposal to Interferometry on stars, which was then implemented by M. Stephen, the then director of the Observatory of Marseilles. It is unclear whether Michelson knew of this preliminary work.
With a slot bracket from an 12 - inch telescope Michelson led by 1891 test measurements of the diameters of the four Galilean moons of Jupiter, which resulted in an excellent accordance with the determined already by other means values. Was an arrangement with the light of approximately 6 meters apart plane mirrors reflected in the 2.5 -meter telescope of the Mount Wilson Observatory: However, it was after these first preliminary work another 25 years until the first Michelson Sterninterferometer was practically used.
With this device, Michelson and Francis Pease led Gladsheim (1881-1938) by the first measurements of stellar diameters. The first such measurement covered the diameter of Betelgeuse, the Michelson and Pearse given in December 1920 to 390 million kilometers. This corresponds approximately to the diameter of the orbit of Mars; so is the red giant Betelgeuse around 300 times larger than the sun. Hale (1921) describes that when a pitch of the slit apertures of 6 feet clearly an interference pattern was visible. At a distance of 8 feet this was already far weaker, and disappeared at a distance of 10 feet. Having a central wavelength of 550 nm could be derived from an angular diameter of 0.045 ".
Six other star diameter followed. In this first success, the construction of an even larger unit, whose levels have now followed all 15 meters apart were. However, with this improved apparatus was only the measurement of a single additional star diameter, and the corresponding series of observations were discontinued in 1931.
Modern interferometers
Since the 1990s, the Michelson Sterninterferomter experiencing a renaissance. This was made possible by the adaptive optics, which allows real-time correction of atmospheric turbulence caused by the impairment. As an example, the system operated in Australia at the Sydney University Stellar Interferometer Observatory Culgoora SUSI discussed that. At Davis et al (1999) is described in detail.
The primary receiving mirror, which define the distance, serve siderostats which always reflect independent of the position of the star in the sky in the same direction. Its diameter is 20 cm, including but effectively contribute only 14 cm, because the starlight is not vertical, but incident at a certain angle. The small size is chosen deliberately: The resolving power of the individual mirror is no longer limited by the atmospheric turbulence, but by the diffraction. Thus, the air turbulence causes no longer as a large telescope a " Wabern " of the constellation, but only one home and wobbling of the same as a whole, which is easier to analyze and correct by the adaptive optics. The instrument has 12 fixed siderostats which are linearly arranged in north-south direction. By selecting a respective different pair of mirrors at different distances from 5 to 640 m can be realized.
Of the siderostats reach the incident beam in a two- parabolic collimator. There, the diameter of the beams from the original 14 cm be reduced to 5 cm and then adapted it to the following, very small optical elements. They then pass through a not further discussed system, which corrects the damage induced by the atmospheric effects of light refraction.
In contrast to the primary columns of the classical interferometer the siderostats are asymmetrically placed at the Culgoora instrument, which initially results in paths of different lengths for the two beams result. This asymmetry is eliminated by a two movable reflectors system. Depending on how far they are away from the optical axis, the path is shortened for a beam, and extended for the other in equal measure. The correction of the light paths also includes an adaptive element a with. The mirror shown with bold lines can be moved and thus reduced in real time, caused by air turbulence image motion.
Now, the two beams pass through a system for correcting the dispersion, which is also explained here in detail. Finally, they reach the heart of the instrument, the Davis et al. (1999) so-called " optical table ."
In this " optical bench ", the beams are caused to interfere. But before they pass each a polarizing beam splitter, which splits it into horizontally and vertically polarized components. The horizontally polarized components move to the interference, the vertically polarized components are wavefront sensors (see adaptive optics) forwarded that analyze the image motion.
To bring the remaining horizontally polarized components of the interference, a further beam splitter is used. It creates two new rays, but which have components of both the original bundle. Prisms direct each of the new jets in each case by a photomultiplier. But this case is not subject to all of the incident energy of refraction, a small part leaves the prism at an unchanged direction. The latter is provided to a third wavefront sensor for one of the two new rays. By comparison with the results of the other two sensors (which the image motion prior to the interference study ), the influence of atmospheric turbulence also be analyzed after the merge of the original beams.
The determination of the angular diameter of the star is now in the process are that the intensities of the two new beams is measured and correlated in time with each other. This correlation measure should not be confused with that of Intensitätsinterferometers. In the latter, the incident rays are converted into intensities before the interference in the Michelson interferometer but only then! The correlation between the two intensities, but showing both types of instruments, the same qualitative behavior. If the distance between the primary receivers of very small, the two intensities are strongly time- correlated. This means in the classical sense, that the interference pattern produced by the Michelson interferometer is clearly visible. If the distance increases, taking the correlation, that is, the visibility of the interference pattern from. The greater the angle of the star diameter, the lower is the required distance in order to achieve a correlation waste.
Due to the numerous Korrekturelemene, especially the adaptive optics system, the SUSI achieved an extraordinary accuracy. Thus determined, Davis et al. (2009) the angular diameter of the Cepheid 1 Car with a measurement error of only 0.00002 "! Thus, it is also possible to observe changes over time caused by pulsation accuracy. It shall however continue down that the nominal angular diameter from the influence of the limb darkening and instructions for any extended stellar atmosphere must be freed.
But modern Michelson Sterninterferometer not only overcome the deficiency of the classical predecessor ( the relatively high uncertainty of the angular diameter ), they also avoid the handicap of Intensitätsinterferometers ( very low sensitivity). While the Intensitätsinterferometer was used up to 2 size only for very bright stars can be measured with SUSI star to the 8th magnitude. For more 10,000 objects of virtually all spectral types are available, their distances also thanks to the measurements of the Hipparcos satellite are mostly known.
From the aperture synthesis interferometry for
Developed at Michelson's Sterninterferometer principles resulted from the 1950s to the development of aperture synthesis radio telescopes by Martin Ryle, and from the 1960s to developments of optical interferometric methods, whose modern descendants are telescopes like the Large Binocular Telescope and the VLT Interferometer.
The basic concept of aperture synthesis is to bring not only two but at least three primary beams for interference. The resulting very complex interference pattern allows not only to determine the angular diameter of the observed object, but also its intensity distribution ( that is, this actually represented as surface appearing body ). In the radio range, interference from the atmosphere do not play a role in this process has been used for several decades. In visible light and near infrared adaptive optics until the aperture synthesis has paved the way. An example is the work of Haubois et al. (2009) called, which dissolved the surface of Betelgeuse in the near infrared with an existing three telescopes interferometer.
In the absence of a solid crust, the question must be considered in interferometric measurements star general, which is a star-filled surface to understand actually. This will be discussed in the relevant article.