Future Prospects

Interferometry and Astrometry In Space

Interferometry is probably the most powerful technique in astrometry. It is a fact of physics that more precise measurements can be made from larger telescopes. Interferometry uses this fact in a very clever way to make the same precise measurements not by building one gigantic telescope, but by combining the light received from a single object at two or more telescopes in an array. Therefore, one gets a measurement which is as precise as if all of the telescopes were actually combined into one huge telescope. This technique has been used in the field of radio astronomy for decades where it is easier to combine the light since the light has a very long wavelength (centimeters, meters, kilometers). In optical astronomy, the light has a very short wavelength (hundreds of nanometers) and so it is tougher to combine the light. It is only recently that technology has enabled astronomers to use optical interferometry and this technique looks promising for future studies.

It is useful for astrometric research to be done from space rather from the ground, as well. Space has the main advantage that there is no atmospheric turbulence or refraction to interfere with images. This is a huge help particularly in interferometry, where the atmospheric distortion limits the effectiveness of the technique. A satellite observatory would also be able to observe the entire sky, since there would be no large, opaque, Earth beneath it. There are problems with astrometry from space, however. Since a satellite observatory is unstable, it may move around quickly and irregularly. There cannot be other experiments on the same platform, either, and at a time when most astronomic satellites are equipped with many experiments, to minimize cost, this solely astrometric instrument would be difficult to be funded.

Hipparcos

The first and so far the only astrometric satellite ever launched was HIPPARCOS (HIgh Precision PARallax COllecting Satellite, also named for Hipparchus who discovered precession and wrote the first star catalog), shown at left. It was launched on August 8, 1987 and stopped observing in March of 1993 after 37 months of observations. It was a highly successful mission: more than 118,000 stars were observed, as well as 48 minor planets, 3 satellites (Europa, Titan, and Iapetus), and one quasar (3C 273). The stars observed were chosen in advance by a consortium of astronomers such that they were well-distributed in space, were astrophysically interesting, and had a limiting magnitude of 7.7-8.7 depending on their Galactic latitudes. The catalogue of results was finally released in 1997 after two independent reductions.

Future Mission Prospects

SIM, scheduled for launch in 2009, will be the first mission to fully exploit the technique of optical interferometry from space. The Hubble Space Telescope Fine Guidance Sensors (FGS) were the first optical interferometers to carry out astrometric observations in space, to a precision of ˜200µas. SIM will determine positions and distances of stars several hundred times more accurately than any previous program, with a goal of 4µas (microarcseconds) positional accuracy. SIM will be a pointed mission, observing only select stars, chosen by teams of astronomers working to answer various questions. One of the most interesting will be the search for planets around nearby stars, but SIM will also carry out studies of stars and the Milky Way Galaxy.

GAIA was proposed in 1995, and it is currently in development as a ESA Cornerstone Mission, with a scheduled launch around 2010. It will consist of three telescopes which will continously sweep the skylarge parabolic mirror, so it is equivalent to a masked single telescope. The mission is set for a five year duration, during which it could obtain measurements down to stellar magnitude 15 to a precision of 10 µas, two to three orders of magnitude more accurate than HIPPARCOS and five orders of magnitude better than ground based telescopes. It would measure both the distance and velocity of approximately a billion stars in the Milky Way Galaxy, or 1% of the total. It could measure radial velocities to an accuracy of a few kilometers per second down to magnitude 17, and overall will have a limiting magnitude of 20, much better than HIPPARCOS.

Very Large Baseline Interferometry (VLBI) on Earth is limited by the size of the planet, but by placing an antenna in space, this baseline can be increased and the spatial resolution can be improved. VLBI Space Observatory Program (VSOP) launched a satellite (HALCA) in 1997 which was operating as of 2003 and is cable of making observations in conjunction with ground-based VLBI antennas with a resolution of much less than a milliarcsecond.

A number of second generation satellites (including RadioAstron and VSOP-2) are being planned.

Photometry

Previous to the year 1930, the only equipment which existed for detecting light was the photograph. It was widely used by astronomers and considered to be the ultimate detector of light. Indeed, many astronomers still prefer photographs for the various advantages they have over more recent detectors. However, from the 1930's through the 1950's, electronic cameras were invented and then developed. These electronic cameras, also known as image tubes or television cameras, worked by converting visible light from an astronomical object into and electrical signal and then detecting the electrical signal. This allowed very faint objects in the sky to be imaged with more accuracy, since the image tubes were much more sensitive even to tiny amounts of light.

Electronic cameras were never widely used by astronomers, but during the 1970's a new detector was developed which has actually revolutionized the way astronomy is done. The charge coupled device (CCD), at right, is a semiconductor doped-silicon chip in which charge (up to 130,000 electrons) is stored and then read out (with only 5 electrons of readout noise) after an image is taken. It is a very efficient device and can detect even minute amounts of light, allowing some of the faintest objects in the universe to be imaged. Additionally, it is a faster technique than either photography or imaging with electronic cameras not only because it is so sensitive to light and thus takes less time to collect light from an astronomical object, but also because the data are read directly into a computer where the images can be analyzed. This eliminates the need for measuring machines, for instance. However, CCDs have the serious limitation of small chip size. The first detectors of this type were 100 × 100 pixel arrays, built in 1973. Currently, astronomers regularly use 2048 × 2048 or 2048 × 4096 pixel arrays; however, making the chips much larger than this would make them highly inefficient, since the read-out time increases with the size of the chip. Once a CCD is exposed to light, it needs to be read out into a computer. This can take a fair amount of time, and increases with the size of the CCD, which sets a limit on the size of the detector.

This limit the size of the chip means that there is a maximum area of sky that can be imaged with a CCD. For very extended objects, like supernova remnants or nearby galaxies, a single CCD cannot be used. However, it is possible to mount a number CCD chips onto a single surface to form a mosaic. This technique allows large areas of the sky to be imaged at one time, and by "dithering" the telescope between exposures, a single image which does not show the small gaps between the chips can be obtained. At left is a mosaic image of the moon using 8 different CCDs which work together in a single instrument on the 4 meter telescope of the Anglo Australian Observatory to produce a single final image. The black lines on the picture is the space between the CCDs where no image is recorded. The largest mosaic array currently in use (2003) is the MegaPrime/MegaCam (below right) at the CFHT, which combines 40 CCDs into a 377 million pixel array covering a 1°×1° area on the sky (with each pixel covering a mere 0.187 seconds of arc, offering excellent resolution).

In the future, engineers and astronomers will work together to build an "ideal detector". This detector will be perfectly efficient, noiseless, have a very large dynamical range so that very faint objects can be seen as well as very bright objects, have a uniform response, and be completely understandable. CCDs are fairly close to this ideal detector. Compared to photography, CCDs do have very high efficiencies, much more uniform responses to light, relatively low noise, and a large dynamic range. There is still work to be done towards reaching this "ideal detector", but the CCD is well on its way.

New high-resistivity CCDs are being developed which have the advantage of greatly increased quantum effeciency, as well as increased spectral coverage (with enhanced sensitivity in the red and infrared) as compared to current CCDs.

Engineers are currently working to develop a superconducting camera detector which might fulfill these requirements. This detector, based on superconducting tunnel junctions (SJTs), would count photons rather than electrons and would therefore have even better sensitivity than a CCD over a broad range in wavelengths, with a spectral resolution of down to 1 Å. By utilizing parallel readout instead of serial, as in a CCD, astronomers could get much better time resolution and lower noise, as well. Such a detector would have to operate at extremely low temperatures (1° above absolute zero), however, which would be difficult in practice for many observatories to accomplish.

Measuring Engines

Although it would seem that photographic astronomy, and thus measuring engines, was going out of fashion, there has still been a strong push in recent years to develop faster and more powerful measuring engines. Photography still has several advantages over other commonly used detectors, most importantly the larger size and so better sky coverage.

The PDS Microdensitometer is the machine currently in use by most observatories in the world for this purpose. The original model was built in the early 1970's and since then several improvements have been made on it. It is fully automated, transfers the images directly into a computer for further analysis, and is more accurate than machines before it. Although it is still widely used by photographic astronomers, it has several limitations which would have to be rectified in future measuring engines. For instance, the instrument is quite slow. It takes many hours to measure a single photographic plate. Additionally, the photometric and positional accuracies could be made better with new technology.

The first of these problems (the slowness) has been addressed by scientists at the Institut National des Science de L'Univers (INSU). They developed the Machine Automatique a Mesurer pour l'Astronomie (MAMA), located at the Observatoire de Paris. MAMA uses arrays of receivers to scan different parts of the plate simultaneously and thus to measure the entire plate much faster. This machine can process as large as 14 × 14 inch plates with positional accuracies of 1 µ (micron) and photometric accuracies of 2% over a range of 3 densities in just a few hours.

Future measuring engines will have to satisfy the following conditions to be useful to astronomers. Otherwise, other detectors such as the CCD will permanently displace photography due to its many advantages. The new engine will lose no information during the digitization process. It will be able to measure an entire plate in a single scan and in minimum time. It will measure with a positional accuracy of less than 1µ and a photometric accuracy of less than 0.1% with very low noise. Finally, there will be some standard reduction procedure so that everyone who uses the engine produces comparable results time after time.

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