Reality → Scale → Macro world → Telescopes
About telescopes
- Optical telescopes. Today, all large astronomical telescopes are reflectors, which
replaced the now obsolete refractor [1] . Mirrors
of the largest telescopes are up to 10.4 m wide if segmented, and
up to 8.4 m if non-segmented. Computer-controlled actuators (active optics) compensate for mirror deformations caused by wind, temperature, and sag at ground level.
Adaptive optics are used to compensate for blurring due to atmospheric distortions. Laser guide stars provide the required reference. Individual telescopes can be combined
to work as interferometer for increased angular resolution [2] .
Spectrographs analyze faint light of galaxies and
quasars from blue to near infrared wavelengths (0.3 to 5 microns).
- Radio Telescopes. Wavelengths from 5 cm (long microwaves) to 10 m
(VHF) can be observed with earth-based
radio telescopes (wavelengths below or above this band are normally absorbed by the earth's atmosphere [3] ).
Costs are lower than for optical telescopes. Size of dishes ranges from a few meters to 305 m
and 500 m. Individual telescopes ('antennae') are grouped together
in arrays for interferometry. The Very Large Array (VLA) of New Mexico consists of 27 telescopes, each with a 25 m dish, arranged in a Y-shape (the telescopes can be moved along the arms of the Y to form different interferometers,
maximum antennae separation is 36 km). The Atacama Large Millimeter Array (ALMA) consisting
of 66 telescopes is a large new international facility in Chile's Atacama desert. The Dutch Low Frequency Array Radio Astronomy (LOFAR)
project uses low-cost dipole antennae in a high-broadband network of supercomputers ('IT telescope').
- Space telescopes. The Hubble telescope circles the earth
in 97 minutes at an altitude of 569 km. Primary mirror: 2.4 m diameter, surface less than 1/35 000 mm deviation from ideal curve. Light
sensitivity: 0.1 - 2.5 micrometer (ultraviolet to infrared). Resolution: 0.007 arcseconds. Data transmission: 120 gigabytes/week. The
James Webb Space Telescope (JWST),
originally scheduled to succeed the Hubble telescope in 2013, has incurred huge cost overruns and delays (now $8 billion vs. $1 billion original cost,
launch not expected before 2020). Mirror: 6.5 m diameter. Light sensitivity: 0.6 - 27 micrometer (infrared). Cooling: 40 K. Position:
Lagrangian Point L2 (=1.5 million km from Earth, behind the moon) to shield highly sensitive
instruments from Sun's radiation and keep 40 K. Observation of x-rays and gamma rays requires satellites and space probes well above Earth's atmosphere
(even a thin layer of air blocks this radiation). X-rays (mainly observed in the 0.1 - 10 nm wavelength band)
and gamma rays (in the picometer band) are emitted by extremely hot (millions of degrees)
and dense objects such as pulsars, neutron
stars, quasars, black holes. Recent
satellite missions include Chandra and Fermi.