Telescope Technologies

Adaptive Optics AO makes use of a powerful laser shot into the atmosphere to act as a sample star. The characteristics of the laser beam are known, so a computer uses the laser “star” as a model to correct for atmospheric seeing. Yuri Beletsky snapped this photo of an AO guide laser at work at ESO’s Paranal Observatory, Chile.

Adaptive Optics Deformable mirrors, driven by small pistons called actuators, frequently change the surface of the mirror This in effect, gives the mirror a “living surface” and untwinkles the stars. Technicians inspect the actuators under a mirror destined for the Large Binocular Telescope (LBT) in the image below. http://archive.news.softpedia.com/news/New-Arizona-Telescope-Uses-Adaptive-Optics-144840.shtml

Honeycomb Mirrors Mirrors are cast over porcelain molds which are then removed, giving the mirror a honeycomb structure. This creates a large lightweight mirror that requires much less glass to make and also is easier to move. In the image at right, Steward Observatory technicians inspect a honeycomb mirror that was cast for the Giant Magellan Telescope.

Honeycomb Mirrors Once mounted on the telescope, air can be blown through the hollowed mirror back in the evening to quickly decrease the temperature of the glass, bringing it down to air temperature. It’s important because a warm mirror would heat the air above it, creating blurry star images. A mirror like this is said to have a lower thermal inertia.

Spin Casting Telescope mirrors used to take much longer to make, since a parabolic shape had to be ground from a disc of glass. Spin casting uses a rotating furnace to melt the glass. The spinning molten glass assumes a parabolic shape, that when cooled creates a surface that takes much less time to grind into a perfect paraboloid. Rotating furnace (left) and lap polisher (above) at the Steward Observatory Mirror Lab.

Segmented Mirrors Building and transporting a single large mirror can be an engineering problem. An alternative is to build the mirror from multiple smaller segmented pieces. An additional advantage is that a segmented mirror can be deformed into a more correct shape, using adaptive optics. The James Webb Space telescope (seen at right) is now being constructed and will be Hubble’s successor. It has a segmented mirror design.

Meniscus Mirrors Think of a meniscus mirror as a sort of “contact lens”. Its thin design allows for it to be controlled by adaptive optics. Its low mass is an advantage, especially in space telescopes. The low mass also creates a mirror with low thermal inertia.

Unblocked Aperture Using an offset parabolic mirror shape, radio waves can be deflected off to the side of the main dish instead of towards its center. This has the effect of increasing the light gathering power. Since none of the aperture is blocked by the receiver. The Green Bank Telescope has a collecting area of 2.3 acres and an offset feed antenna.

Space Telescopes Putting a telescope in space has these advantages: It can be used 24/7 (no daytime) It does not have to deal with the effects of the atmosphere (wind, clouds, seeing). All wavelengths can be observed. Our atmosphere only passes visible light, radio waves, and a small fraction of IR and UV wavelengths. The Hubble Space telescope, seen at right, has served astronomers since 1993.

IP Techniques: Stacking Much of the improvement in imaging in recent years has come about as a result of computer image processing techniques. Stacking and averaging images is one of the most common routines. Multiple images are taken, then combined and averaged. Averaging has the advantage of removing flaws that might appear in one frame, such as a cosmic ray or satellite trail. Poor individual images can be rejected, leaving only the best images to be used to create the final image. This technique also optimizes the signal to noise ratio in the image by increasing the overall imaging time.

Interferometry Interferometry uses the technique of combining 2 or more telescope mirrors to form one image. This creates one large “virtual mirror”. This requires intensive computing power and also precise timing mechanisms to combine the wavefronts of incoming light. The ALMA radio telescope (a portion of it is pictured above) consists of 66 individual dishes, most of them 12 meters in diameter in the Atacama Desert in Chile. The array can be configures to extend over distances from 150 meters to 16 km. In compact grouping, ALMA sees a wider area of the sky at lower resolution. Spread over a large area, the resolution increases because the virtual dish size increases, but the light-gathering power remains constant.