Telescopes

Basic function of a telescope: extend human vision Collect light from celestial object Focus light to create image or spectrum of the object Use larger aperture than the human eye Expose for longer than the human eye Achieve better resolution than human eye Observe at wavelengths the eye is not sensitive to (i.e. beyond 400 – 700 nm) Examine spectral information in detail

Light Hitting a Telescope Mirror huge mirror near a star small mirror far from 2 stars In the second case (reality), light rays from any single point of light are essentially parallel. But the parallel rays from the second star come in at a different angle.

If the mirror is a particular shape, a paraboloid, light rays from a distant source, parallel to the "mirror axis" all meet at one point, the focus.

Image Formation Light rays from a distant, extended source are all focused in the same plane, the "focal plane" creating an image of the source. "focal plane"

Optical telescopes Kinds of optical telescopes: 1) Refractor – uses a lens that light passes through, to concentrate light. Galileo’s telescope was a refractor. Like your eye – brings parallel rays together on retina to make sharp image.

Problems with Refracting Telescopes <-- object (point of light) image at focus Lens can only be supported around edge Some light absorbed in glass (especially UV, infrared) Air bubbles and imperfections affect image quality "Chromatic aberration"

Lens - different colors focus at different places. Chromatic Aberration Lens - different colors focus at different places. white light blue focus red focus Mirror - reflection angle doesn't depend on color.

Largest Refracting Telescope Built Yerkes 40-inch (about 1 m).

2) Reflecting telescope Solution: 2) Reflecting telescope use concave mirror (shape is ideally parabolic), not lens, to focus light. Newton built first one. Big, modern research telescopes are reflectors. Gemini South 8-m reflector.

Reflector advantages Mirrors can be large, because they can be supported from behind. No chromatic aberration Less light lost and fewer image quality problems Largest single mirror built: 8.4 m diameter for the Large Binocular Telescope Mt. Graham, AZ.

There are 10 m telescopes, but in segments Keck 10-m telescope

focus options or Nasmyth focus Prime focus – fewer reflections, loss of light, distortions. or Nasmyth focus

Nasmyth focus platforms

Characteristics of telescopes Light gathering power:  area, or D2 Main reason for building large telescopes! Image with telescope of twice the diameter, same exposure time. Image of Andromeda galaxy with optical telescope.

INT 2.5 m. 800-1000 s per field. 91 fields

Characteristics of telescopes, cont. Magnification: angular diameter as seen through telescope/angular diameter on sky Typical magnifications 10 to 100 Field of View: how much of sky can you see at once? Typically many arcminutes – few degrees. Resolution: The ability to distinguish two objects very close together. Angular resolution: θ = 2.5 x 105 /D where θ is angular resolution of telescope in arcsec,  is wavelength of light, D is diameter of telescope objective, in same distance units. Example, for D=2.5 m, λ=500 nm, θ = 0.05” Resolution is limited by an optical effect called diffraction. Magnification. Explore more in lab. Depends on eyepiece used. Skip here.

Two light sources with angular separation larger than angular resolution vs. equal to angular resolution Airy rings due to diffraction in the telescope. Very faint usually.

But, “seeing” limits resolution for ground-based optical telescopes * Air density varies => bends light. No longer parallel Parallel rays enter atmosphere No blurring case. Rays brought to same focus. Sharp image on CCD. * CCD Blurring. Rays not parallel. Can't be brought into focus. Blurred image. resolution limited to about 1”

fuzziness you would see with your eye. detail you can see with a telescope on ground.

Example: the Moon observed with a 2.5 m telescope 1" => 2 km Hubble Space Telescope image, 0.05” resolution Ground-based telescope image, 1” resolution

Detectors Quantum Efficiency = how much light they respond to: Eye  2% Photographic emulsions  1-4% CCD (Charge coupled device)  80% Can be used to obtain images or spectra CCDs also provide data directly in digital form – easier to process.

Photographic film CCD Same telescope, same exposure time!

Spectrographs: light spread out by wavelength, using prism or “diffraction grating” Reflection grating – many grooves in it so light diffracts off the grating. Get interference like in transmission case.

Some future optical telescopes Large Synoptic Survey Telescope (LSST): 8-m telescope with large field of view (3.5°). Will survey entire sky repeatedly. Site in Chile. First light 2019. Thirty Meter Telescope (TMT): segmented design, like Keck. First light 2022. Built by consortia of universities and countries.

Effelsberg 100-m (Germany) Radio Telescopes Large metal dish acts as a mirror for radio waves. Radio receiver at prime focus. Surface accuracy not so important, so easy to make large one (surface shouldn’t have irregularities that are larger than 1/16 ). But angular resolution is poor. Remember: θ = 2.5 x 105 /D Effelsberg 100-m (Germany) Andromeda galaxy – optical D larger than optical case, but  much larger (cm's to m's), e.g. for  = 1 cm, diameter = 100 m, resolution = 20". Andromeda radio map with Effelsberg telescope

Green Bank 100-m telescope (WV) Arecibo 300-m telescope (Puerto Rico) Parkes 64-m (Australia) Jodrell Bank 76-m (England) Green Bank 100-m telescope (WV) Arecibo 300-m telescope (Puerto Rico)

So how can we get better resolution? Interferometers – e.g., VLA Use interference of radio waves to mimic the resolution of a telescope whose diameter is equal to the separation of the dishes

Interferometry A technique to get improved angular resolution using an array of telescopes. Most common in radio, but also limited optical interferometry. D Consider two dishes with separation D vs. one dish of diameter D. By interfering the radio waves from the two dishes, the achieved angular resolution is the same as the large dish.

VLA and optical image of Centaurus A Example: wavelength = 1 cm, separation = 2 km, resolution = 1" Very Large Array (NM). Maximum separation 30 km (only about 1km in this configuration). Very Long Baseline Array. Maximum separation 1000's of km VLA and optical image of Centaurus A

Atacama Large Millimeter Array 18,000 ft elevation plateau in Chile USA/Europe/Japan collaboration Started observing in 2011 with a few dishes 66 dishes eventually Molecules radiate at mm wavelengths. Molecular gas is fuel for star formation. So this is to study how stars form, now and in the early universe.

UNM is building its own array for =3-10m: the Long Wavelength Array (LWA) Far larger than the VLA, to give same resolution. “Stations” of 256 antennas, to be spread across NM What if you want to do this for the longest radio waves? Some sources emit waves down to 1 MHz or less (many meters wavelength)

Square Kilometer Array, currently being designed, will be 50 times collecting area of VLA, with baselines to 1000’s of km

Optical-to-mm-wave Telescope Sites Site requirements Dark skies (avoid light pollution) Clear, dry skies Good “seeing”, stable atmosphere High, dry mountain peaks are ideal observatory sites, for optical to mm waves.

USA at night

Mauna Kea Observatory, Hawaii Kitt Peak National Observatory, Arizona

Radio Telescope Sites Away from radio interference is most important. Radio astronomy can be done in cloudy weather, day or night.

Telescopes in space Pros – above the atmospheric opacity so can work at  impossible from ground, above turbulence, weather, lights on Earth Cons – expensive! Repairs difficult or impossible.

Spitzer Space Telescope - infrared Shorter infrared wavelengths allow you to see through dust. Dust is good at blocking visible light but infrared gets through better. Trifid nebula in visible light Trifid nebula with Spitzer

Herschel Longer infrared wavelengths allow you to see radiation from warm dust in interstellar gas

FERMI – Gamma Ray Telescope Gamma rays are the most energetic photons, tracing high-energy events in Universe such as “Gamma-ray Bursters”.

Hubble Space Telescope and its successor-to-be: the James Webb Advantage of space for optical astronomy: get above blurring atmosphere – much sharper images. Center of M51: HST (left; 0.05” resolution) vs. ground-based (right; 1” resolution)

The JWST Mock-up of JWST JWST more optimized for near IR. Distant galaxies, planets, star forming disks, etc. Mock-up of JWST Diameter 6.5 meters (vs. HST 2.5 meters) – much higher resolution and sensitivity. Will also observe infrared, whereas Hubble is best at visible light. Expected launch 2018.