A. THE ELECTROMAGNETIC SPECTRUM

B. TELESCOPES: GENERAL

• Invented: 1608 (Lippershey, Holland). [Note: microscope invented 1654, also in Holland.]

• First astronomical use: 1610, by (Galileo, Italy). Utterly transformed astronomy.

• Collect more light from source --- most important

• Magnify source

• Resolve more detail in source

• An objective or primary optical element forms an image (i.e. an accurate representation of original scene) at a usable focus, where it can be studied by eye, recorded by film or other detectors (as in a camera), or fed into yet other instruments

• Lens: transparent glass shaped to refract (or bend) light rays to a common focus. The picture below shows how a flat glass surface bends light rays. The shorter the wavelength, the stronger the bending.
  Refraction of Light By a Prism
  (click for descriptive animation)

  
  

• Mirror: shaped glass which reflects light rays off its front surface to a common focus. See picture below.

• For distant objects (including all astronomical objects), the incoming rays from each point on the source are parallel to each other. In this case, the image is formed at a point which is one focal length from the objective.

• A small lens called an eyepiece is used to magnify the image at the focal point and make the rays parallel again to allow the eye to form a sharp image of it. (Although your eye contains an adjustable lens, which can focus on nearby or distant objects, the light beam from the objective of a telescope in the absence of an eyepiece is converging or diverging too strongly for this lens to correct.)

C. TELESCOPE PERFORMANCE CHARACTERISTICS

• f/ = (Objective Focal Length)/(Objective Diameter).

• The smaller is f/, the more concentrated is the light in the focal plane and the easier it is to see faint extended objects like nebulae. (Same as for the aperture setting on a camera---the smaller the f/ number, the higher is the brightness in the focal plane.) Typical small telescopes have f/ in the range 5-20.

• Defined to be the increase in the apparent angular size (measured, e.g., in degrees) of the image

• Mag = (image size in degrees)/(object size without scope in degrees).

• For a given telescope, magnification is determined by the focal length of the eyepiece:
  Mag = (Focal Length Telescope)/(Focal Length Eyepiece)
  Mag = 2000 mm/(FLE) for the Meade 8-in scopes
  Note: high powers (> 150x) are not necessarily better, except for specialized applications (e.g. planets). Susceptible to image blurring by atmosphere, telescope vibration. etc.

• Defined to be the original angular diameter of the region viewed through the telescope. Field of view decreases as magnification increases.

• E.g. with a 20 mm eyepiece, the Meade 8-in scopes produce a field which is 20 minutes of arc in diameter. (Compare to the full Moon, which is 30 min of arc.)

• Most important attribute of a telescope

• Light collected is proportional to the objective diameter².

• Compare capability of 8-in Meade scope to human eye:
  Assume pupil diameter of dark-adapted eye is 5 mm. Meade objective is 200 mm. Light gathering capability of Meade is then (200/5)² = 1600x larger than eye.
  If eye can detect magnitude 5 stars, the Meade will detect stars of 13th magnitude. There are over 5,000,000 of these, compared to the 2,000 or so visible with the naked eye!

• Quantitatively defined to be the smallest measurable detail in an image (in seconds of arc). Depends both on telescope optics and Earth's atmosphere.

• Basic limit is set by the physics of light: since it is a wave phenomenon, light spreads out or diffracts. The picture above shows how light spreads out when it passes through a narrow aperture, such as the objective lens of a telescope. Because of the interference of light waves from different parts of the aperture, the larger the aperture, the more concentrated the emerging beam, implying better resolution.

• The resolution is proportional to (Wavelength/Objective Diameter). The larger the telescope, the better the resolution.

• A 10-in diameter telescope with perfect optics will produce a 1 arc-sec diameter image of a star (assuming a perfectly stable atmosphere). [Note that all stars are so distant that they are point sources in a such a telescope.]

• Seeing: the Earth's atmosphere also refracts light, and because it is constantly moving, there is always a blurring and jittering of images in a scope. Astronomers call this "seeing," and quantitatively define it to be the diameter of a star image (in seconds of arc) caused by atmospheric turbulence. Seeing actually dominates diffraction in most cases. Expect typical seeing of 2-4 seconds of arc at the Student Observatory. Seeing can be measured by observing a double star of known separation (see writeup for Lab 3).

• Below is an enlarged image of the bright star Betelgeuse seen though a large telescope. It is a large blob, broken up into smaller near point-like units. Click on the image for a video of the seeing effects:
  "Seeing" Produced by Earth's Atmosphere

D. TELESCOPE TYPES

• Refracting: objective is a lens; bends rays. Galileo's of this type. McCormick 26-in of this type. Largest: 40-in diameter (built 1896).

• Reflecting: objective is a mirror; reflects rays. Invented by Gregory; improved by Newton. All large telescopes are reflectors. Largest 400" (10-m) diameter (built 1993).

• Catadioptric: combines lenses and mirrors, e.g. to produce a larger well defined field of view. Most famous: Schmidt wide field survey telescopes. Meade 8-in scopes are of this type.

• Lenses produce chromatic aberration: light of different wavelengths comes to focus at different points.

• Mirrors need be figured only on one side

• Mirrors easy to support accurately from behind; lenses require support at edges, tend to sag.

• Harder to support heavy lens mechanically at top end of tube than mirror at bottom end.

• Folding action of primary and secondary mirrors (see below) means that telescope tube is much shorter than in "straight through" refracting design.

• More discussion

• To produce a good image, telescope optics must be figured to a minimum tolerance of about 1/4 of the wavelength. For optical telescopes, this is 10^-5 cm. Small! Good polishing/test techniques not developed until 19th century.

• Scale comparison: if a 320" (8-m) diameter telescope mirror were scaled up to the size of the continental United States, i.e. about 3000 miles diameter, then the maximum ripple allowed in its polishing would be about 2 inches!

Note that in three of the designs shown, a "secondary" mirror at the top of the telescope tube is used to redirect the light beam. Although the secondary does block part of the primary, this has only a small effect on the net image quality; in particular, it does not produce a "hole" in the center of the image. In the Cassegrain design, a hole is actually made in the primary iteself. Your 8-in telescopes have a Cassegrain design.

• Altitude-Azimuth (Alt/Az) mount: one vertical axis and one horizontal axis. Lowest cost but require computer control for large scopes since must move in two axes simultaneously to track stars.

• Equatorial mount: two axes, but polar axis is tilted to parallel the Earth's rotation axis. Motion around this one axis then tracks stars. Harder to engineer, easier to operate. Most telescopes use equatorial mounts but largest ones are Alt-Az. Your 8" telescopes are equatorials.

E. TELESCOPE MILESTONES: (details in Lecture 7)

• 40-in refractor 1896; largest of this type
  
• Hubble Space Telescope: 94-in reflector in space (launched 1990); most precise yet manufactured.

• Keck Observatory: Two 400-in (10-m) telescopes consisting of 36 individual mirror "segments" each (1993, Hawaii). See image at right and diagram.

• Very Large Telescope (VLT): Four 320-in monolithic mirror telescopes (2000, Chile)

• Other EM spectral bands. Astronomers now exploit wide regions of the full electromagnetic spectrum. First instruments outside the visible range were radio telescopes (1950's). Now: radio (e.g. National Radio Astronomy Observatory, headquarters in Charlottesville), microwave, infrared, ultraviolet, X-ray, gamma-ray telescopes. Here is a list of major telescopes in all EM bands

F. BINOCULARS

• Additional optics are used so that the view is "right side up."

• The view for nearby objects is 3-dimensionsal; special prisms may be used to increase the separation of the objective lenses for better 3-D resolution. Astronomical objects are so distant that there can be no 3-D effect. However, views with binoculars can be especially vivid because simultaneous use of both eyes produces less strain, and the optics of binoculars usually allow for easy centering of the eyes on the emergent beam.

• Binocular optics are usually classified in the form "7 x 35". Here, the first number is the net magnification of the binoculars, and the second is the diameter of the primary lenses in millimeters. Good types for general astronomical observations are 7x35 or 8x50. 10 power and higher binoculars require tripods for stability. The field of view is also often marked on the binoculars, typically given as the diameter in feet for objects at a distance of 1000 yards.

• Binoculars produce some of the best views of the Moon, rich star fields, comets, and the Milky Way.

G. LAB REPORTS & OBSERVING FORMS

• Appendices D and E in the ASTR 130 Lab Manual describe how you are expected to use the standard observing forms to record observations with binoculars or telescopes and how to write up your lab reports. Read these sections carefully.

• Blank observing forms and a sample, filled-out form can be found here.

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Last modified 28 January 2001 by rwo

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Text copyright © 2001 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 121 at the University of Virginia. Images shamelessly stolen from the Web---sorry, guys!