Disclaimer: links to web sites are ever-changing.  It turns out to be a Sisyphus task to keep them updated all the time.  Therefore, either try a different "spelling" of the hyperlink, look for it on google.com and/or let me know about an outdated link by writing an e-mail to aveh@wncc.net .
see especially: Telescopes by Nick Strobel (http://www.maa.mhn.de/Scholar/telescopes.html) and Orion Telescope Center (http://www.telescope.com/default.asp?page=Technical/Choosing_Telescope.html).

Telescopes


In order to do meaningful astronomy, it's imperative that we keep the sky as dark as possible.  Thus I promote the work of the International Dark-Sky Association .

../verschie/finder.jpg Every telescope has a finder (also called spotting scope). You need to start out with this to find the right object (makes sense, right?). The finder gives you a wider field of view: Since stars etc. move slower through the field when you slew the telescope, it's easier to point at the right star/ planet/ nebula/ galaxy.
Looking through the telescope magnifies more than the finder does, giving therefore a smaller field of view. Stars etc. move faster and it would be very tricky to find a particular object.
When pointing at an object, you must place it into the middle of the finder's field. Then you will see it through the telescope as well, if ...
... can you see why finder and telescope should be perfectly aligned?




Types and Design

Draw a refractor, a Newtonian and a Cassegrain.  Look into your textbook or go to the Orion Telescopes & Binoculars (click on Learning Center, then Equipment Basics 101, then How to Choose a Telescope).  Also look at the drawings of some different types at Mt. Wilson's .  


../verschie/refract.jpg
The best telescope for observations of planets (so they say) is the refractor . In fact, this refractor gives a really sharp image of Saturn despite its small aperture.  The eyepiece is always at the end of the tube and that's where the observer needs to be - in this case biology instructor Dave Nash (a star in Sagittarius).
The refractor is also the first telescope that was invented. It's the main design used for terrestrial observations.






../verschie/lightpath_refractor.gif
../verschie/lightpath_newtonian.gif

../verschie/orannewt.jpg

A Newtonian, whose tripod is inconveniently small.

../verschie/lightpath_cassegrain_king.gif ../verschie/whitnewt.jpg Most major telescopes nowadays (especially professional ones) are reflectors . Since here the objective mirror is always at the end of the barrel, it can be easily supported, which is a major advantage over refractors. All reflectors (except prime focus) have a secondary mirror in common. Its purpose is to redirect the received light out of the telescope, so an object can be viewed.
This reflector exhibits the Newtonian design. The eyepiece is directly behind (and to the side) of the opening.
 ../verschie/telescope_Doug_Rice_Dobsonian.jpg
The "Dob" uses the same design as a Newtonian but it uses a mount placed on the ground.Here my former colleague

Doug Rice (Spanish Intructor at Blue Mountain Community College in Penldeton, Oregon) assembles his oversized 18'' (450 mm) Dob.  The photo shows the part and trusses holding the diagonal secondary mirror at the front of the telescope.  The huge mirror is on the ground.  (The picture links to a Sept. 2002 article in Pendleton's local paper. )

The header links to a Dob building web site (it's popular because with its simple mount it can be built by any amateur astronomer).  And this links to a web site featuring some Dob designs.
And this links to John Dobson's web site , the inventor of this type of telescope, founder of the " Sidewalk Astronomers ", and apparently philosopher.





../verschie/lightpath_cassegrain.gif

The other reflector type commonly used by amateurs is the Cassegrain which has the eyepiece at the end of the barrel (most major professional telescopes are of this type). This means that there must be a hole in the objective mirror. This is not a disadvantage, since light is focused from all over the mirror. It simply cuts away a small percentage (less than 10%) of brightness, but that was taken away by the secondary mirror, which is in the way anyway.
This particular reflector is actually called a Schmidt-Cassegrain , because it has a correcting, thin lens covering the opening. Nothing is perfect, and so Cassegrains with spherical mirrors do not focus the image at exactly one point. The Schmidt lens gets rid of this blurring.  (The college owns a C-8 and a C-5.)

miscell/mer-trn4.jpg This is another Schimdt-Cassegrain, a C-5.  My wife Kate operates is correctly with a solar filter on the front since this was November 15, 1999, the day of the Mercury transit.
Find a description and design image of these telescopes at Celestron.  And some images of this telescope at   Orion.
The Coude design is rare and only used with large professional telescopes, such as the Ondrejov telescope in the Czech Republic (just Southeast of Prague; jointly used by Slovak astronomers as well).  Some telescopes can be easily redesigned to change them to a Coude fcous (third drawing on this web site).

The term "Prime focus" is often used by astrophotographers, meaning to replace the eypiece with a camera.  This is possible with any telescope.  But a real "Prime Telescope" is rare: see this one in the Caucasus Mts. in Russia (near the Black Sea) and notice how this astronomer would hunches in the prime focus cage of California's Palomar telescope .


 


Telescopes used in the Astronomy labs at WNCC in Scottsbluff and Sidney, Nebraska,
and at KPC in Soldotna, Alaska




 Design (Mount)  Aperture  Focal length  f-stop
(divide focal length by aperture)		  apprx. price (includes tripod)
small orange
(mine in Alaska)		 Newtonian (alt-azimuth) 4'' (100 mm) ~ 400 mm
 f/4
 $ 170
large orange
(Scottsbluff)		 Schmidt-Cassegrain (equatorial) 8'' (200 mm) 2000 mm f/10 $ 1200
large black
(mine in Alaska)
 ../verschie/telescope_sch_cas_8.jpeg
 Schmidt-Cassegrain (equatorial)
 8'' (200 mm)
 2000 mm
 f/10
 $ 1000
small red
(KPC in Alaska)
 ../verschie/telescope_mak_cas_102.jpeg
 Maksutov-Cassegrain (equatorial)
 4'' (102 mm)
 1300 mm
 f/12.7
 $ 390
white
(Scottsbluff)		 Newtonian (equatorial) 4.5'' (114 mm) 900 mm f/8 $ 460
white
(Sidney)		 Refractor (alt-azimuth) 2.4'' (60 mm) 900 mm f/15 $ 200
white
(Scottsbluff)		 Cassegrain (alt-azimuth) 5'' (125 mm) 1250 mm f/10 $ 700 (w/o tripod)

 

  

Where are the largest telescopes in the world?  See the Sky & Telescope July 1993 issue, I think.
 

Eyepieces available:
.965": 6mm, 9mm, 12mm, 18mm
1.25": 17mm, 25mm, 40mm

 What property does an eyepiece determine?

 Other equipment available (and its use):
Barlow-lens, color filter, Oxygen III filter

 look through hand-outs (from Orion Catalog , including prices)
 
 

Telescope Size (Properties)

 List the (optical) features of a telescope: Aperture, ...

(see King Observatory in Lincoln)
 

Powers of a telescope

  1. Light gathering power

    2.     Stars, nebulae, galaxies, quasars are faint.  So the most important purpose of an astronomical telescope must be to make them as bright as possible.  A bigger telescope (larger aperture) collects more light, so telescopes are judged by their size (aperture).

    (The owl has large pupils, so that it can gather more light (of the little that's there) during its night hunting.)
     
     
     
     

    A camera, which usually has an aperture of 35 mm, can gather more light by opening the shutter for a longer time. The left of these 2 images of Scorpius was exposed for 2 sec, the one on the right for 20 sec.
     

    Images (c) Andreas Veh.
     
     
  2. Resolving power

    3.     Larger aperture also produces sharper images.

    (The eagle has large pupils, so it's able to distinguish its prey from the surroundings as it's soaring high above ground.)
     
     
     
     
     
     
     
     
     
     
    (c) Martin Reble.  One of my favorite objects, M57, the Ring nebula in the constellation Lyra (whose really bright star Vega is part of the summer triangle).  A telescope needs to have good resolution, i.e. large aperture, before one can magnify an object to enjoy it more.

    For a nice example of resolving power and Hubble's superior vision, check out my favorite Hubble images . The first image shows Pluto and Charon imaged by Hubble before its focus was restored. Compare this to the best photographs obtained by ground-based telescopes and to Hubble's refocusing.
     
     

  3. Magnification

    4.     The most misunderstood power takes only third place.  Before you magnify something, make sure that you get a sharp image.  Otherwise, what good does it do you when a planet appears big, but blurry?  And that's the case for cheap department store telescopes.  Resolving power depends on aperture, which thus sets a limit to magnification.  Beyond that ... blurry image.
     
    (c) Hubble space Telescope.  I altered the one in the middle.  A small aperture refractor can show Saturn as it appears in the left image.  The middle image shows how blurry it becomes when a higher power eyepiece is used on the same cheap department store refractor.  The image on the right shows the best image that can be achieved with the college's 8 inch (aperture; 2000mm focal length) Schmidt-Cassegrain with a 2x Barlow lens and a 17mm eyepiece, at a magnification of 220x, during a night of exceptional seeing (no atmospheric turbulence) and Saturn being in opposition.  Any more magnification would blur this image as well.

    Therefore large magnification can work only for large aperture telescopes.
        Magnification depends on the telescope's focal length and the eyepiece's focal length.  Since the eyepiece can easily be changed, magnification is changed as well all the time, and therefore is not an inherent property of a telescope (the question "how much magnification does this telescope have?" doesn't make sense).
     

    (c) Royal Observatory Edinburgh.  The Pleiades (also called Seven Sisters) in Taurus are a beautiful sight for the naked eye, binoculars and low power telescopes.  A large aperture telescope can only show parts of the Pleiades because its focal length is too long (and thus its magnification too large).

        Magnification and Field of View are in a trade off situation.  You like to see some objects in their entirety, so why keep magnifying them?  (Do you want to see how many enemies you have across the trench, or if one of them has a pimple on his forehead?)   When looking at objects, you also always start out with a low power eyepiece, so you can easily find them.
     
     

Other Astronomies

Look at the following figures in your textbook:
(Chaisson/McMillan p.55), Fig.3.5 Electromagnetic spectrum
p.57, Fig.3.6 Electromagnetic spectrum
p.61, Fig.3.9 The Sun in different parts of the EM-spectrum
p.101, Fig.4.30 The Milky Way in diff. parts of the EM-spectrum

and these figures on the internet .


To my WNCC Astronomy home page .