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What Telescope is Best?

What Telescope is Best?. Will Sager BVAC. Six Things I Think I Think About Scopes. There is no perfect scope – different types have different strengths A cheap scope is worse than no scope at all You don’t have to spend a fortune to get a good scope A good scope can’t beat a bad sky

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What Telescope is Best?

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  1. What Telescope is Best? Will Sager BVAC

  2. Six Things I Think I Think About Scopes • There is no perfect scope – different types have different strengths • A cheap scope is worse than no scope at all • You don’t have to spend a fortune to get a good scope • A good scope can’t beat a bad sky • When it comes to aperture, bigger usually is better • Your use of a telescope is inversely proportional to how hard it is to use

  3. optics Telescope Components finder tube • Optical tube • Eyepiece • Mount • Finder • Tracking motor • “Go to” system eyepiece motor go to mount

  4. optics Telescope Rule of Thumb finder tube • Need good quality • Optics • Mount • Eyepiece • If any of these are poor, your results will be also eyepiece motor go to mount

  5. How a Telescope Works • Optics gather light • Focus at point to form image • Eyepiece allows you to examine image detail • Both telescope lens (mirror) and eyepiece are rated by aperture and “focal length” eyepiece objective lens

  6. Telescope Flavors • Three main types: • Refractor– uses glass objective lens • Newtonian reflector – collects light with a curved mirror • Catadioptric– also collects light with a mirror, but has folded light path

  7. Refractor • Light goes from lens directly to the eyepiece • No obstruction in light path – gives high contrast • Most expensive scope per inch of aperture Objective lens eyepiece focuser diagonal

  8. Refractor • Refractor must have special 2 or 3 element lens if it is to focus all colors of light at same point – or it must have long focal length • Apochromatic lens – expensive color corrected objecitve • Refractors also tend to have long tubes, causes wobble Takahashi TOA-130 5” objective lens $5500

  9. Newtonian Reflector eyepiece focuser • Concave mirror focuses light • Secondary mirror pushes light cone to side to the eyepiece • Tube is open; can have air currents (bad) • Secondary mirror blocks some light, causes diffraction, and lessens contrast • Mirror must be collimated • Most Newts have off-axis coma secondary mirror primary mirror spider But biggest aperture bang for the buck

  10. Newtonian Reflector Orion 6” Newtonian on equatorial mount - $240 Orion 12” Newtonian on Dobsonian mount - $1000

  11. Catadioptric Reflector • Has primary mirror like Newtonian, but secondary mirror folds the light back toward primary • Light passes through hole in primary to eyepiece • Secondary mirror curved to make scope have longer focal length • Some have corrector plate at front to correct optics corrector plate secondary mirror primary mirror focuser diagonal & eyepiece

  12. Schmidt Cassegrain Telescope - SCT Small Meade ETX SCT – 3.5” - with go-to mount - $300 Celestron 11” SCT - with GPS go-to mount - $2800

  13. Most Popular SCT Celestron 8” SCT on half-fork go-to mount - $1500 Meade 8” SCT on fork go-to mount - $1999

  14. Mount Types • Altaz – one axis vertical (azimuth) and the other horizontal (altitude) • Equatorial – one axis aligned with Earth’s rotation axis (right ascension) and the other perpendicular (declination) • Fork – popular mount style for SCT scopes – can be set altaz or equatorial • German equatorial – GEM – popular type for many scopes – has one axis pointing at celestial rotation axis (right ascension) and another perpendicular (declination) Altaz Dobsonian Fork-mounted equatorial

  15. Mounts Refractor on GEM Refractor on altaz Newtonian on GEM SCT on GEM Star motion around celestial axis

  16. Eyepieces • Eyepieces change the magnification of your telescope • Magnification = focal length of objective divided by focal length of eyepiece • Ex: 2000 mm f.l. objective with 20 mm f.l. eyepiece, M=100 • Many different types of eyepieces with different lens combinations • Most good eyepieces have 1.25” or 2” barrels Eyepieces must be good quality too

  17. Telescope Power 90 mm = 3.5” max ~ 175x • Laws of physics put a limit on resolving power of telescope • Dawes limit: R = 4.56/D • R = limit in arcseconds • D = aperture diameter in inches • This says that you need a bigger aperture for higher resolution • Rule of thumb: 50x per inch • 3-inch objective – 150x • 8-inch objective – 400x • Practically, atmospheric conditions rarely allow >200x • High power requires steady mount and tracking 200 mm = 8” max ~ 400x

  18. Finder • Usually finder is a small telescope mounted on side of big scope • Finder scope can be confusing because image is backwards • Many scopes have a “unit power” finder because it is easy to use

  19. Computerized Mount “go-to” • Biggest revolution in telescope mounts in a generation • Mount has motors that are controlled by computer • Computer tells mount where objects are located • Great for finding hard-to-find objects • Not idiot-proof. Still need to know some stars for calibration Newtonian on go-to GEM SCT on go-to fork

  20. Telescope Recommendations • Don’t buy a cheap scope – it’s better to have less scope with fewer bells & whistles but with good quality optics • Don’t go too big – don’t go too small (6”-8” sweet spot) • Go to star parties and look through lots of scopes –talk with other astronomers • Don’t want to spend $$? - get a good pair of binoculars (7 x 50) • Figure out what you want in a scope • Keep expectations realistic Nikon 7 x 50 binos $80

  21. Basic Telescope • Under $500 • Dobsonian reflector • GEM reflector No frills Orion 8” Dob $370 No frills Orion 6” Newt $240

  22. A Bit Better • Under $1000 • Bigger Dobsonian • Small go-to SCT Celestron Nexstar 6” go-to $999 Orion 10’ Dob with object locator

  23. Pretty Nice • Under $1500 • 8” SCT go-to • 12” Dob • 6” achromatic refractor Meade 8” SCT with go-to GEM mount - $1499 Celestron 8” SCT go-to $1299 Meade 6” refractor on go-to mount $1200

  24. Conclusions • Incredible variety of good telescopes on the market • Look for good quality scope • Buy good quality eyepieces and accessories • Get a star planisphere and star atlas • Get Sky & Telescope magazine to see what’s going on • Read reviews from other astronomers • Check www.astromart.com for used scopes • Learn the sky and how to use your scope. Check local astronomy club for help • Get away from light pollution • Turn on your sense of awe and wonder

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