1 / 24

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

chaman
Download Presentation

What Telescope is Best?

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related