What Telescope is Best? Will Sager BVAC. Six Things I Think I Think About Scopes There is no perfect...
-
Upload
jessica-wells -
Category
Documents
-
view
215 -
download
0
Transcript of What Telescope is Best? Will Sager BVAC. Six Things I Think I Think About Scopes There is no perfect...
Six Things I Think I Think About ScopesSix Things I Think I Think About Scopes
• There is no perfect scope – different types have There is no perfect scope – different types have different strengthsdifferent strengths
• A cheap scope is worse than no scope at allA cheap scope is worse than no scope at all• You don’t have to spend a fortune to get a good You don’t have to spend a fortune to get a good
scopescope• A good scope can’t beat a bad skyA good scope can’t beat a bad sky• When it comes to aperture, bigger usually is When it comes to aperture, bigger usually is
betterbetter• Your use of a telescope is inversely proportional Your use of a telescope is inversely proportional
to how hard it is to useto how hard it is to use
Telescope Telescope ComponentsComponents
• Optical tubeOptical tube
• EyepieceEyepiece
• MountMount
• FinderFinder
• Tracking motorTracking motor
• ““Go to” systemGo to” system
tube
eyepiece
finder
optics
mount
motor
go to
Telescope Rule of Telescope Rule of ThumbThumb
• Need good qualityNeed good quality– OpticsOptics– MountMount– EyepieceEyepiece
• If any of these are If any of these are poor, your results poor, your results will be alsowill be also
tube
eyepiece
finder
optics
mount
motor
go to
How a Telescope WorksHow a Telescope Works
• Optics gather lightOptics gather light• Focus at point to form imageFocus at point to form image• Eyepiece allows you to examine image detailEyepiece allows you to examine image detail• Both telescope lens (mirror) and eyepiece are Both telescope lens (mirror) and eyepiece are
rated by aperture and “focal length”rated by aperture and “focal length”
eyepieceobjective
lens
Telescope FlavorsTelescope Flavors
• Three main types:Three main types:
• RefractorRefractor – uses glass objective lens– uses glass objective lens
• Newtonian reflector Newtonian reflector – collects light with a – collects light with a curved mirrorcurved mirror
• CatadioptricCatadioptric – also collects light with a mirror, – also collects light with a mirror, but has folded light pathbut has folded light path
RefractorRefractor
• Light goes from lens Light goes from lens directly to the directly to the eyepieceeyepiece
• No obstruction in light No obstruction in light path – gives high path – gives high contrastcontrast
• Most expensive scope Most expensive scope per inch of apertureper inch of aperture
Objectivelens
diagonal
eyepiece
focuser
RefractorRefractor• Refractor must have special 2 Refractor must have special 2
or 3 element lens if it is to or 3 element lens if it is to focus all colors of light at same focus all colors of light at same point – or it must have long point – or it must have long focal lengthfocal length
• Apochromatic lens – Apochromatic lens – expensive color corrected expensive color corrected objecitveobjecitve
• Refractors also tend to have Refractors also tend to have long tubes, causes wobblelong tubes, causes wobble Takahashi TOA-130
5” objective lens$5500
Newtonian ReflectorNewtonian Reflector• Concave mirror focuses lightConcave mirror focuses light• Secondary mirror pushes Secondary mirror pushes
light cone to side to the light cone to side to the eyepieceeyepiece
• Tube is open; can have air Tube is open; can have air currents (bad)currents (bad)
• Secondary mirror blocks Secondary mirror blocks some light, causes diffraction, some light, causes diffraction, and lessens contrastand lessens contrast
• Mirror must be collimatedMirror must be collimated• Most Newts have off-axis Most Newts have off-axis
comacoma But biggest aperture bang But biggest aperture bang for the buckfor the buck
primary mirrorsecondary mirror
eyepiece
spider
focuser
Newtonian ReflectorNewtonian Reflector
Orion 6” Newtonian on equatorial mount - $240
Orion 12” Newtonian on Dobsonian mount - $1000
Catadioptric ReflectorCatadioptric 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
primary mirrorsecondary
mirror
diagonal & eyepiece
corrector plate
focuser
Schmidt Cassegrain Schmidt Cassegrain Telescope - SCTTelescope - SCT
Small Meade ETX SCT – 3.5” - with go-to mount - $300
Celestron 11” SCT - with GPS go-to mount - $2800
Most Popular SCTMost Popular SCT
Celestron 8” SCT on half-fork go-to mount - $1500
Meade 8” SCT on fork go-to mount - $1999
Mount TypesMount 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
MountsMounts
Refractor on GEM
Refractor on altaz
Newtonian on GEM
SCT on GEM
Star motion around celestial axis
EyepiecesEyepieces• Eyepieces change the Eyepieces change the
magnification of your magnification of your telescopetelescope
• Magnification = focal length Magnification = focal length of objective divided by focal of objective divided by focal length of eyepiecelength of eyepiece– Ex: 2000 mm f.l. objective Ex: 2000 mm f.l. objective
with 20 mm f.l. eyepiece, with 20 mm f.l. eyepiece, M=100M=100
• Many different types of Many different types of eyepieces with different lens eyepieces with different lens combinationscombinations
• Most good eyepieces have Most good eyepieces have 1.25” or 2” barrels1.25” or 2” barrels Eyepieces must be good quality too
Telescope PowerTelescope Power• Laws of physics put a limit on Laws of physics put a limit on
resolving power of telescoperesolving power of telescope• Dawes limit: R = 4.56/DDawes limit: R = 4.56/D
– R = limit in arcsecondsR = limit in arcseconds
– D = aperture diameter in inchesD = aperture diameter in inches
• This says that you need a bigger This says that you need a bigger aperture for higher resolutionaperture for higher resolution
• Rule of thumb: 50x per inchRule of thumb: 50x per inch– 3-inch objective – 150x3-inch objective – 150x
– 8-inch objective – 400x8-inch objective – 400x
• Practically, atmospheric Practically, atmospheric conditions rarely allow >200xconditions rarely allow >200x
• High power requires steady High power requires steady mount and tracking mount and tracking
90 mm = 3.5” max ~ 175x
200 mm = 8” max ~ 400x
FinderFinder• Usually finder is a small Usually finder is a small
telescope mounted on telescope mounted on side of big scopeside of big scope
• Finder scope can be Finder scope can be confusing because image confusing because image is backwardsis backwards
• Many scopes have a “unit Many scopes have a “unit power” finder because it is power” finder because it is easy to useeasy to use
Computerized Mount “go-to”Computerized Mount “go-to”• Biggest revolution in Biggest revolution in
telescope mounts in a telescope mounts in a generationgeneration
• Mount has motors that Mount has motors that are controlled by are controlled by computercomputer
• Computer tells mount Computer tells mount where objects are where objects are locatedlocated
• Great for finding hard-Great for finding hard-to-find objectsto-find objects
• Not idiot-proof. Still Not idiot-proof. Still need to know some need to know some stars for calibrationstars for calibration
Newtonian on go-to GEM
SCT on go-to fork
Telescope RecommendationsTelescope Recommendations• Don’t buy a cheap scope – it’s Don’t buy a cheap scope – it’s
better to have less scope with better to have less scope with fewer bells & whistles but with fewer bells & whistles but with good quality opticsgood quality optics
• Don’t go too big – don’t go too Don’t go too big – don’t go too small (6”-8” sweet spot)small (6”-8” sweet spot)
• Go to star parties and look Go to star parties and look through lots of scopes –talk with through lots of scopes –talk with other astronomersother astronomers
• Don’t want to spend $$? - get a Don’t want to spend $$? - get a good pair of binoculars (7 x 50)good pair of binoculars (7 x 50)
• Figure out what you want in a Figure out what you want in a scopescope
• Keep expectations realistic Keep expectations realistic
Nikon 7 x 50 binos $80
Basic TelescopeBasic Telescope• Under $500Under $500• Dobsonian reflectorDobsonian reflector• GEM reflectorGEM reflector
No frills Orion 6” Newt $240
No frills Orion 8” Dob $370
A Bit BetterA Bit Better• Under $1000Under $1000• Bigger DobsonianBigger Dobsonian• Small go-to SCTSmall go-to SCT
Orion 10’ Dob with object locator
Celestron Nexstar 6” go-to $999
Pretty NicePretty Nice• Under $1500Under $1500• 8” SCT go-to8” SCT go-to• 12” Dob12” Dob• 6” achromatic refractor6” achromatic refractor
Celestron 8” SCT go-to $1299
Meade 8” SCT with go-to GEM mount - $1499
Meade 6” refractor on go-to mount $1200
ConclusionsConclusions
• Incredible variety of good telescopes on the marketIncredible variety of good telescopes on the market• Look for good quality scopeLook for good quality scope• Buy good quality eyepieces and accessoriesBuy good quality eyepieces and accessories
– Get a star planisphere and star atlasGet a star planisphere and star atlas– Get Sky & Telescope magazine to see what’s going onGet Sky & Telescope magazine to see what’s going on
• Read reviews from other astronomersRead reviews from other astronomers• Check Check www.astromart.com for used scopes for used scopes• Learn the sky and how to use your scope. Check local Learn the sky and how to use your scope. Check local
astronomy club for helpastronomy club for help• Get away from light pollutionGet away from light pollution• Turn on your sense of awe and wonderTurn on your sense of awe and wonder