What Telescope is Best?What Telescope is Best?

Will SagerWill Sager

BVACBVAC

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