14 th March 2007Terry Evans, Bridgwater Astronomical Society Buying a Telescope

14 th March 2007Terry Evans, Bridgwater Astronomical Society Buying a Telescope What For? What’s a Telescope? Telescope types Mount Types Other Bits and Pieces

14 th March 2007Terry Evans, Bridgwater Astronomical Society What For? First Telescope Advanced Visual Imaging

14 th March 2007Terry Evans, Bridgwater Astronomical Society First Telescope – First Advice Don’t Buy a Telescope!! Buy:- Binoculars Star Chart / Planisphere Astronomy magazines / on-line service Warm coat & hat Alarm Clock!

14 th March 2007Terry Evans, Bridgwater Astronomical Society First Telescope -What For? See the basic bright targets - Moon, Planets, Sun (with care – see later) See some of the brighter deep sky objects (DSOs) –Should be able to find all the Messier objects

14 th March 2007Terry Evans, Bridgwater Astronomical Society First Telescope - problems I can’t find anything It’s always indoors –Not portable –Site not suitable –Takes too long to set up Everything’s too small / too faint

14 th March 2007Terry Evans, Bridgwater Astronomical Society Advanced Visual – What For? See more detail on the Planets and Moon See more in the bright DSOs See fainter DSOs Track Asteroids, see Supernovae, etc Patrick Moore’s law –4” refractor, 6” reflector minimum

14 th March 2007Terry Evans, Bridgwater Astronomical Society Advance Visual - Problems It still stays indoors all the time! –Bigger and heavier than a first telescope –Takes longer to cool down –More things to fiddle with rather than observing Still can’t find the faint objects Everything bounces about Things don’t look like they do in the magazines

14 th March 2007Terry Evans, Bridgwater Astronomical Society Imaging Scope – What For? Take wide field images of brighter DSOs with digital SLR Take images of moon and planets with a webcam Take deep exposures of DSOs with cooled CCD camera

14 th March 2007Terry Evans, Bridgwater Astronomical Society Imaging Telescope Forget the scope – it’s all in the mount! –A small scope on a good mount is better than a big scope on a poor mount You can probably use your existing scope Visual niggles become big issues! –Mirror flop, Meridian flip –Stiffness and collimation

14 th March 2007Terry Evans, Bridgwater Astronomical Society What’s a Telescope? The Optical Tube Assembly (OTA) The Mount Eyepieces Other Accessories

14 th March 2007Terry Evans, Bridgwater Astronomical Society Telescope (OTA) Types Refractor –Uses a Lens to gather the light Reflector –Uses a mirror Catadioptric (compound) –Uses a mirror and a full aperture lens

14 th March 2007Terry Evans, Bridgwater Astronomical Society Points to Consider Resolution is determined by Aperture Magnification is determined by the eyepiece –Magnification = f/l Objective / f/l eyepiece –e.g. 1000/25 = 40, 1000/12.5 = 80 –Maximum useful magnification is 50x per inch –e.g. 200x for a 4” Contrast affected by obstruction and quality

14 th March 2007Terry Evans, Bridgwater Astronomical Society Telescope Types - Refractor Uses a Lens to gather the light The Eyepiece magnifies the image

14 th March 2007Terry Evans, Bridgwater Astronomical Society Refractor Points  Simple construction  No obstruction – good contrast. Best for planets and the moon  Little to go wrong  No adjustment  Can be used for solar projection  Long tube  Relatively expensive per inch  Can show chromatic aberration (false colour). Expensive (Apochromatic) ones don’t

14 th March 2007Terry Evans, Bridgwater Astronomical Society Telescope Types - Reflector Newtonian Reflector. The Parabolic Primary Mirror gathers the light The Flat Mirror reflects the light An Eyepiece magnifies the image

14 th March 2007Terry Evans, Bridgwater Astronomical Society Newtonian Reflector Points  Simple construction  You look in through the side (convenient)  Weight mostly at the back (mirror)  Inexpensive (cheapest for aperture)  No chromatic aberration  Long Tube  You look in through the side (imbalance)  Obstruction reduces contrast, esp. planets  Needs Collimation  Limited Backfocus  Coma (can be corrected)

14 th March 2007Terry Evans, Bridgwater Astronomical Society More Newtonian Points  Diffraction spikes useful for focussing  Diffraction Spikes reduce contrast  Mirror takes time to cool  “Tube currents”

14 th March 2007Terry Evans, Bridgwater Astronomical Society Cassegrain Reflector The Parabolic Primary Mirror gathers the light The Hyperbolic Secondary Mirror reflects the light back down the tube An Eyepiece magnifies the image

14 th March 2007Terry Evans, Bridgwater Astronomical Society Cassegrain Points  Short tube  Free from Spherical Aberration  High quality image  More Expensive than Newtonian  Narrow field of view  (Very) uncommon

14 th March 2007Terry Evans, Bridgwater Astronomical Society Schmidt Camera The Spherical Primary Mirror gathers the light The Corrector Lens corrects for spherical mirror Detector (film) at the prime focus

14 th March 2007Terry Evans, Bridgwater Astronomical Society Schmidt Camera Points  Very wide field of view  Camera only  Curved focal plane  Very Rare!

14 th March 2007Terry Evans, Bridgwater Astronomical Society Schmidt Cassegrain Telescope The Spherical Primary Mirror gathers the light The Corrector Lens corrects for spherical mirror The Convex Secondary Mirror reflects the light back down the tube An Eyepiece magnifies the image

14 th March 2007Terry Evans, Bridgwater Astronomical Society Schmidt-Cassegrain Points  Short Tube  Look in through the back (stable but can be awkward)  Spherical mirror much cheaper than parabolic  Huge range of accessories  Relatively heavy  Fairly expensive  Corrector dews up easily  Needs collimating  Curved focal plane (can be corrected)  Long Cooldown time

14 th March 2007Terry Evans, Bridgwater Astronomical Society SCT Focussing Points  Very long backfocus  Can attach diagonals  Can attach long optical train for imaging, spectroscopy, etc  “Focus shift”  Mirror can “flop” An SCT focuses by moving the Primary Mirror

14 th March 2007Terry Evans, Bridgwater Astronomical Society Maksutov Telescope The Spherical Primary Mirror gathers the light The Meniscus Lens corrects for spherical mirror The Convex Secondary Mirror reflects the light back down the tube An Eyepiece magnifies the image

14 th March 2007Terry Evans, Bridgwater Astronomical Society Maksutov Points  Short Tube  Very high quality images (smaller obstruction than SCT)  No collimation  Focussed using Primary mirror  Long focus  More expensive than SCT  Corrector dews up easily  Very long Cooldown time (fan cooled)  Focussed using Primary mirror

14 th March 2007Terry Evans, Bridgwater Astronomical Society Other Telescope Types Folded Refractors Enhanced Refractors (Petzval) Ritchey-Cretien (Hyperbolic Primary & Secondary) Meade “Advanced RC” (SCT with different corrector to give a flat focal plane) Corrected Newtonians (Tak Epsilon, Cape Newise) Maksutov-Newtonian

14 th March 2007Terry Evans, Bridgwater Astronomical Society Mounts Dobsonian Alt-Az (Altitude & Azimuth) German Equatorial Fork Mount Computerised “Go-To” mounts

14 th March 2007Terry Evans, Bridgwater Astronomical Society Dobsonian Mount

14 th March 2007Terry Evans, Bridgwater Astronomical Society Dobsonian Mount  Very Cheap  Can take large Newtonians (“Light Buckets”)  Very simple to set up  Won’t track the stars Can add a tracking platform – expensive  Field Rotation  Finder position can be awkward

14 th March 2007Terry Evans, Bridgwater Astronomical Society German Equatorial Mount  Usually high quality  Tracks the stars  Good polar alignment using Polar Alignment Scope  Can take heavy loads  Can change scopes easily  Fairly expensive  Needs to be balanced  Not easy to set up  “Meridian Flip”

14 th March 2007Terry Evans, Bridgwater Astronomical Society Fork Mount (Alt-Az)  Fairly easy to set up  Fairly inexpensive  Tracks through the meridian  Limited to short tubes  Drives two motors  Field rotation  Limited at the zenith

14 th March 2007Terry Evans, Bridgwater Astronomical Society Fork Mount (Equatorial)  Just needs a Wedge  Fairly inexpensive  Tracks through the meridian  Limited to short tubes  Non trivial polar alignment  Limited at the Pole  Can’t change scopes  Arm flexure  Gear wear

14 th March 2007Terry Evans, Bridgwater Astronomical Society Computerised Go-To Mounts Can be Fork or GE mount Need to be able to identify some bright stars Get a good finder or a wide field eyepiece Cheap versions need to be “trained” Forks can work in Alt-Az mode (very well)

14 th March 2007Terry Evans, Bridgwater Astronomical Society Telescope / Mount Combinations Large Newtonian on Dobsonian mount Small / Medium Newtonian on GE Mount Refractor on GE Mount SCT / Maksutov on Fork Mount SCT / Mak on GE for imaging

14 th March 2007Terry Evans, Bridgwater Astronomical Society Imaging Considerations Has to be Equatorial and Polar Aligned Fork Mounts are too unstable GE Mount needs to take scope + weights Portable rig needs to be easily aligned Periodic Error Correction a “must have” Allow for a guidescope Small telescope, big mount !

14 th March 2007Terry Evans, Bridgwater Astronomical Society Other bits and pieces Finder scope –Right angle can be unintuitive –Straight through can be awkward to use on Newtonian Good range of eyepieces –Get a wide field (low magnification) first –Barlow lens rather than high magnification eyepiece

14 th March 2007Terry Evans, Bridgwater Astronomical Society More bits & pieces Battery / Power supply Heater straps to prevent dewing Electric focuser Filters Full aperture Solar filter Adjustable chair Handwarmers & Thinsulate gloves

14 th March 2007Terry Evans, Bridgwater Astronomical Society Examples Not necessarily recommendations!

14 th March 2007Terry Evans, Bridgwater Astronomical Society Small Refractor, GE Mount Bresser Skylux 70 £90 Undriven mount, no upgrade

14 th March 2007Terry Evans, Bridgwater Astronomical Society Mid-size Refractor, GE Mount Sky-Watcher Evostar 102 4” £270 Undriven mount, Upgradeable

14 th March 2007Terry Evans, Bridgwater Astronomical Society Small Newtonian, GE Mount Sky-Watcher SkyHawk-1145PM £150 Light Mount, Driven

14 th March 2007Terry Evans, Bridgwater Astronomical Society Small Newtonian, GE Mount Sky-Watcher Explorer 150P £250 Undriven Mount

14 th March 2007Terry Evans, Bridgwater Astronomical Society Small Newtonian, GE Mount Orion Europa £450 Heavy Mount which could take bigger scope Driven

14 th March 2007Terry Evans, Bridgwater Astronomical Society Dobsonian Light Bucket Orion OD250L 10” £650 Orion 16” £2,000

14 th March 2007Terry Evans, Bridgwater Astronomical Society Dobsonian Light Bucket Meade Lightbridge 12” £650 16” £1,500

14 th March 2007Terry Evans, Bridgwater Astronomical Society Small Maksutov Meade ETX90 £550 Meade ETX125 £850 Excellent optics Good Go-To

14 th March 2007Terry Evans, Bridgwater Astronomical Society Mid-size SCT Celestron CPC 8” £1900

14 th March 2007Terry Evans, Bridgwater Astronomical Society Mid-size SCT Meade LX90 8” £1,700

14 th March 2007Terry Evans, Bridgwater Astronomical Society Big SCT / “RC” Meade LX200R 12” £4,000 16” £10,000

14 th March 2007Terry Evans, Bridgwater Astronomical Society Basic Imaging Rig HEQ5 Mount £800 William Optics ZS66 SD £250 Canon 300D Body £250 (E-Bay)

14 th March 2007Terry Evans, Bridgwater Astronomical Society Advanced Imaging Rig Paramount ME TEC 140 APO Refractor Pentax 75 SDHF Petzval Starlight Xpress SXV-H9, Filter Wheel & Filters

14 th March 2007Terry Evans, Bridgwater Astronomical Society Resources Sky at Night Magazine (Reviews) Cloudy-Nights (Reviews) US Based Excelsis (General Ratings site)

14 th March 2007Terry Evans, Bridgwater Astronomical Society Dealers Telescope House SCS Astro (Wellington) MC2 (Frome) Ian King Green Witch (Cambridge) Optical Vision Ltd (Sky-Watcher scopes)

14 th March 2007Terry Evans, Bridgwater Astronomical Society Second Hand E-Bay (mainly cheap tat! - careful)