Zen of Telescope Buying Lee Keith of the Milwaukee Astronomical Society

Magnification & FOV Review: Meade 6600 6” diameter, DO = 152mm fO = 762mm fe = 25mm FOVe = 52°   Once you understand the principle, the calculation is pretty easy. Don't underestimate the importance of being able to find this number, though, it is fundamental to the determination of most other performance parameters of the telescope. So let's try a couple of quick examples. My first telescope was a Meade 6600 -- they don't make it any more -- it's a 6-inch f/5 Newtonian scope. It came with a 25mm eyepiece. So... what was the magnification I was getting with this scope? Incidentally, notice how I characterize the scope and the eyepiece... the scope by its diameter and f-ratio, the eyepiece by its focal length. This is common parlance. It's also common to call out the scope diameter in inches, but for our purposes we will need to convert to mm. So the value we want for DO is 6 × 25.4 = 152.4mm. Wait a minute... the magnification is the focal length of the objective divided by the focal length of the eyepiece... so -- don't I need to tell you the focal length of the objective? Ah, but I have. Since the f-ratio is the focal length of the objective divided by the diameter of the objective, fR = fO/DO, then the focal length of the objective is found from fO = DO×fR = 152.4 × 5 = 762 mm. Then the magnification is fO/fe = 762/25 = 30.48, which we would just call 30. The eyepiece has a field of view of 52°, so the field of view for the telescope at this magnification will be 52 ÷ 30 = 1.7°.  

Scope of the Telescopes Targeting entry/mid level telescopes Used for VISUAL use only Rules are more stringent (expensive) for imaging Inexpensive beginner telescopes not suited for serious deep sky imaging Can attach phone for pictures of Moon, maybe small images of Jupiter, Saturn, Venus This presentation on milwaukeeastro.org.

Ownership Experience & “Looking Under the Hood” Don’t just consider how cool it looks, how many gadgets it has, mileage or price. Consider what happens AFTER purchase. Use a “holistic” approach; look deeper. For a car, you would also consider: Fuel cost Maintenance record/costs Insurance cost Safety record & devices “Usefulness”: Passengers and/or cargo

Telescope Myths Need a telescope with high magnification. Telescopes are always long, thin & white. Any telescope will show all objects well. Celestial objects will look like Hubble images. If it looks cool, it will work well also. I want the biggest one possible – NOW! Telescopes see through clouds. I can see Apollo astronauts on the Moon. Good telescopes must be expensive.

Telescope Realities If you can’t lift it, it won’t get used. You may have to find the objects yourself. Light pollution limits what and how well you can see objects. Need to find dark skies. Smallest/cheapest scopes only see Moon well; planets & few very brightest objects poorly. It will be dark. It may be late. It may be cold. There may be mosquitoes!

Telescope Advice Don’t buy one just because it seems in/expensive. Think about what you want to observe most. Don’t buy one that is too big or heavy. Think about how you will store & transport it. If you don’t know the constellations or time is short, get a computerized (GOTO/PushTo) scope. Be skeptical of hyperbole in advertising. They’ll never tell you what they’re NOT good for.

Telescope Advice If you are not sure, use MAS telescopes first. We also have many experts on astronomy and telescopes with decades of experience. Save hundreds (thousands?) by not buying the “wrong” telescope prematurely. Find out if you really like the hobby first. If 1st scope, consider getting small (4-6”) scope now and larger one later. This can then be a portable “travelscope”.

Toy Telescope Warning! Costs less than $200 or so Crows about how much “power” it has (>500x) Shows Hubble pictures of objects on box Has fuzzy optics, shaky mount & blurry, soda straw eyepieces Sold in toy, dept store or Menards Typically a 60mm refractor or 3” reflector See only Moon, moons of Jupiter & rings of Saturn poorly assuming you can find them & keep them in view in the scope. What did you expect for a toy?

INTERMISSION For those who can’t wait to buy a telescope, any telescope ASAP and feel they have enough information to get a head start on those who want to buy the best telescope for them. While we’re waiting…

Magnification & FOV Review: Meade 6600 6” diameter, DO = 152mm fO = 762mm fe = 25mm FOVe = 52°   Once you understand the principle, the calculation is pretty easy. Don't underestimate the importance of being able to find this number, though, it is fundamental to the determination of most other performance parameters of the telescope. So let's try a couple of quick examples. My first telescope was a Meade 6600 -- they don't make it any more -- it's a 6-inch f/5 Newtonian scope. It came with a 25mm eyepiece. So... what was the magnification I was getting with this scope? Incidentally, notice how I characterize the scope and the eyepiece... the scope by its diameter and f-ratio, the eyepiece by its focal length. This is common parlance. It's also common to call out the scope diameter in inches, but for our purposes we will need to convert to mm. So the value we want for DO is 6 × 25.4 = 152.4mm. Wait a minute... the magnification is the focal length of the objective divided by the focal length of the eyepiece... so -- don't I need to tell you the focal length of the objective? Ah, but I have. Since the f-ratio is the focal length of the objective divided by the diameter of the objective, fR = fO/DO, then the focal length of the objective is found from fO = DO×fR = 152.4 × 5 = 762 mm. Then the magnification is fO/fe = 762/25 = 30.48, which we would just call 30. The eyepiece has a field of view of 52°, so the field of view for the telescope at this magnification will be 52 ÷ 30 = 1.7°.  

Ready to Make a Rational, Informed Decision? Telescopes should not be an impulse buy. Ready to get educated? Let’s get started…

Telescope Terms Objective Aperture Focal Length (fl) f/ratio Light gathering lens or mirror Aperture Diameter of objective Focal Length (fl) Distance from objective at which image forms f/ratio Ratio of focal length/aperture Proxy for focal length Useful mostly in astro imaging of nebulae

Refractors (Lenses) Sedans & Sports Cars Pros Good ones have very sharp images Portable Cons Expensive per inch aperture Achromatic ones can show color fringes Types: Achromatic (2 elements ;corrected 2 colors) ED apochromatic (2 elements; extra-low dispersion) Apochromatic (3 elements; corrected 3 colors) Achromatic refractors not recommended.

MAS Refractors 80mm f/7 Hα (Red/Solar only) 5” f/5 Achromatic (White/Solar only) 100mm f/5.8 Apochromat (F Scope /Toeller Obs./Imaging only)

Reflectors (Mirrors) Minivans Pros Best bang for buck Cons Mirror collimation Types: Newtonian Dobsonian (Newtonian on alt/az mount) Highly recommended.

MAS Reflectors 12.5” f/8.86 Newtonian (“A” scope) 12.5” f/7.4 Newtonian (“B” scope) Two 18” F/4.5 Dobsonian (C & D Shed)

Catadioptrics (Mirror & Lens) SUVs Pros Very compact for aperture & focal length Cons Can be expensive, but less than refractors Most have large central obstructions Types: Schmidt-Cassegrain (SCT) Maksutov-Cassegrain (Mak-Cass) Maksutov-Newtonian (Mak-Newt) Schmidt-Newtonian (SNT)

MAS Catadioptrics (Schmidt-Cassegrains; SCTs) 10” f/6.3 (Ray Zit Obs/Visual/Piggy Back) 12” f/10 (Tangney Obs./Planets) 14” f/10 (G Scope/Z dome/Planets/Deep Sky)

Mounts – Altitude/Azimuith Simple – up/down, left/right horizontal movements Generally less expensive Can be less massive Does NOT follow objects as they move, unless computerized Generally not suited for photography

Mounts - Equatorial Automatically follows objects as they move Take a Alt/Az and tilt toward NCP and you have an equarorial German equatorials need counterweights Fork type does not Good for astrophotography

Computerized Mounts Can be motorized (GOTO) or not (Push To). Spend more time observing, not hunting. Good for adults short on time or experience.

Dobsonian Pick Up Trucks Newtonian scope on alt/az mount Best “bang for buck”. Can be bulky in larger sizes

Dobsonian Variations Classic Solid Tube Collapsible Tube Removable Truss Tube Ultralight (12”-30lbs!) Push-to or GOTO

Finders Optical (Small refractor telescope) Inverted, but bright image 50mm recommended on >=6” scopes Zero Power Reflex (“Telrad”) Projects red dot or target onto sky Naked eye view only No magnification; 1x

Focusers Rack & Pinion Crayford Helical Simple, classic focuser Roller-bearing design provides zero image-shift through focusing range - significantly improves performance compared to rack and pinion focusers Helical Inexpensive Rotate to focus Not recommended

Binoculars/Spotting Scopes (2 telescopes in one!) Great for large swaths of the sky (Milky Way) Large field of view (>5° vs <1° for telescope) Larger, brighter star clusters (Pleiades) Larger, brighter nebulae (Orion Nebula) Get at least 50mm (2”) aperture (7x/10x50) Andromeda Galaxy Comets Moon/eclipses Portable Good for daytime use

Telescope Truths More aperture is (usually) better! See dimmer objects See bright objects better See more details Mount must be solid!!! View will be jiggly and objects will not be seen as well as scope is capable of. Harder to find objects Not to mention that it is frustrating! Can spend as much or more for mount as scope

Telescope Truths Magnification is irrelevant when buying (!) Limited mostly by atmosphere Up to 50x per inch under good conditions with good optics; smaller in larger scopes e.g. 4”:200x 6”:300x 8”:400x 10”: 500x Use exit pupil to judge powers on larger >8” 8”: 200x 10”: 250x 12.5”: 318x High powers used only for Moon, planets and close double stars Any telescope gives “high” magnification with short enough eyepiece.

Putting It All Together: s Purpose What do you want to observe? No telescope shows all object types well. Portability Can you lift it? Can you transport it? Will you want to after carrying it 10x? In the cold with gloves? Price After picking scopes based on the above, narrow down by price. Don’t exclude your perfect scope based only on price. Don’t sacrifice quality for aperture!

Questions to consider when buying a telescope 1st P: What do you want to look at? The moon, planets, bright nebula, double- stars, galaxies? 2nd P: Will it fit in your vehicle? Room for gear? 2nd P: How far will you need to move the telescope to setup and use it? 3rd P: How much do you want to spend?

⇐ Astro Object Spectrum ⇒ Dim/Large Large Aperture Low Magnification Mostly Stars Mix of Both Most Forgiving Bright/Small/ Detailed High Resolution High Magnification Galaxies Globular Clusters Planetary Nebulae Bright/Emission Nebulae Galactic/Open Clusters Sun, Moon & Planets Dark Nebulae Variable Stars Multiple Stars

What Scope is Best for Object? Scope Type <4” <8” >=8” Short FL Long FL Lens Mirror Cat Object Sun x Moon Planets Star Clusters Nebulae Galaxies Dbl Stars Comets Scope not needed for aurora or meteors. Long FL>900mm/36in

Face Off: “A” vs “B” Scope Each appear superficially similar. Each are 12.5” diameter but… “A” is f/8.86 (111 inches fl) (large image scale/magnification) “B” is 7.4 (92.5 inches fl) (long fl but 20% wider field of view than “A” & 42% “faster”) “A” has smaller secondary for higher contrast view of planets. (Thanks Paul!) Now do you see that they are not the same?

Are My 5 Telescopes Too Many? 6” f/6 Dobsonian (Ground mirror myself!) Most portable widest field low power scope. 180mm (7.1”) f/15 Mak-Cass Portable high power planetary imaging 10” f/4.5 Newtonian Deep sky; wide field; still portable 60mm f/6.67 Coronado Hα Solar scope Above scopes all share one GOTO mount!! 16” f/4.5 Dobsonian (Push-to) Deep sky light bucket (big guns)

“Best” Scope for Visual Observation of Moon & Planets According to Gerald North, author of “Observing the Solar System” 8-12” long focus Newtonian Economical but not portable >=5” circa f/8 ED achromatic refractor Great image quality; expensive 6-10” long focus Mak-Cass or Mak-Newt Close behind ED refractor but more economical 8-14” f/10 SCT (9¼” preferred) Has large central obstruction

Recommendations $200: 50-70mm binoculars/spotting scope View comets, Milky Way vistas, star clusters $500: 4-5” computerized scope or 6-8” basic Dobsonian. $1000: 6-8” Schmidt Cassegrain (SCT) or 5- 6” computerized newtonian/Mak-Cass or 8- 10” computerized Dobsonian. See them at American Science & Surplus at 68th & Oklahoma Ave. Can buy new or used online.

Magnification & FOV Review: Meade 6600 6” diameter, DO = 152mm fO = 762mm fe = 25mm FOVe = 52°   Once you understand the principle, the calculation is pretty easy. Don't underestimate the importance of being able to find this number, though, it is fundamental to the determination of most other performance parameters of the telescope. So let's try a couple of quick examples. My first telescope was a Meade 6600 -- they don't make it any more -- it's a 6-inch f/5 Newtonian scope. It came with a 25mm eyepiece. So... what was the magnification I was getting with this scope? Incidentally, notice how I characterize the scope and the eyepiece... the scope by its diameter and f-ratio, the eyepiece by its focal length. This is common parlance. It's also common to call out the scope diameter in inches, but for our purposes we will need to convert to mm. So the value we want for DO is 6 × 25.4 = 152.4mm. Wait a minute... the magnification is the focal length of the objective divided by the focal length of the eyepiece... so -- don't I need to tell you the focal length of the objective? Ah, but I have. Since the f-ratio is the focal length of the objective divided by the diameter of the objective, fR = fO/DO, then the focal length of the objective is found from fO = DO×fR = 152.4 × 5 = 762 mm. Then the magnification is fO/fe = 762/25 = 30.48, which we would just call 30. The eyepiece has a field of view of 52°, so the field of view for the telescope at this magnification will be 52 ÷ 30 = 1.7°.  

Astrophotography/Imaging Appropriate telescope (were you listening?) High quality, stable equatorial mounting DSLR or CCD astrocamera & laptop Do either planetary and/or deep-sky imaging Don’t worry, the MAS has everything you need Try it, you’ll like it!

Decisions, decisions, decisions! Better Eyepieces Astro Camera (another time) Accessories galore! Telescope and/or eyepiece case Filters: Light Pollution (LPVs), Moon & Planet Red light/headlamp Collimator Barlow lens Anti-dew heaters Star Atlas & other reference materials It never ends… Next time…

Zen of Telescope Buying Lee Keith of the Milwaukee Astronomical Society