1. Zen of Telescope Buying
Lee Keith of the
Milwaukee Astronomical Society

2. 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 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 = × 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°.

3. 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.

4. 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

5. 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.

6. 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!

7. 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.

8. 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”.

9. 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?

10. 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…

11. 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 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 = × 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°.

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

13. 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

14. 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.

15. 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)

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

17. 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)

18. 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)

19. 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)

20. 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

21. 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

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

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

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

25. 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

26. 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

27. 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

28. 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

29. 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.

30. 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!

31. 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?

32. ⇐ 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

33. 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

34. 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?

35. 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)

36. “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

37. 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.

38. 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 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 = × 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°.

39. 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!

40. 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…

41. Zen of Telescope Buying
Lee Keith of the
Milwaukee Astronomical Society