Thinking About a New Scope? Think Big!


Made in the USA



20" f/5$7,695

20" f/4$9,495

Plus Shipping & Crating

The Universe has just come into view!
The ultimate large aperture value
More light gathering area per dollar than all other models

This is truly big aperture.

Thousands of deep sky objects will tempt you. In fact, nearly everything in Uranometria 2000 is within your grasp!

The Obsession 20" is the scope that in 1989 revolutionized the Dobsonian.

Our user friendly design, craftsmanship second to none, one person set up and super smooth operation have since made Obsession the standard by which all other Dobsonians are compared.

The 20" is the ultimate blend of cost, ease of use, and visual impact. If you want an instrument that will provide you with a lifetime of celestial enjoyment, the Obsession 20 is it. It's simply the telescope of choice for deep sky enthusiasts.

20" f/5 vs 20" f/4:

The 20" Obsession can be ordered as an f/5 or as an f/4.

Eyepiece height at zenith for 20" f/5 is 8 feet.
Eyepiece height at zenith for 20" f/4 is 6.5 feet.

Advantages of f/5

Advantages of f/4

In short, both scopes provide dramatic and exciting views. The f/5 is the ultimate personal scope. It yields better coma free images, but requires two more ladder steps. The f/4 is easier for public viewing, but requires a coma corrector like the Paracor.

6 times the light gathering power of an 8 inch Schmidt-Cassegrain Obsession
Limiting stellar magnitude: 16.5 14.0
Rayleigh Resolution Limit: 0.27
arc seconds
arc seconds
Contrast limit:
(minimum diameter before a
nebular object fades from view)
arc minutes
arc minutes

Customer Comments

Keith B., 20" owner in Mason, Ohio

Dear Dave,

I took my scope out to its first star party.

I need to mount a paper towel dispenser on it to help viewers with their drooling.

Keith B.

Read more 20" customer comments


by Glenn Schaeffer

Bubble Nebula

Bubble Nebula
by Glenn Schaeffer

NGC 3628

NGC 3628
by Glenn Schaeffer

Sculptor Galaxy

Sculptor Galaxy
by Glenn Schaeffer

Dumbbell Nebula

Dumbbell Nebula
by Glenn Schaeffer


by Alex Zarvis


by Glenn Schaeffer


by Alex Zarvis

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20" Specifications:

Metric info shown in italics

Primary Mirror:

  • 20" - 508 mm GALAXY, or OMI 2 inch thick mirror
  • 96% reflectivity
  • Enhanced Aluminum Coating and Dielectric Overcoat (a supreme coating)
  • Center marked for precise collimation
  • plus Interferometry certification

see Optics page for more info

Secondary Mirror:

  • f/5: 3.1" - 79 mm United Lens 1/10 wave or better
    f/4: 3.5" - 89 mm United Lens 1/10 wave or better
  • Interferogram included
  • 96% reflectivity at 550 nm
  • Super enhanced Brilliant-Diamond non-fading multi-dielectric coating

see Optics page for more info

Focal Length:

  • f/5: 100" (± 2") - 2540 mm (± 50 mm)
    f/4: 80" (± 2") - 2032 mm (± 50 mm)

Eyepiece Height at Zenith:

  • f/5: (approximately) 8' - 245 cm
    f/4: (approximately) 6'6" - 198 cm

Mirror Cell:

  • 18-point

Truss Pole:

  • f/5: Length: 70" (± 2") - 180 cm (± 5 cm)
    f/4: Length: 55" (± 2") - 137cm (± 5 cm)
  • Diameter: 1.25" - 32 mm
  • Wall Thickness: 0.035" - 0.9 mm

Upper Assembly:

  • Diameter: 25" - 63 cm
  • Height: 13" - 33 cm
  • Weight: 11 lbs. - 5 kg

Lower Assembly:

  • A= 31" - 79 cm | B= 29" - 73 cm | C= 25" - 63 cm | D= 27" - 68 cm

see diagram

Mirror Box weight:

  • The heaviest component (what you don't lift): 100 lbs - 46 kg
  • Weight at Handles: (what you lift at Wheelbarrow handles): 25 lbs - 11 kg

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Mirror Options:

Obsessions are built with premium diffraction limited optics from Galaxy and OMI (Optical Mechanics Inc.). See Optics page for more info.

Choose from Galaxy or OMI mirrors when ordering.

Interferometry certification included on all mirrors, both primary and secondary. Ion deposition 96% enhanced coatings standard on all mirrors.

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Standard Features and Accessories:

Choose from four great focusers

The JMI EV3 comes standard on all Classic Obsessions. Or upgrade to the EV1, or EV1 with motofocus, or the awesome Feathertouch by Starlight Instruments. See the Focuser Options page for more information.


JMI EV3 (standard on all Classics)

JMI EV1 with motofocus

JMI EV1 with motofocus



Feathertouch by Starlight Instruments

by Starlight Instruments

Wheelbarrow handles

Comes standard on all Obsessions 12.5" through 25" at no extra charge.

Primary Mirror Coating

Enhanced Aluminum - 96% reflectivity, with dielectric overcoat (a supreme coating) standard on all primary mirrors. See Optics page for more info.

Secondary Mirror Coating

An Obsession exclusive - Super enhanced Brilliant-Diamond is standard on all Obsession secondary mirrors. This premium multi-dielectric coating has 96% reflectivity at 550 nm. The ultimate coating for the deep sky obsessed. This coating is standard on all Obsession secondaries at no extra charge. Brilliant-Diamond is just one of the reasons Obsessions out perform all others.

With 96% enhanced on the primary and our standard 96% Brilliant Diamond secondary, the optical system is equivalent to an extra 2" of aperture!

Optional Accessories:

See the Accessories page for more info on these items

Counterweight kit

(you supply the lead shot or sand) Recommended if you intend to use heavy barlow/eyepiece combinations, a Paracorr coma corrector, a traditional finderscope, or the Argo Navis digital setting circles.


Ripstop Nylon light shroud

Barlow-Laser Collimator 2" barrel

(Highly recommended). Includes 2" magnetic barlow lens and mask for the new barlowed laser collimating technique. Great visual aid and no better way to accurately align fast Newtonians. learn how

Argo Navis digital setting circles
with super res 10K encoders

more info

ServoCAT Goto Drive System

more info

Dew heater for secondary mirror

Secondary mirror heating system automatically senses the air temperature and the mirror temperature, then maintains the secondary at a preset (adjustable) temperature above the ambient air temperature to prevent dewing or icing.

more info

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Ordering Information:

A Deposit is required to place an order ($2,000 for 20" Classic)

Mail the deposit to:

Obsession Telescopes
PO Box 804
Lake Mills, WI 53551

You can e-mail your order, or mail it with the deposit.

  • With your order, please list any options and/or accessories you want, your mailing address, e-mail address if any, and phone number(s).
  • After we receive it, we will send you an order confirmation letter.
  • Delivery is usually 1 to 2 months from time of order.
  • Please e-mail for current delivery times.
  • Balance due prior to shipment.
  • Prices do not include shipping and crating. We ship throughout the world via UPS.
  • Any questions, please call or e-mail.
  • NOTE 10% order cancelation charge.