Thinking About a New Scope? Think Big!
Telescopes
20" f/5 & f/4 Classic
- 20" Specifications
- Mirror Options
- Standard Options and Accessories
- Optional Accessories
- Ordering Information
20" f/5
20" f/4
Obsession 20" Review by Alan Dyer - Astronomy Magazine
Obsession 20" Review by Ed Ting of Scopereview
Science at NASA announces Comet Petriew discovered with 20" Obsession
20" f/5$6,995
20" f/4$9,995
Plus Shipping & Crating
The Universe has just come into view!
The ultimate large aperture value
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
- Crisper images, no Paracor coma corrector needed. Coma is inversely proportional to the cube of the focal ratio. In other words, the coma free field of view in a f/5 is twice as big as in an f/4.
- Easier to achieve precise collimation of primary mirror.
- Cost is less than an f/4.
- Slightly smaller 3.1" central obstruction (3.5" on f/4)
Advantages of f/4
- Smaller ladder, two steps shorter than an f/5.
- Easier to use for public viewing.
- Rich field of view. Great for star cruising the Milky Way and large deep sky objects. Easier to keep object in field of view.
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 20" |
Typical 8" |
|---|---|---|
| Limiting stellar magnitude: | 16.5 | 14.0 |
| Rayleigh Resolution Limit: | 0.27 arc seconds |
0.68 arc seconds |
| Contrast limit: (minimum diameter before a nebular object fades from view) |
4.3 arc minutes |
11.0 arc minutes |
M51
by Glenn Schaeffer
Bubble Nebula
by Glenn Schaeffer
NGC 3628
by Glenn Schaeffer
Sculptor Galaxy
by Glenn Schaeffer
Dumbbell Nebula
by Glenn Schaeffer
M57
by Alex Zarvis
M104
by Glenn Schaeffer
M51
by Alex Zarvis
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
- 98% 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
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
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.
Obsessions are also available without optics if you already have your mirrors. Call, fax, or email for price and delivery.
Standard Options 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
Feathertouch
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 98% 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 98% 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.
Telrad
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
ServoCAT Goto Drive System
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.
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
USA
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.
Customer Comments
Mark K., 20" f/5 owner in Jacksonville, Florida
The star images in the OMI Mirror are pinpoints. The view of the moon with a 13mm Ethos was mind blowing. The 6" refractors just cant keep up. Nothing like the power of a large, well made scope and mirror.
After 6 months of crummy skys I finally got to take the 20" with OMI optics to our dark site. There are no other Dobs that comes close to the silky movement of this scope. It moves equal in any direction with no backlash. The views simply blew me away easily twice as bright as my 13" dob. There is no match for World Class Dobs and this is what you deliver. Thanks a Million.
Read more 20" customer comments