Thinking About a New Scope? Think Big!
20" f/5 & f/4 Classic
- 20" Specifications
- Mirror Options
- Standard Features and Accessories
- Optional Accessories
- Ordering Information
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)
- The 20" f/5 Classic Obsession is by far the best value. You get more square inches of aperture per dollar than any of the other models. In fact, if the 20" was priced similarly it would be nearly $1,500 more! If you want the most photon power per dollar the 20" f/5 Classic can't be beat.
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
|Limiting stellar magnitude:||16.5||14.0|
|Rayleigh Resolution Limit:||0.27
(minimum diameter before a
nebular object fades from view)
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.
Metric info shown in italics
- 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
- 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
- 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
- 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
- Diameter: 25" - 63 cm
- Height: 13" - 33 cm
- Weight: 11 lbs. - 5 kg
- 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
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.
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
by Starlight Instruments
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!
See the Accessories page for more info on these items
(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
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.
A Deposit is required to place an order ($2,000 for 20" Classic)
Mail the deposit to:
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.