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CN Report: The 18" f4.2 Obsession UC - Birth of a New Classic


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Obsession UC
The 18" f4.2 Obsession UC - Birth of a New Classic
Tom Trusock

6/07




Albert Highe, Mel Bartels, Greg Babcock, John Hudek, and others have spent years trying for that perfect portable dob. If you're active in the ATM scene, you've had glimpses of several different lightweight designs, but for us poor schmucks who don't know a hammer from a drill, well, we're stuck dreaming. Yes, there's always been trade offs for an ultra compact design: shroud coverage, flexture, and an exposed mirror but for many of us the biggest is the lack of a major vendor. Till now, if you wanted a large aperture ultra compact scope you had some rather limited choices.



Obsession Telescopes has been around for a while. Owner Dave Kriege has been given his due for his hand in the current dobsonian revolution, and Obsession design is often imitated by ATM's. Some might not be too happy about that, but Dave's attitude is just the opposite - he helps where ever he can, even to the point of writing a how-to book and supplying parts to the do-it-yourself crowd.



The classic Obsession design grew out of a desire to make large aperture telescopes portable, and there is no doubt it's weathered the test of time quite well. Unfortunately, city dwellers can attest that skies have gotten worse, gas prices have increased, and the size of the average vehicle is decreasing to cope. Yes, taking an 18" classic Obsession to a dark site in a Prius is an entertaining thought, but reality is a bit more frustrating. Especially if like so many of us, you just don't seem to be getting any younger.



So say hello to Dave's little friend: the Obsession 18 f4.2 Ultra Compact or UC. The UC's aren't intended to replace the original Obsession (henceforth referred to as the "Classic"), but they are an addition to the product line for those who need an extremely portable scope.



Obsession Telescopes unveiled the design publicly at Texas Star Party this year, but only recently finalized the prototype. They've now entered the production stage, and we've got an exclusive first look at this revolutionary telescope. Early this summer, I spent 5 days at a gathering in Michigan's UP with some good folks - Obsession Telescopes Dave Kriege among them. He brought the final UC production prototype along for the ride, and not only proceeded to put on an excellent dog and pony show, but better yet - he let me and other amateurs actually observe with the scope.

But before we get there, here are the stats:

OBSESSION TELESCOPES 18" f/4.2 UC (Ultra Compact)
from private correspondance with Dave Kriege

The 18" f/4.2 UC is not a replacement for the Classic Obsession 18 f/4.5. The UC is aimed at users that have very limited hauling or storage capacity. The UC will fit in any car and thru any doorway. It will appeal to people that like a minimalist style. The Classic Obsession 18 is aimed at users that want maximum stability and ruggedness and do not have transport or clearance issues. It will appeal to people that like the warmth and beauty of a fine wood instrument. Optically they are nearly identical except that a Paracorr is recommended on the UC.

No compromises. The UC uses the same proven design criteria and materials that made the Classic Obsession so successful.

Eyepiece height at zenith
  • UC is 71 inches
  • Classic is 78 inches
Set up time including nylon light shroud
  • UC is 2 or 3 minutes
  • Classic is 4 or 5 minutes
  • Add another minute or so for either to place nylon light shroud
Ease of use (setup, take down, collimation)
  • Same for both
Ease of Motions (hand guiding)
  • Same for both
Construction/materials
  • Same for both. Appleply and Baltic Birch.
  • UC components covered with satin black laminate.
  • Classic components are stained and have six coat polyurethane finish.
Bearing geometry and surfaces
  • Same for both, Ebony star on etch virgin teflon.
Light baffeling
  • Classic is best with 360 degree upper tube assembly baffle.
  • UC 160 degree baffle opposite focuser.
Optics (OMI or Galaxy)
  • UC f/4.2 primary mirror requires a Paracorr coma corrector for optimum views.
  • Classic f/4.5 Paracorr optional but not needed.
  • Same 3.1 secondary mirror for both.
  • Central obstruction and Image contrast the same for both.
Coatings
  • Same for both. 96% on primary, 98% on secondary.
Mirror cell
  • Same for both, 18 point.
Wheelbarrow handles
  • Standard on both.
Truss poles
  • UC Six 1 inch diameter. All six interconnected at apices.
  • Classic Eight 1.25 inch diameter. All eight separate.
Stability
  • Classic has the edge with 8 pole design and 1.25 inch diameter poles.
  • UC has 6 pole design and 1.25 inch diameter poles.
Dimensions of lower unit for transport
Includes mirror box nested in rocker
  • UC 24 x 24 x 13.5 high (will fit in any trunk or hatchback)
  • Classic 30 x 30 x 30 high
Dimensions of entire telescope nested for transport
Includes mirror box nested in rocker with upper tube assembly on top
  • UC 24 x 24 x 19.5 high (will fit in any trunk or hatchback)
  • Classic - Not possible. Upper Tube Assembly cannot be placed on top.
Custom foam lined travel case
  • UC uses a case 26 x 26 x 20 high (see below)
  • Classic - No way.
Upper Tube Assembly weight (with Feathertouch focuser)
  • UC 5 pounds
  • Classic 11 pound
Weight of heaviest component
(includes 2 inch thick standard thickness mirror)
  • UC is 65 pounds. Includes the virtural mirror box, mirror, and dust cover
  • Classic is 82 pounds Includes mirror box, mirror, and dust cover
Weight at wheelbarrow handles
  • UC about 10 pounds
  • Classic about 15 pounds
Weight of complete scope
  • UC about 90 pounds
  • Classic about 140 pounds
Clearance width required with wheelbarrow handles attached.
(or minimum size of doorway needed to roll in or out with wheels attached)
  • UC 27 inches
  • Classic 30 inches
Price and options:
  • Same for both.
  • The Obsession 18 f/4.2 UC is $6340 and includes the Feathertouch focuser, Telrad, external light baffle, and counterweight kit. This is the same price as the Classic Obsession 18 f/4.5 with same items.
  • Optional Paracorr is $295. (Highly recommended at f/4.2)
  • Optional ripstop nylon light shroud is $149. (Highly recommended)
  • Optional Argo Navis DSC with 10K res encoders is $795.
  • ServoCAT GoTo drive available late 2007.
  • Optional white 26 x 26 x 20 thermo-formed, high-density polyethylene shipping container foam lined. Case meet the Air Transport Association's(ATA) Specification 300, Category 1 requirements. is $495.
    • (Having scope shipped in this case saves you about $200 in packing costs).
A thank you to Obsession Telescopes for allowing CN to reprint this correspondance in the article...




We were impressed. But more on that later.

As I've mentioned, there are always design trade-off's involved in an instrument. Previous ultralight designs have sacrificed weight for stability - resulting in both flexture and, well, a skittishness in the motions. Lets face it, current non-ultralight scopes have spent 10-15 years evolving and are quite good. To take it to the next level as per portability, it seems logical that you'd have to give up something.

The UC gives up very little - except size.



Collapsed, the scope is absurdly small. The rocker box isn't more than a couple of inches off the ground (if that), and there really isn't a mirror box. (Kriege referrers to it as a virtual mirror box.) The bearings are reinforced with steel plate, and fold into two pieces for the ultimate in portability. All exposed steel is stainless. The primary support system utilizes a Kevlar sling, and an 18 point cell that has a small cooling fan attached. The mirrors are f4.2 and will be supplied by long time associates Galaxy and OMI. They are (what is termed as today) standard thickness mirrrors.


As you can see, the Obsession Ultra Compact is a six pole, single ring upper truss design. The six poles are fastened together at the ends and accordion to make life easier during transport - no loose poles floating around. They also do a darn good job of eliminating flexture. This scope stays in collimation both when parked or slewing.



On the single ring upper truss assembly (UTR) you'll spot a couple of interesting things. First off, the FeatherTouch focuser is standard, and it's mounted on top of the UTR. The foam covered handle is an excellent touch and is in the perfect location for most observers. The Telrad is mounted on a post that also projects above the UTR and in an interesting design move, the post is hollow and is used to hold the counterweights - three of which are supplied with the system. This setup allows the user to go from lightweight mono eyepieces to heavy binoviewers with a minimum of balance issues.



Those of you wishing to computerize the setup need not fear about clearance - a recess is provided in the ground board for the AZ encoder and provisions have been made for a virtual encoder mount for the ALT. The computer itself is mounted to a clip that attaches to the truss poles, placing it in a very natural position.



One of the major concerns dealt with the initial apparent lack of a shroud. Dave Kriege is well aware that most of us consider a shroud necessary (for a number of reasons), and has devised a system that keeps the shroud tight over the scope and out of the light path. The shroud will be an option for the production units. Because the focuser is mounted above the single ring UTA, a light shield is included as standard equipment.



The secondary sports tool less adjustments, and features an Astrosystems dew guard kit. (No, the high tech secondary cover shown in the images above is not included. You'll have to use your own sock.) The shroud ends a couple of inches above the primary, but thermal inertia should work fairly well to keep dew from forming on the primary until way into the early hours - at which point you'll most likely be asleep anyway.



As you can see, the mirror is more exposed to incidental dust and dirt than in a traditional design, but that's one of the few trade-off's for the ultra light weight and portability of the Obsession UC.



Wheel barrel handles are available for the observer who wants to keep the scope fully assembled and roll it out to observe. The scope can be locked into position using a Velcro strip. Additionally, the front and back edges of the rocker box are padded to help prevent you from dinging up the corners in storage or transport.

While it's not a done deal, Obsession is considering offering the transport case shown in the photos for resale. While increasing the room the scope takes up, it allows for "no worry packing" - a boon for those of us who have vehicles that are already crammed to the gills.

Initial setup and collimation took a matter of mere minutes (probably less time than it took you to look at the pictures). The collimation adjustments for the secondary are easily reached from the eyepiece, making that last bit of tweaking a snap.

Observing with the scope was a blast. The 18" UC is - hands down - the best star party scope I've had opportunity to use.




The shroud and shield worked well - contrast was very good, and there were absolutely no dew issues. Another fear; unwary observers would kick sand and dirt on the mirror proved to be pretty much a non-issue. But as the optics arrived a little dirty (this is the 5th prototype this optic set has seen light in) Dave Kriege demonstrated how the open design lends itself to cleaning the primary.

At f4.2 you really do yourself a disservice if you're not using a paracorr but with it, images are spectacular. The 31mm Nagler in particular makes an excellent low power eyepiece for picking off targets. It provided a true field of just over a degree and magnification around 70x.

We spent the first part of the evening with the telescope, and I had ample opportunity to put it through it's paces. The scope was at a decent height - I'm 6 foot 2 inches, and most of my observing was done with my feet on the ground. For targets within 10-15 deg of zenith however, a small step was necessary. In use, the UC didn't feel like an ultralight. For one reason or another, the ultralights I've used have been a little skittish. Not much of that here. And you're probably wondering about the patented "Obsession" motions - while the motions weren't quite as smooth as the classic Obsession, they were as smooth as or smoother than most of the other dobsonians on the market. I expect they can be tweaked a bit further, but as it stood, they were most certainly good enough. There was a slight bump evident in the alt motion at one vertical position, but Dave Kriege tells me this will be pretty much eliminated from the production scopes. The only other differences will lie in an improved fit and finish.



And speaking of production: the 18" Ultra Compact is expected sometime this fall. Pricing is (tentatively) set around that of the Classic with the FeatherTouch upgrade.

All in all I was very pleased with the performance and portability of the telescope - so much so that I'm giving serious consideration to one of the planned 15" versions for my own personal ultimate travel / star party scope. The 18" was lightweight, quick to setup and easy to use with very few drawbacks as compared to the classic design. Everything that a telescope is supposed to do, it did. And quite well at that.



Twenty years ago, Dave Kriege helped revolutionize the dobsonian telescope.

He's doing it all over again.








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