转自CLOUDYNIGHTS：
The 18" f4.2 Obsession UC - Birth of a New Classic
Tom Trusock
6/07
Al Highie, Mel Bartels, and many others have spent years trying for that perfect portable dob. If you're active in the ATM scene, you'll have had glimpses of several different lightweight designs, but for us poor schmucks who don't know a hammer for a drill, well, we're stuck dreaming. There's always been trade offs for an ultra compact design, a few would include: shroud coverage, flexure, exposed mirror but for many of us the biggest is - lack of a major vendor. Till now, if you wanted a large aperture ultra compact scope you had some rather limited choices.
Without question, Obsession Telescopes has been around for a while. Owner Dave Kriege has been given due credit for the current dobsonian revolution, and his Obsession design has been often imitated by ATM's. Where some might not be too happy about that, Dave's attitude is just the opposite - he helps where ever he can, going beyond writing a how-to and supplying parts.
The classic Obsession design grew out of a desire to make large aperture dobsonians 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 car has decreased. Taking an 18" classic Obsession to a dark site in a Prius is an entertaining thought, but reality is 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 is 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 design. 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.
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, the current non-ultralight design has spent 10-15 years evolving and is really quite good. To take it to the next level, it seems logical that you'd have to give up something.
The UC gives up very little - except size.
Collapsed, the telescope 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.
As you can see, the Obsession Ultra Compact is a six pole, single ring upper truss design. The six poles as fastened together at the ends and accordion out 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. 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. This places 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 the 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 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. 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 versions. The only other differences will be an improved fit and finish.
And speaking of production: the 18" Ultra Compact is expected sometime this fall, and pricing is tentatively set identically to 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 seriously considering 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.
[ 本帖最后由 我爱祖祖 于 2007-6-20 17:19 编辑 ] |
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