Hi,
I have used the Obsession 18" UC on several occasions and the first prototype 18" SDM UC Peter Read from SDM Telescopes produced, over 3 nights. Each under superb observing conditions at Coonabarabran. Gary Kopff from Wildcard Innovations and Andrew Murrell have also used them both. Consider my advise worth what it costs you, but at least take comfort from the fact that I have used them both, in addition to several other classic style Obsession and SDM telescopes.
Let me explain the origins of both scopes. John Hudek from Galaxy Optics in the US built himself a scope on the ultracompact design using a mirror cell as the virtual mirror box many years ago. That's where things sat with the concept for several years. John had a photograph of that scope on the homepage of his Galaxy Optics website for many years. Dave Kriege from Obsession Telescopes buys mirrors from John Hudek (amongst others) and got to thinking there may be a market for a scope based on John Hudek's design. Dave Kriege set about to improving the UC design based on maximising portability and transportability. He made a large number of innovative changes to John Hudeks' design and ended up with an improved scope. Dave is the first to acknowledge that John's UC scope was his inspiration. Peter Read who builds his classic style scopes based on the classic Obsession design, saw photographs of Dave's UC scope and figured he may as well do something similar, with his own improvements/modifications where he thought fit. The Obsession 18 UC is designed to be as light and tranportable and portable as possible and able to be rapidly assembled/dissassembled. The SDM UC is designed to be a compromise between portability/transportability and stability of the observing platform.
Let me say you will be very happy irrespective of which scope you buy, you can't make a bad decision here as they are both exceptional telescopes of outstanding quality.
Some of the differences between them:-
The most important is that the Obsession 18" UC uses a single plane plywood ring with the focuser fitted to a right angle accessory board and then a kydex light shield behind the focuser.
http://www.iceinspace.com.au/forum/a...se.php?a=34203
You can clearly see the single plane ring at the top of the scope in this photo. The kydex light baffle is not attached in the photograph. It just attaches to the ring with velcro.
The SDM UC uses a traditional type of Upper Cage Assembly (UCA) only it is a bit narrower than on a standard Obsession style scope. It is about 6 inches from top to bottom of the UCA.
The Obsession UC uses 6 thin poles all joined together at the top and they attach with 3 butterfly nuts to the UC ring. This makes them easy to transport. The SDM UC uses 8 standard sized poles that attach to the UCA in the traditional manner with camming clamps.
Both of the above differences make the Obsession 18 UC a little more portable than the SDM UC, but the SDM is more stable at the top end and a slightly better observing platform IMO. The Obsession UC flexes slightly when you drag it quickly in azimuth. The SDM is rock solid in every respect. It comes down to what is the most important issue to you, portability/tranportability or stability of the scope when you are observing.
I would expect you will get the Obsession 18 a bit faster than you will get the SDM UC 18. That is certainly the case with the classic style scopes. Whilst delivery of an Obsession 18 UC takes a lot longer than an Obsession Classic, delivery is still very fast for a custom made premium scope.
With the Obsession 18 UC your only choice of optics is an 18"/F4.2 mirror from James Mulherin at OMI. These are excellent optics. With the SDM UC you can select the mirror supplier from a list of premium mirror makers, including James Mulherin at OMI. Some will offer alternative F-Ratios as well, so you are not locked into an F4.2 scope only with SDM.
Something to consider is that because of the high number of scopes Obsession produce annually they cannot offer a scope in a non standard aperture, or F-ratio. SDM on the other hand being a small boutique manufacturer can build whatever you want. You may decide that a 16"/F4.5 or a 20"/4 is really what would suit your needs best. Peter Read can custom build either scope for you based on his ultra compact design. I will also add that Peter Read is a perfectionist and nothing leaves his shop that is not 100% perfect in every way.
In terms of accessories and options, anything you can have fitted to an Obsession UC you can also have fitted to an SDM UC, including Argo Navis and Servocat, which I recommend without hesitation. In fact because SDM are smaller they can offer some additional accessories not offered by Obsession, including the "UCA Dew Heater system" as opposed to just the Astrosystems "secondary" dew heater fitted to the Obsession.
I think you are wise in your choice of accessories. I would also order the Astrosystems barlowed laser collimater. Whilst you cannot collimate the telescope from scratch with this device alone, it saves an enormous amount of time once you have the scope properly set up and secondary and primary tilt are the only collimation adjustments required.
If you were to order the SDM UC I would also order the UCA Dew control system which includes the Thousand Oaks Dew controller, the Astrosystems secondary dew heater and dew heater straps for the finderscope, telrad and eyepiece. I would also get a 7 or 8x50 RACI finder fitted if it is not standard, in addition to a Telrad.
If you wish to attach a camera of any description to this scope for imaging purposes the SDM design is more solid in this respect IMO, although I am sure the Obsession UC would also hold a camera fine.
Cheers,
John B