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First Look: Obsession 15" UC

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First Look: Obsession 15" UC
Tom Trusock

Last year, Dave Kriege of Obsession Telescopes released the Ultra Compact 18".  At a clearance height of 14", the UC fits into places where large dobs have never fit before.  However for various reasons 18" may still be a bit more than some folks want to handle, so this year Obsession is introducing a 15" version of the UC. 

And I had a chance to play with the prototype last weekend at a gathering up in Michigan's UP. 

To make a long story short, if you've seen the 18", you probably have a good handle on what the 15" is all about.  It's smaller, lighter and less expensive - just what you probably suspect.  The mirrors will be coming out of OMI as Galaxy makes nothing smaller than 18".  The 15" UC follows the 18" UC at the f4.2 focal ratio.  As compared to the 15" Classic (which I've also used), it takes up roughly half the height in transport and weighs around 25 lbs less.

15" Obsession Ultra Compact
Focal Ratio
Focal Length
Tools Required
Captive Hardware
Available Option
None (Assembly)
Northern Hemisphere Late Fall 08
Southern Hemisphere Late Spring 08)

According to Dave Kriege, the only significant differences between the prototype I used and the production models coming out in the fall are:
  • Rocker box - the rocker box was NOT production, simply something that was slapped together quickly for a photo shoot and the trip, and frankly, it showed in both finish and utility.  I'm told that the scope will be quite a bit more stable on the production base than it was on the prototype base.
  • Lack of lower light shield - the 15" UC will have the same light shield around the primary as the 18" UC, there was no shield on the prototypes.
  • Lack of shroud - the shroud was not ready for the prototype. Let me take a minute and note something - In this neck of the woods, shrouds are important not only for their ability to stop glare, but also as dew shields. Fortunately, in this case, the one evening we used the scope, the conditions were extremely dry and the primary did not dew over. However, a shroud is strongly recommended.
I had a chance to see Dave set it up, give it a good going over and then use it over the course of an evening.  On the whole I was pretty impressed.

Mechanical Notes:

The scope can be quickly and easily setup by one person (see video at bottom of page), and stores into a relatively small space.   While the (optional) storage container wasn't as small as I'd hoped (it appeared very similar to the storage unit for the 18" UC) it did appear to provide very good protection and assuming you have the clearance in your vehicle, allowed one to pack around it.  If you have a tiny vehicle, then the storage unit probably won't be an option.  

As standard equipment on the UTA you'll find the 2" FeatherTouch focuser, reverse counterweight system, foam covered handle (a thoughtful touch), light baffle and Telrad.  The secondary is 2.6" - the same size as on the Classic - and uses three thumbscrews for adjustment.    Note that a dew heater is not standard equipment.  A quick glance at their website reveals that Obsession now sells a dew heater which automatically keeps the temp of the secondary a few degrees above ambient.  (Most likely it's the same one as sold by Astrosystems, and I highly recommend it.)  Further note that the 15" UC uses a three spider vanes rather than four, so on bright targets the diffraction spikes might be a little different than what most folks are used to.   The light baffle velcros on, and does an effective job of blocking light directly across from the focuser.

The scope uses an "offset" six pole design where the tubes are already connected.  The hardware is captive so you don't have to worry about getting to your dark site and having lost something.  In the same train of thought, all assembly and collimation is "no tools" (excepting your laser, cheshire, etc..).  The scope went together quite simply and in a matter of minutes.

Height to the eyepiece when you're observing at zenith is around 60".  For me, this was a little on the short side and would require me to bring a chair.  But then again, I'm a standard deviation (or three) to the right of the bell, so take that into consideration.  In the photo above Dave Kriege (around 6' 1") gives a good size reference for the scope.

If you go with the Argo Navis option, you will be able to anchor it to one of the truss poles for easy access.   I was informed the ServoCat will also be available as an option.

Moving down to the virtual rocker box, lets take a look at the cell.  It's a nine point floating mirror cell, with three adjustments.  Basically, its of the same design that's found in the other ~1,700 Obsessions that are in the field.  It uses a Kevlar sling for edge support, and sports a small fan mounted in the middle of the cell.  Note the fan does not come hooked up to anything.  If it were me, I'd either buy a set of rechargeable batteries and attach them to the cell, or I'd opt for simply using a 9 volt battery.  In my opinion, the key to this scope lies in keeping things as simple as possible.  If you noticed that the rocker box in the photos is mismatched to the bearing size - remember again, this isn't a production rocker, rather it was simply something knocked together quickly with materials at hand.

You'll note the mirror cell attaches directly to the bearings.  The bearing system utilizes the same ebony star and Teflon as found on other Obsessions, and the scope (for the most part - more on that latter) has inherited the standard Obsession motions, which is to say, there ain't much smoother.  The bearings fold for transport and storage.  In addition, they sport a set of handles to assist you in moving the scope, and holes for the (standard) wheel barrel handles.  When moving the scope via the wheel barrel handles, there are going to be 4 attachments for the bearings and rocker box.  Once again, not present on the non-production rocker.

The mirror cover sits atop the mirror and while somewhat flexible is quite thick and easily capable of supporting small objects which may get placed upon it during transport.  It's not exactly pretty, but there's little danger of this one snapping or cracking - and that's really what you want.

Usage Notes:

One of the nice things about this little get together at a remote location in Michigan's Upper Peninsula was the gear.  For a small gathering, there were several amazing products; a 30" Obsession, the new Binocular Photon Machine and the 15" UC Prototype just to name three.   Although it wasn't a new moon weekend, we had about an hour and forty five minute window for true deep sky observing (1:45am to 3:30am), and a much longer observation window with the moon up.   After moonset, we noticed that there was a slight aurora going on in the north but since (most) of the targets we were interested in were in the south it was of little import.  Temps were around 45-50F, humidity was low, SQM was 21.56, and seeing was around Pickering 4.

Observations made with the moon up showed the glare you'd expect from the lack of a shroud.  Although it's listed as an option, it's a necessity for anything other than dark, dark sky viewing.  If you're thinking about one of these, add the cost of the shroud into the price.

Another item you'll want to budget for - a coma corrector like the Paracorr.  My personal "œmust have Paracorr" point is when the focal ratio becomes faster than f5, although I can deal with it (if I have to) down to around f4.5.  At f4.2 however, I feel that most observers really aren't getting everything they can out of  the system without some form of coma correction.

With the moon down, the scope was an absolute joy to use.   The spiral arms in M51 were obvious.  M13 was a brilliant blue white ball of stars, resolved across the face.   M71 was superb, as was the Ring.

Balance was not an issue with the two eyepieces we used that evening: Nagler 31 and Ethos 13.

While the lack of a Paracorr was obvious, the views (on axis) were sharp and had wonderful contrast.  Fifteen inches is a significant amount of light gathering to use from a dark site, and this little guy makes for a wonderfully portable galaxy grabber and glob buster.   Every OMI mirror I've looked through has been excellent, and this one looked to hold to that standard.

One issue I've noted with other ultra-light /  ultra-compact designs is that due to the lack of mass, they often become a bit skittish and jump around as you move them.  The UC exhibits that tendency, but to a lesser amount - most likely in part due to the 2" thick mirror.  Note that Obsession touts this scope as Ultra Compact not, Ultra Light.  In my opinion, everything is a trade off - the sheer act of making the telescope  compact and light means you're going to lose some stability.  While Obsession didn't manage to pull off a miracle, the 15" UC is actually better in this regard than I expected, and I'm told with the production rocker it should be better yet.

General Thoughts:

The UC is designed to provide a method for one person with a small vehicle to get a decent amount of aperture (and the rest of their stuff) to a dark site.  It does so with aplomb.  If you're like me; all thumbs when it comes to amateur telescope making, but still want a portable large aperture telescope and looking hard at the UC, then the natural question is which one to pick.  If the height to your eye is 61 or 62 inches or more, and you absolutely do not want to haul a step stool to your dark site, then the 15" becomes the logical choice.  If money and size aren't issues, personally I'd opt for the 18" UC.  In terms of the views - 15" is nice, but 18" is better.   Aperture rules.

The UC line has come on the market at an auspicious time.  Now that we're in the realm of $4+ for a gallon of gas, I suspect we're going to see the end of many of the large vehicles we've used to transport our toys to dark sites.  It's good to know that when the time comes to get rid of my truck I'll still be able to take along a large aperture telescope.  I rather suspect these will be a sizeable success for Obsession.

And special thanks to Brent for the loan of his digital camera after mine decided to eat my memory card....


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