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Obsession 25” f/5 – Rebuilding a Classic

Posted by Dave Bonandrini   06/19/2010 05:50PM

Obsession 25” f/5 – Rebuilding a Classic

The 25 inch f/5 telescope is Obsession's flagship product. It’s the largest telescope Obsession makes. You can throw your Parracor into the weeds, because the scope is f/5 and does not need it. But because it is f/5, the scope is very tall; as in eleven feet tall.

This scope has been in production for 20 some years, so you see a lot of them, in varying states of condition, at every star party.

The Obsession 25" is a real man's telescope. I don't say this in some chauvinistic kind of way; I mean it really is a manly machine. It is not going to fit in the back of your Honda; you are going to need a van or trailer to move it. You are going to need a healthy body to push it up the ramps. You are going to need tools to maintain it. You are going to need an 8 or 10 foot ladder to safely view at zenith. If you are the kind of man who can successfully get your kid's Frisbee out of the gutter rather than have to wait for the lawn care guy to do it, this scope is for you.

Two years ago, the scope in this article was picked up new from Obsession. The owner loves it, but wanted to be able to assemble it by himself without carrying the heavy UTA up the ladder. He also felt that it was just too hard to wrestle the 8 truss poles into their sockets that high off the ground. It is easy to reach across an 18” scope to position the trusses; but a 25" at eleven feet up is another story completely.

The Fix

The fix for this dilemma is some newer style hardware, and a device called a Connecting Ring.

The Connecting ring allows all of the trusses to be clamped together before the UTA is attached. As a bonus, you can attach the Connecting Ring and the UTA with your feet flat on the ground!

Fixing up the UTA

The original UTA had 4 truss sockets that require you to place two truss poles into each one. Then you flip a cam lever to hold the trusses in place. The cam levers are not captive, so you often see owners use string, wire of fishing line to keep them from backing out and getting lost in the vehicle.

In order for the Connection Ring to sit flush against the UTA, first we need to remove all of the old hardware.

The Truss sockets are removed with two small Philips screws. The ceiling tile hanger bolts simply pull through.

Next, using a sharp chisel, remove the plastic cap from the two drywall screws. Remove the plastic collar also and discard.

Using a countersink bit, enlarge the holes and replace with stainless steel screws.

Remove the strut bolts. The washers under them appeared to have been set when the polyurethane was still wet, so use caution so you don't pull off a big chunk of wood. Use a sharp chisel, and take your time.

Countersink the hole and spray on some new polyurethane. If you don't the wood may turn grey from getting dew on it every night.

After the poly dries, replace with a stainless steel flush bolt.

The Connecting Ring is a Baltic Birch ring made to the same diameter as the original Obsession ring size. The matching stain is Minwax Colonial Maple. Spray with four coats of polyurethane.

The Registration pins were made from one inch aluminum tubing.

The Toggle Clamps came from Reid Tool Supply:

The UTA Brackets and Clamps came from Webster Telescopes:

Fixing up the Mirror Box

The old split clamps were removed from the Mirror Box. Four drywall screws were removed from the corners and one 3/8 bolt needed to be tapped back into the box and removed from the inside. One clamp had a broken clamping finger where the wood had separated between the ply.

It seemed a shame to take the scope apart and not optimize it for maximum contrast. The light baffle was glossy wood (anything reflecting stray light back into the eyepiece is bad design). The wood was masked off and the entire light baffle was spray painted flat black. All of the inside hardware was also sprayed flat black.

While the ALT bearings were off, they were cleaned and waxed with a fresh coat of TurtleWax paste wax.

The Rocker Box was flipped over and the Ground Board was removed. There was found the strangest sight; the Ground Board had ten pieces of Teflon! It had the normal three large pads and the central pivot pad, but it also had three little scraps placed around the pivot and three more around the outer edge. The pads were glued not with contact cement, but foamy glue that looked like polyurethane.

The bottom of the Rocker Box was covered in FRP (Fiberglass Reinforced Panel) , a good choice for a scope of this weight. The FRP had a strange, sticky coating that was like baby powder and Vaseline mixed together. The sticky coating was cleaned off and a new coat of TurtleWax was put in its place.

New Webster truss clamps were installed on the mirror box, and the scope was reassembled.

Before returning the mirror to the scope, the mirror was washed and rinsed. One could see light shining through holes where the enhanced coatings have begun to fail. Even brand new mirrors can have hundreds of pin holes, but these holes were the size of a nickel.

People love to argue about mirror coatings. Conventional wisdom says that enhanced coatings are more reflective, but do not last as long as standard aluminum coatings.

Whatever the case, these coatings clearly did not last more than two years.

The Connecting Ring was installed and the clamps were adjusted to keep the same distance between the UTA and the top of the Mirror Box as the scope always had.

It was raining like a SOB, so the truss poles were left long until an actual star could be brought to focus to be sure all of his eyepieces came to focus. I know, it should be the exact same distance, but you know how Murphy's Law and telescopes interact these days.

The trusses will be trimmed after "first light".

The scope already had three counterweight tubes on the mirror cell and to make up for the weight of the Connecting Ring it will need one more.

The scope came out great. It was easy to assemble without climbing any ladders.

Dave Bonandrini