Jump to content


- - - - -

Teeter Truss Conversion

Lurch Reborn: Rob Teeter Truss Structure
(or Beauty and the Beast)
Tom Trusock – ttrusock@hatchet.badaxe.k12.mi.us

 This is Beauty, where is the Beast?

The acquisition.

A while back I happened to come into possession of a rather unique telescope.  The optics (consisting of a 10” f7.5 Swayze primary) were simply exquisite, the focuser a  work of art (Feathertouch), the 80+ inch OTA, well;  no matter what else it was a conversation piece (California Tubular designs), and driven (dob driver II) to top it off.  I had only two gripes with the scope:

1)    the mount was badly in need of reworking.
2)    have you ever tried to transport a 10” f7.5 sonotube scope? 

I had thought it would be a test to see if I like observing from a platform, 6 - 8 inches off the ground.  I intend on getting a larger scope some day and wanted to determine just how large I could go.  I had initially thought I would keep it for 8 months to a year and then sell it to fund my upgrade to a bigger scope.  As time wore on, I wound up liking the optics so much, I decided I wanted to keep it.  I quickly realized, that if I were going to keep it, the mount would need a MAJOR overhaul. 

 There’s the beast! The 10” F7.5 before restructuring – the old
 8” f6 shown in the foreground gives a sense of scale.

To give you an indication of just how bad the mount was let me simply say that some of you may have seen an observing report, and recognize the scope by it’s name: Lurch.  The name was quite appropriate.  The motions, while probably acceptable at one time, had degenerated badly.  The stiction was so great that if you attempted to move the scope manually, at anything other than the lowest powers, you simply overshot the target.  The rocker box and ground board were made of 1" aircraft plywood, and stood just under 3 feet tall. The base alone was extremely heavy, and  while sonotube is convenient, 90+ inches of it made the scope a little difficult to transport.  It did fit in my medium sized SUV, but only if my daughter, wife and I agreed to have a large stellar post run through the middle of the vehicle and allowed it to rest on our shoulders.  Provided I wanted to keep the optics, everyone agreed that a truss would make life a little easier.

Why would anyone prefer a long focal ratio?

While the mainstay in days past, long focal ratio dobs are fairly uncommon today, due in large part no doubt to the talents of optical magicians like Carl Zambuto and structure design/manufacturing gurus like Rick Singmaster.  If you have ever looked through a well collimated Starmaster with a Zambuto primary, then you know that a short focal ratio scope can do extremely well on the planets, so you don't tend to sacrifice much at all.  Short focal ratio scopes have another advantage in usability.  Many can be used while seated or standing flat footed.  Long focal length scopes with decent aperture usually require some sort of platform to stand on while observing near the zenith.  

Every single telescope design is a trade off.  You simply have to decide what you are willing to live with.

If you are willing to cope with the fuss of a long focal ratio, it does have advantages to its shorter counterparts. (This is assuming we are talking similar quality optics and mechanics for both the long and short scopes.)

A long focal ratio scope is much easier to collimate than it’s shorter counterparts.  This can be easily seen by using formulas found in Nils Olof Carlins - FAQ about Collimating a Newtonian telescope (http://www.atmsite.org/contrib/Carlin/collimation/).  If you apply Sinnotts ¼ wave tolerance (this is where coma introduces a wave front error of ¼ wave),  and Sidgwicks’ strict definition (where coma just begins to effect the airy disk) the practical collimation tolerances of an f7.5 mirror fall between 3.04mm and  7.43 mm in diameter, while those of an f4.3 mirror lie somewhere between .57mm and  1.40mm.  Right along with this you can easily see that in a fast scope, coma begins to become intrusive (image degrading) a very small distance from the sweet spot.  You can also think of these definitions in terms of how large of a field stop your eyepiece can have and still be diffraction limited (or close to it) across the field.  This is a powerful argument for tracking in fast scopes that are used for planetary observing.

The effects of coma can be vastly reduced, and nearly eliminated. It could be argued that you can simply use a paracorr in a short scope, but consider that the long focal ratio scope does not need a corrective lens. This is one less piece of glass in the system - something that many observers, myself included, prefer. 

The longer focal length allows for a smaller secondary which in turn improves contrast because of the smaller central obstruction.  You may have heard long focal length dobs referred to as APO-eaters, and indeed they can compare very favorably (I nearly cried when I compared a little 6” f8 Zambuto to my TV-102; but that’s another story).  While contrast may not be everything, it’s certainly a big chunk.  Some makers of short focal length scopes trade off the fully illuminated field for a smaller central obstruction; this design will certainly improve planetary performance, but raises other issues.

The increased magnification at the eyepiece due to the longer focal length of the scope can also be a bonus to many individuals, particularly lunar and planetary observers.  It allows you to get to higher magnifications without resorting to  a barlow or Powermate, giving fewer elements to scatter light and reduce contrast.  Eye relief is maintained by the use of longer focal length eyepieces. Additionally, simpler eyepiece designs perform better on longer focal length scopes.  Many individuals believe that for planetary observing, a quality simple eyepiece such as an ortho is superior to a more complex eyepiece such as a radian (for several reasons, all beyond the scope of this article).

Greater depth of field – this translates into a scope that is easier to focus, and this can lessen the amount of refocusing you need to do because of atmospheric conditions.

Finally, if you are interested in figuring your own mirror, it is easier to get a good figure at a longer focal ratio.

But, in spite of all this, they aren’t nearly as popular (useability can be a very big issue), and calls to every manufactures I could find advertising truss conversions left me empty handed.  No one wanted to touch a 10” f7.5.  The situation sat for months.  I even considered making my own structure, but was forced to acknowledge my woodworking skills are sorely deficient.

Enter Rob Teeter.

In December of 2002, a friend dropped me a note about this “new guy” out in NJ.  Rob Teeter was running a little shop out there called “Teeter’s Telescopes”  Being the internet junkie that I am, I was surprised to find that a prolonged search brought up nearly nothing, only the company web page.  What was there looked intriguing.  Rob specialized in truss conversions and long focal length dobsonians from 8” up to 20” called “Teeter’s Planet Killers”.  I even noted that he offered a 10” f7 dob.

Satisfied, I asked him if he would be willing to undertake a 10” f7.5 truss conversion. He agreed and in late December 2002, I packed up the optics, mirror cell, focuser, diagonal, spider, and a check and sent them all his way.

The Construction

 The UTA nests inside the mirror box for transport

The pictures he sent were superb and served to whet my appetite, but I did wonder –what would it look like in person?

Looking at it now, I can tell you the pictures didn’t lie.  Rob’s workmanship is simply excellent. He used 1/2" Baltic Birch plywood with a maple veneer throughout the scope, except for the altitude bearings which are 3/4" Baltic Birch and the mirror cover which is 1/4" domestic pine (used for contrast).

All hardware is solid brass including large drawer-pulls, corner protectors and personalized name plate mounted on the mirror box, nuts and bolts on the upper tube assembly, and the chest handles on the rocker box.

All exterior wood surfaces have been stained using a Colonial Maple stain (my choice, Rob offers several options) and overcoated with three coats of Gloss Marine Spar Polyurethane (brushed, not sprayed, for a more handmade final appearance).   All interior wood surfaces (ie. interior of mirror box) are finished with several coats of Krylon Ultra Flat Black paint.

Because one of my primary requirements was portability, Rob designed the mirror box in such a way as to allow the Upper Tube Assembly (UTA) to sit inside of the mirror box and not harm the primary mirror during transport.  Rob liked this design so much, he decided to offer it on future Planet-Killers and Truss Conversions.

Rob’s fit and finish is first class.

The modified Novak 9 point mirror cell is removable, using four 1/4-20 stainless steel bolts with large diameter knurled heads, that thread into four 1/4-20 T-nuts fastened in the mirror box.  Additionally, the primary cell is easily adjusted by hand, and allows for a no-tools collimation.  In an interesting note, Rob found this cell on Astromart saw it as an upgrade to mine, and called to ask if I wanted it.

Rob used Moonlite ball and socket Truss connectors,  and Moonlite 1" diameter clear anodized Truss poles.  The connectors and blocks are made from black delrin plastic which is very sturdy and provides a solid connection between mirror box and upper tube assembly, while providing a no tools assembly.

Other components used in include Astrosystems' teflon kits (azimuth and altitude), central azimuth pivot kit.  Additionally, Rob provides a custom fit Astrosystems’ shroud, shipped directly from Astrosystems.

I decided to have Rob install 4000 tic encoders (using the Sky Engineering Mounting Kit) for use with my Sky Commander.

Rob also added a custom eyepiece holder for the front of the rocker box.

We kept my 10” F7.5 Swayze primary, secondary mirror (Astrosystems), secondary holder (also Astrosystems), Rigel Quick-Finder and Starlight Instruments' Feathertouch focuser.

As update followed update, eventually we reached the point where there was nothing left to do.  Rob dropped the scope off for shipping, and I cringed – not wanting to hear the bill.  Surprisingly, it was only $160 via the local UPS store.

Rob keeps you updated every step of the way.

A week later, the scope arrived, dropped in my garage by the boys in brown.  I unpacked two rather large boxes and several smaller ones, overjoyed to find that nearly everything came through shipping unscathed.  Initial setup (even though I had never seen the scope, never setup a truss, and there were no written instructions provided) was quick and easy.  The mirror quickly slipped into it’s cell.  Encoder tests to check that the encoders and computer were functional came out perfectly.  I found the motions of the mount to be extremely smooth, and required only a slight fingertip pressure for movement in either direction.  Collimation was easily accomplished in a matter of minutes, and  I was ready to observe.

Unfortunately, the weather did not cooperate.  It was to be 8 days until I would have a chance to take my revamped scope out.

First (re)Light

Notes from my first session with the scope follow:

“The stability and motion of the truss structure is fantastic. Stiction is practically non-existent, and hand guiding at 200x+ was a breeze, requiring only a fingertips worth of pressure.  The mount is so stable I didn't even have to take my hand off the UTA while observing – heck, I can move the scope by hand and keep right on looking.  Rob also did a superb job of making the structure orthogonal.  The Sky Commander worked impressively well on the *very first* try.  I've owned 5-6+ computerized scopes (using Autostar, Nexstar, Celestron AAM, and JMI miniMax) and NEVER had results like that from the start.  Later Keith and I were plopping DSO's dead center at 190x (13mm naglers in the binoviewer) – there was absolutely no need to drop to a low power finder eyepiece!  Further, every eyepiece we tried last night (and we tried a TON ) came to focus. The mount is so much better than what I was using it’s not even funny.  Rob also managed to lower the eyepiece height by several inches, and cut multiple pounds off the overall weight of the scope.”

While poor seeing (Pickering 2 improving to Pickering 4 over the evening - see http://uk.geocities.com/dpeach_78/pickering.htm for details) limited the magnifications that we could use that evening, it was nice to be held back by the seeing, rather than the motions of the scope or the optics.

 Awaiting the night

Since then I've been out a couple of other times, and continue to be very pleased with the new structure.  I knew the optics were good before I sent it off to Rob, so that was never a concern.  My success with the Sky Commander has continued to be nothing less than phenominal.

One of my primary worries had lain in the ability of the truss to hold collimation while pointing at different sections of the sky.  I’m happy to say, this proved to be a completely groundless concern.  Tests with my kendrick laser show that the scope holds perfect collimation at any angle.

The added portability of the truss structure ensures that the scope will now travel to new locations and meet new people.  It’s also quite a bit lighter which makes it easier to haul out of the garage.  It will get more use than it ever did. 

Rob Teeter delivered an excellent structure to me, and did it in a fairly short amount of time (4 months).  He was excellent in keeping me updated every step of the way, and even included pictures so I could follow every step of construction.  This is a policy that more scope manufacturers should adopt, in my opinion.  Rob kept his eye out for both new and used upgrades, and consulted me at each junction about what to modify.  I came away with the very real impression that Rob worked with me as opposed to for me to build the scope I wanted, all the while keeping costs as low as possible.

Rob Teeter can build me a scope anytime.

Tom T.

Tom is extremely happy with his new scope, but will admit wondering about how to get his wife to agree to a 16” Planet Killer.


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics