42 replies to this topic

[Olivier Biot]

Rethinking Kriege & Berry - The Double Truss

By Jeff Morgan

[jg3]

Congratulations, Jeff, for success on your scope, and for providing a clear and aware presentation on what the double truss structure offers.

Some other capabilities of the double truss:

1) The two trusses can be split, and bolted together, as in the Royce design I used liberally (not literally) in 2006. The heavier part is small, and the bulky part is light. Great for quick assembly and tear-down at home. It holds collimation after separation and re-assembly.

2) Mine is on an equatorial presently, but on my to-do list is to attach bearings and build a rocker box. Then it will be easily set up in either Dobsonian or equatorial mounting - whatever I feel like doing that evening.

3) Many mirror cells come with three mounting bolts. I was able to attach the lower truss poles directly to the cell mounting bolts, eliminating the mirror box as a source of deflection.

4) Jeff made hexapods work with a square center box. Many might prefer the simpler geometry of Royce's double 8-pole trusses on square boxes. I went for hexapods with hexagonal boxes, which arose naturally from the three cell bolts.

My 12.5-inch f/6.3 split truss with boxes mostly of 1/4-inch plywood also weighs close to 40 lbs. Its poles are just flattened at the ends and bolted to the boxes, as in Royce's. Not the easiest to assemble in the field from a complete teardown, but it has good home and pickup truck portability, and no free-play slop.

The Kriege-Berry design is instructive and inspiring, but shouldn't be an orthodoxy. Maximized moment arm length, and counterweights right in the already-heavy mirror box, are not its strong points.

[astroneil]

A very thorough and well written article Jeff. This will certainly provide plenty of food for thought for ATMers wanting to take Dobsonians in different directions.

Well done indeed!

Neil.

[Jeff Morgan]

Thanks Neil. After building my 5th or 6th scope I realized I was in quicksand as far as making the balance numbers work. The harder I worked ... the deeper I was stuck, culminating in the carbon fiber scope. Working with carbon fiber was fun, but not very practical unless one is very experienced in composites. And it was hardly cost effective either. At one time I even gave up my heavy 2" eyepieces - but that didn't last very long and all the other issues remained.

[LateViewer]

Outstanding article. A really really well done piece.

Clear without being too wordy. The article gets right to the points.

Well done sir!

[CarlDD]

A very interesting 'scope and excellent article Jeff.

I'm no ATM'r, so I'll have to wait for one of the dob vendors in world takes this approach, if it could be available in kit form that would be even better.

Thanks and Best Regards
Carl

[Doug76]

Nicely done, and a real cure for what ails a Dob.
Doesn't hurt that it looks better too!
Doug

[jrbarnett]

Jeff:

Intriguing findings. I like the double truss concept quite a lot.

Regards,

Jim

[Starman1]

It seems the Serrurier truss works fine for his f/6 mirror.
However, the primary mirror seems inadequately protected from wind-blown dust or dew. Perhaps the author uses a plastic shield in the lower section?

But it seems to me the extra complication of building is simply eliminated if the length of the upper tubes are reduced by going to a shorter f/ratio scope.

In my 12.5" f/5 dob, I have heavier 1.25" diameter poles for stiffness, a conventional 2-ring upper with finders, heavy eyepieces, (all heavier-than-needed sub-assemblies)and the mirror box weighs 49.5 lbs with fan, wires, large handles, solid altitude trunnions, and a 2.2" thick mirror (that weight is less than my 8" LX200's upper section was, and I never tear down at night anyway since I'm still observing).

If the scope were f/4, the mirror box would be lighter.

So it seems the cure for the lever arm on the conventional dob is a shorter f/ratio.

Does having to use a coma corrector seem like too big a burden? I can tell you from experience it's not a burden at all if you simply take it for granted it will be there.
And there is no reason a shorter f/ratio scope cannot also be well baffled for good contrast. With primary mirror baffle, shroud, and UTA light shield, you can achieve good baffling and control of light even at f/3.

As for rigidity in the conventional design, this can be enhanced. The parallel-pairs structure has problems but this can be "repaired" by making the pole pairs non-parallel. The loss of collimation (as seen in an autocollimator) with altitude change can be mitigated or even eliminated with some structural changes.

Reducing the weight of the sub-structures seems like a valid reason to pursue this design, but (other than the fact the author puts his focuser on the wrong side:grin:) it seems like a solution that is really only important for longer focal ratio newtonians. And then you're back to the ladder problem.

It is a nicely-constructed solution to the problems the author encountered with his 12.5" f/6, and his construction details were elegant in many ways. I just don't see the return to longer f/ratio newtonians above 10", though.

[Jeff Morgan]

It seems the Serrurier truss works fine for his f/6 mirror.
However, the primary mirror seems inadequately protected from wind-blown dust or dew. Perhaps the author uses a plastic shield in the lower section?

The performance issue in this environment is mirror cooling. The open primary structure is quite helpful here. No dew problems in Arizona. We do get frost issues this time year. Last session three scopes shut down due to frosting eyepieces. (We were all a bit unprepared that time!) However, no frost on the primary. I doubt it would be an issue in more humid climes due to the (unfortunate) thermal inertia of the primary. In any event, a fan can create enough breeze to take care of that. (You may have noticed dew does not form on breezy nights.) OTOH, dust is inevitable in this environment, whether solid tube, truss, reflector, refractor. Handled with periodic cleaning.

But it seems to me the extra complication of building is simply eliminated if the length of the upper tubes are reduced by going to a shorter f/ratio scope.

In my 12.5" f/5 dob, I have heavier 1.25" diameter poles for stiffness, a conventional 2-ring upper with finders, heavy eyepieces, (all heavier-than-needed sub-assemblies)and the mirror box weighs 49.5 lbs with fan, wires, large handles, solid altitude trunnions, and a 2.2" thick mirror (that weight is less than my 8" LX200's upper section was, and I never tear down at night anyway since I'm still observing).

Been there. Done that. Attached is a photo of an earlier scope more according to "The Book" (it even uses genuine Obsession tube clamps). This one happens to be 10" f/5. One still gets a mirror box much heavier than need be (heavy/high density/hard to carry) and differential balance sensitivity. A better situation than f/6, but not much better because the design itself (not the focal ratio) constrains the balance point to a relatively low location and maximizes focuser moment arm.

I took a quick look at the numbers (for my current 12.5"), f/5 would shave around 10 pounds from the mirror box, and the eyepiece balance issue improves - slightly. Of course, I did not account for the fact that I would likely want a Paracorr .... another 1 pound of weight at the least favorable location ....

OTOH, the double truss layout shaves 23 pounds and I can barlow a 31 Nagler and then switch directly to a featherweight Brandon. If you have a spreadsheet you can quickly demonstrate this for yourself. Weight * Arm = Moment tells no lies.

So it seems the cure for the lever arm on the conventional dob is a shorter f/ratio.

Does having to use a coma corrector seem like too big a burden? I can tell you from experience it's not a burden at all if you simply take it for granted it will be there.
And there is no reason a shorter f/ratio scope cannot also be well baffled for good contrast. With primary mirror baffle, shroud, and UTA light shield, you can achieve good baffling and control of light even at f/3.

Well, that just gets us to the well-discussed topic of Short vs. Long. I think everyone is aware of the trade-offs. For convenience, one goes Short. For optical performance, one goes Long. Each owner picks his preferred place on continuum. My other scopes are refractors (D&G, Astro-Physics) and the only reflectors that approach "refractor-like" views are the long ones. I certainly wouldn't want to go to a major star party with a short focus Newtonian and find out I was set up next to the Astro-Physics tent! OTOH, give me a f8+ scope, and I'll give Mr. C. a run for his money.

Structurally, the faster truss scopes naturally get better balance and more rigidity because the truss geometry gets away from the long skinny structures that rule the day now. No secret there. Of course using the "Sidwick Rule" one sees that allowable flexure is "relaxed" at f/6, microscopic at f/3. The double truss approach gives vastly increased rigidity but doesn't sacrifice the view. And it gives one a great place to mount a serious finder scope.

BTW, on the 10" f/5 I did use a Paracorr as you might be able to discern in the photo. It has it plusses and minuses (also well-discussed elsewhere). In the article I alluded to an accident involving the equatorial platform where the f/5 mirror fell from its sling cell and was scratched. Bob Royce happened to have a f/6 mirror available and made me a decent (great!) deal on it. For that case, the added moment arm of the Paracorr was a wash with going f/6. I lengthened the structure and was happy to get rid of the Paracorr. YMMV.

As for rigidity in the conventional design, this can be enhanced. The parallel-pairs structure has problems but this can be "repaired" by making the pole pairs non-parallel. The loss of collimation (as seen in an autocollimator) with altitude change can be mitigated or even eliminated with some structural changes.

Reducing the weight of the sub-structures seems like a valid reason to pursue this design, but (other than the fact the author puts his focuser on the wrong side:grin:) it seems like a solution that is really only important for longer focal ratio newtonians. And then you're back to the ladder problem.

It is a nicely-constructed solution to the problems the author encountered with his 12.5" f/6, and his construction details were elegant in many ways. I just don't see the return to longer f/ratio newtonians above 10", though.

Ladders certainly have drawbacks, but one can still bring some serious focal length to the observing field in light, easy to manage components. Improving designs, and design offshoots are all part of the fun of telescope making, no? It's fun to see the variety of approaches people use, and fun to read the ATM articles of decades past. Funny thing is that even 60 years ago they were talking about many of the same issues we discuss today.

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[Starman1]

Jeff,
Your points are well-taken.
However, I have never seen evidence with my own eye that contrast in the eyepiece depended on focal ratio. If you assume that all long focal length mirrors will be better figured than their shorter brothers, that would be a false assumption. I have seen poorly figured mirrors of all focal ratios.
I would gladly set up next to the AstroPhysics tent with a well-made 16" f/4 newtonian. It would produce images none of his products were capable of.
Of course, in deference to AP, the number of 16" f/4 scopes constructed anywhere nearly as well as a typical AP would be very small in number.

One of the ways that 10" could have been lightened would have been to significantly shorten the UTA and add a lightweight extension to the UTA for light shielding, say by having the lining of the UTA extend upward another 10" beyond the upper tube ring. That would have lightened the focuser board, the aluminum tubes in the UTA and taken a fair amount of weight out of the UTA. Then the LTA could have been lighter as well.

While I'm not a fan of the ultralights that dot the landscape of ATM scopes (I haven't seen one yet that can maintain collimation with altitude change), one can significantly lighten a UTA without sacrificing baffling or rigidity. That 10" could have used a little bit of that.
However, it does look rigid! OMG.

And, while I'm at it, your comments about collimation tolerances and rigidity in conventional construction at super-short f/ratios are well-taken. Here is a link that discusses those problems from the standpoint of rigidity and collimation tolerances:
http://www.astrosyst...ShortFRatio.htm
I begin to see that short doesn't equal lightweight.
Still, the proper choice of materials in the UTA can go a long way toward decreasing mirror box mass.

I thank you for once again sparking my interest in a dob-building project. It's given me a lot of ideas.
This is fun!

[Jeff Morgan]

Thanks for the link for the super-short components. Amazing how the free market moves to fulfill demand. Who hasn't penciled out a few sketches of a telescope optimized around the 21 Ethos? I'll probably be joining you guys at the f/3 party in a few years, after letting the Brave Ones figure out the bugs. But first I'm thinking about stretching the 12.5" out to f/8 or even f/10. Mars oppositions are starting to get better. Too many projects, too little time.

As to focal ratio and contrast, there are certainly a large number of fast refractors that support your view. I used to own a Jaegers f/5 that gave amazing views on low surface brightness targets. The entire NGC 7000 complex was marked out in white paint with that scope, no filters required (or desired).

I've been re-reading the Sky & Telescope archive and just finished a series of ATM articles on the construction of Newtonians that ran over most of the 1945 issues. (As an aside, you can read about an amateur f/3 Newtonian in the November 1944 issue.) They bemoan the point you brought up - the refractor/reflector comparison begins with a refractor invariably built by a master optician and assembled by a precision instrument maker. Hardly a fair fight, but we can still try!

To digress a bit more, when reading the old ATM columns one can't help but feel that builders of years past had a more solid grounding in optics, mechanics, and mathematics than we do as a group today. Materials and commercial accessories have gotten better. But while we consider our creations as more "modern", I'm not so sure. The "old guys" knew quite a lot.

[seryddwr]

Where do you find those spring washers that you used in your mirror cell?

[Jeff Morgan]

Where do you find those spring washers that you used in your mirror cell?

McMaster-Carr.com a treasure trove for the ATM. Try part number 9715K82. Since you will most likely need to stack them to get acceptable travel, get a couple of 10-packs.

[llanitedave]

Jeff, I've got somewhat similar principles planned for my upcoming 16" rebuild, so your article was of great interest -- and very well done, I might add.


The key is the raising of the rocker box and the movement of the center of gravity closer to the center of the OTA. The benefits I see are in increased stiffness, as you pointed out, reduced weight overall, as you also pointed out. Another consideration is the span of the swing diameter, both in altitude and azimuth. Even though my scope is an f/4.5, these cantilever issues are still similar. The higher balance point means there's less of a height difference between viewing at a range of elevations, and the eyepiece doesn't cover as wide a swath when following objects across the sky or swinging from one to another. This allows fewer posture changes, chair adjustments, and other interruptions. It's a practical compactness that is more obvious in observing than in transport.

For scopes housed in an observatory, like mine, it also means less of the floor space is taken up by the motions of the scope -- and you can fit a larger scope in a smaller building.

I used an idea similar to your hollow uprights for my shorter rocker box. It works well. I'll definitely do hollow uprights for my rebuild, unless I decide to continue with the fiberglass/foam composite theme.

The main difference between my idea and yours is that my lower optical assembly will be solid panels rather than triangular struts. That's an aesthetic choice rather than a structural one, and I expect (and hope) that the engineering results will be the same.

Again, it was good to read your article -- it vindicated a lot of the things I'd been mulling over, although I'm still in the design phase for mine.

[Jeff Morgan]

Another consideration is the span of the swing diameter, both in altitude and azimuth. Even though my scope is an f/4.5, these cantilever issues are still similar. The higher balance point means there's less of a height difference between viewing at a range of elevations, and the eyepiece doesn't cover as wide a swath when following objects across the sky or swinging from one to another. This allows fewer posture changes, chair adjustments, and other interruptions. It's a practical compactness that is more obvious in observing than in transport.

Thanks Dave. Funny thing is, I also own a 8" refractor and I use a 80 mm scope as a finder there too. Partly because it makes a great finder, but just as much to alter the balance point and reduce the swing of the eyepiece. But I never directly considered that aspect with respect to the reflector! Mainly I was concentrating on spreading out the mass from the mirror box, reducing the focuser moment arm, and overall rigidity.

IIRC, you built he Eye of Sauron with the altitude bearings to swing completely through the zenith. Always thought that idea was clever. Are you keeping it?

[llanitedave]

IIRC, you built he Eye of Sauron with the altitude bearings to swing completely through the zenith. Always thought that idea was clever. Are you keeping it?

Definitely. The mirror cell allows it, so there's no reason not to keep it. In fact, it should work better with a shorter cantilever.

To be honest, though, I haven't used that feature as much as I'd expected, simply because so few objects cross the zenith. And I have a definite preference for which side of the UTA the Telrad is on, (the top is better than the bottom) so the design still isn't perfectly symmetrical.

It would probably have its greatest advantage if the scope had a two-axis drive, which might be a future add-on.

[Olivier Biot]

Where do you find those spring washers that you used in your mirror cell?

McMaster-Carr.com a treasure trove for the ATM. Try part number 9715K82. Since you will most likely need to stack them to get acceptable travel, get a couple of 10-packs.

Instead of those curved disc spring washers you could also go for Belleville washers. That's what I did for collimating my cell.

[Jeff Morgan]

Instead of those curved disc spring washers you could also go for Belleville washers. That's what I did for collimating my cell.

Bob Royce had told me about the Belleville washers, then gave me that specific part number. Is there a difference?

[Olivier Biot]

Yes, there's a big difference.

Belleville washers can create virtually any spring constant by stacking them in different ways. The curved disc spring washers don't have this ability, they basically stiffen linearly when stacking them.

There's a neat article about Belleville washers on Wikipedia.

What I also like about Belleville washers, is that they are conical, hence they take the load everywhere around the axis, which is not the case with those curved disc spring washers. I use a washer at both ends to distribute the compression load.

Belleville washers can be pricey, I try to find them on eBay. Apparently they're easier to get in the US than over here.

[Olivier Biot]

Here's an image of a stack of Belleville washers used as a collimation compression spring on my 14" f/5 Dob:

[Jeff Morgan]

So they are. I'll include a set on my next M-C order.

[Olivier Biot]

You'll regret not having known them earlier

Credits go to Chriske as I was ingnorant on their existence.

[GeneT]

Are any of the vendors, that you know of, planning to offer a version of your design for sale? You have convinced me that it is time to rethink the traditional Dob design.

[Jeff Morgan]

No Gene, I am not aware of a commercial builder doing this. It is not a hard thing to do (or do at home), but as long as people are willing to lug around a 50+ pound mirror box the safe bet for the commercial guys is to stick with the formula. Especially in this bad economy.

Forget about the massive increase in structural rigidity. This design is all about ergonomics and convenience. Convenience has always sold well. The only issue it does not solve is ladders, (the solution there is a refractor or Cassegrain). If a manufacturer did offer a double truss, I'm sure people would love to have their heaviest scope component weight cut in half and lose the eyepiece balance issues.

Doesn't seem like Rocket Science, does it?

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