Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Why are truss scope UTAs so shallow?

  • Please log in to reply
33 replies to this topic

#1 25585

25585

    Voyager 1

  • *****
  • topic starter
  • Posts: 14,793
  • Joined: 29 Aug 2017
  • Loc: In a valley, in the SW UK. 51°N

Posted 22 October 2021 - 07:49 AM

To me one of the big disincentives of truss scopes are their shallow UTAs. While I understand ultra light OTAs need to be that, standard truss scopes look to need more top solid tube.

 

Focuser, bigger finder, Telrad fitting would have more placement options. The UTA would be more rigid. Secondary & spider could be more recessed. Lots of advantages. Shorter stronger poles would help with general OTA rigidity. 

 

 


Edited by 25585, 22 October 2021 - 07:51 AM.


#2 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 96,161
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 22 October 2021 - 08:23 AM

To me one of the big disincentives of truss scopes are their shallow UTAs. While I understand ultra light OTAs need to be that, standard truss scopes look to need more top solid tube.

 

Focuser, bigger finder, Telrad fitting would have more placement options. The UTA would be more rigid. Secondary & spider could be more recessed. Lots of advantages. Shorter stronger poles would help with general OTA rigidity. 

This is another one of those "loaded questions."  A more objective way to ask the question:

 

What factors are important in determining the dimensions of the Upper Cage Assembly of Truss Dob?  

 

A few thoughts:

 

- A taller UTA is probably less rigid. 

 

- In general, recessing the spider and secondary isn't a big advantage.  Truss UTAs are generally setup so the sky is not directly visible through the focuser.

 

- There is generally plenty of room for the finders.  Far more than is needed, particularly in a larger scope.  

 

StarSplitter with Drum Fan.jpg

 

StellarVue finder on 22 inch.jpg
 
Tall UTAs do not travel as well and are necessarily heavier.
 
Jon

  • Bill Jensen, John Huntley, Markovich and 4 others like this

#3 wrvond

wrvond

    Soyuz

  • *****
  • Posts: 3,731
  • Joined: 25 Sep 2014
  • Loc: West Virginia

Posted 22 October 2021 - 08:25 AM

On my build the secondary, spider, heater, holder and screws alone weigh 31.5 ounces - half an ounce away from two pounds. When I add in four aluminum poles and hardware, two rings, a drum tube, focuser board and focuser plus hardware to attach the truss poles. I don't need any extra weight in the form of a cylinder longer than it needs to be to protect the secondary.


  • Jon Isaacs and ShaulaB like this

#4 sixela

sixela

    Hubble

  • *****
  • Posts: 16,174
  • Joined: 23 Dec 2004
  • Loc: Boechout, Belgium

Posted 22 October 2021 - 08:57 AM

I happen to think that the height necessary to attach your spider, finders and focuser and protect the secondary is all that is needed, provided you also use a light shield opposite the focuser (often a lot light and more compact to store than an overly large UTA). More than that and you're making the scope less compact when you're traveling and are making it harder to balance anything but really small f/ratio scopes unless you make the Alt bearings humongous (which means you often have to remove them for transport and they become clumsy to transport). You're also going to make the UTA less rigid.

 

So this is what I like (I'm talking about the length of the UTA , not the colour ;-) ):

Montdenier-Canopus-450-mm.jpg

 

Yes, of course extending the UTA downwards will shorten truss poles, but you can also make the truss stiffer by making the poles wider or changing the material (you can also make the triangle base wider but that too will have implications on portability.

That 'optimisation' of making the tube/box part larger so it serves as baffling and shortens truss poles goes for the mirror box as well; once you no longer care about portability there is no "local minimum" optimal UTA and mirror box size at all and you might as well just not have a truss scope at all. Yet split tube Dobs are almost extinct, so that must mean that portability is important for most truss Dobs.

 

Of course this is more important for longer focal lengths. There's not a bone in my body that would like a 1200mm focal length scope to be a truss scope (barring special applications, i.e. if the scope would be too heavy for the application). On the other hand, I certainly would not like to have a 2000mm focal length tube Dob, not even a split tube Dob.


Edited by sixela, 22 October 2021 - 09:11 AM.

  • Jon Isaacs, John Huntley, Don H and 4 others like this

#5 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 96,161
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 22 October 2021 - 09:04 AM

Yes, of course extending the UTA downwards will shorten truss poles, but you can also make them stiffer by making them wider or changing the material.

 

 

I have not done the analysis but I believe that generally the issue with truss scopes is not diameter and length of the truss tubes but rather the design of the ends and their mounts.  

 

Jon


  • 25585 and Dana in Philly like this

#6 sixela

sixela

    Hubble

  • *****
  • Posts: 16,174
  • Joined: 23 Dec 2004
  • Loc: Boechout, Belgium

Posted 22 October 2021 - 09:08 AM

Not always (e.g. I have a 10" Alkaid prototype and for this f/5-ish one the choice of pole diameters wasn't optimal; current series production scopes have wider trusses and are better. And it really is the trusses, and you can feel it instantly when move the scope by pulling on the trusses rather than the UTA). Of course I was assuming well engineered attachments at both ends.

 

But yes, overly wide truss poles with poorly engineered attachments do make me cringe (I should know, I had them for years until I rebuilt the scope).


Edited by sixela, 22 October 2021 - 09:09 AM.


#7 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 52,824
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 22 October 2021 - 03:13 PM

It helps if the focuser is deep enough in the UTA for the UTA to shield the bottom of the focuser from light.

This also helps prevent dew on a secondary mirror.

And, it is more aesthetically pleasing than a short UTA with a large plastic light shield sticking out the top.

 

It does, of course, change the balance point, but this can be accommodated.

 

I suppose the logical approach is a medium length upper tube assembly with a detachable upper light shield, similar to 

post #2 with a modest-length light shield added.

The scope in post #4 needs a shroud, for a variety of reasons, but is certainly a more logical construction than any single ring upper.


  • Markovich and 25585 like this

#8 MitchAlsup

MitchAlsup

    Fly Me to the Moon

  • -----
  • Posts: 5,942
  • Joined: 31 Aug 2009

Posted 22 October 2021 - 03:15 PM

To me one of the big disincentives of truss scopes are their shallow UTAs. While I understand ultra light OTAs need to be that, standard truss scopes look to need more top solid tube.

Then DON'T make the UTA so compact.

 

My 20" F/4 had a single ring and while "adequate" it was a source of frustration and of vibration (and maybe part of a collimation issue). 

All SetUp.jpg

 

My newer 13" F/3 has a much stiffer UTA and is much better at collimation and vibration control

assembly25.JPG

 

All my future builds will use the circular I-beam architecture.


  • ShaulaB, wrvond and 25585 like this

#9 a__l

a__l

    Gemini

  • -----
  • Posts: 3,293
  • Joined: 24 Nov 2007

Posted 22 October 2021 - 04:07 PM

It is preferable that the spider is attached between the rings and not over the top ring. It is preferable for transport that the bottom of the secondary is slightly higher than the lower ring. This defines the minimum dimensions for the height of the UTA.


  • Bill Weir, wrvond, 25585 and 1 other like this

#10 25585

25585

    Voyager 1

  • *****
  • topic starter
  • Posts: 14,793
  • Joined: 29 Aug 2017
  • Loc: In a valley, in the SW UK. 51°N

Posted 23 October 2021 - 06:34 AM

Thank you all for your answers. My question was intended to be loaded.

 

Like most, I think 12" aperture is the limit for solid tube Dobs, so over that size a truss type is inescapable, thus confronting the issues such designs have for the first time.



#11 niallk

niallk

    Vostok 1

  • -----
  • Posts: 154
  • Joined: 04 Dec 2014

Posted 23 October 2021 - 06:44 AM

I don't find any issue in practice with the UTA design on my classic 15" Obsession.

#12 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 96,161
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 23 October 2021 - 06:54 AM

Thank you all for your answers. My question was intended to be loaded.

 

Like most, I think 12" aperture is the limit for solid tube Dobs, so over that size a truss type is inescapable, thus confronting the issues such designs have for the first time.

 

From what I've seen,  loaded questions nearly always get off track and end up in arguments rather than discussions. That's why I pointed out that this was a loaded question.. 

 

Bottom line: some UTAs are short, some aren't. Short has advantages and disadvantages..the primary disadvantage is baffling, additional baffling may be required. Single rings have issues of their own.

 

Jon


  • Bill Jensen, John Huntley and Markovich like this

#13 CrazyPanda

CrazyPanda

    Gemini

  • *****
  • Posts: 3,273
  • Joined: 30 Sep 2012

Posted 23 October 2021 - 09:34 AM

I have not done the analysis but I believe that generally the issue with truss scopes is not diameter and length of the truss tubes but rather the design of the ends and their mounts.  

 

Jon

Maybe.

 

One night I took my scope to a dark sky site, and used it all night while completely forgetting to clamp the lower truss blocks. I never noticed all night long. It was only when I went to pack up the scope and drive home that I noticed.

 

After that, I decided to do a collimation test without the lower truss blocks clamped down. The laser holds steady against the primary mirror from ~15 degrees to zenith.

 

These are what the lower truss connectors look like:

 

https://www.cloudyni...11062_48527.jpg

 

Granted there is reasonable support there when they are seated in their holes, but apparently they have no need to be clamped in place (and they can independently shift about 2-3 degrees in those seats)

 

The upper truss connectors are also very simple and "bad" according to basic analysis, but they do not introduce any problems.

 

https://www.cloudyni..._11062_3536.jpg

 

They are "bad" because the connection point is offset from the axis, which can put the tube into bending more easily, and they are not connected to a single point, meaning they can pivot independently (i.e. they are not a triangle)

Still, with 1.25" / 0.049" poles, the truss assembly is perfectly stiff. I'm sure that if I connected the upper ends properly, I could probably get away with narrower tubes, but that shows that sub-optimal connections can be easily compensated for with wider tubes. Stiffness goes up by the cube as diameter increases, so it doesn't take much to compensate for a bad connection design.


  • 25585 likes this

#14 MitchAlsup

MitchAlsup

    Fly Me to the Moon

  • -----
  • Posts: 5,942
  • Joined: 31 Aug 2009

Posted 23 October 2021 - 11:54 AM

Bottom line: some UTAs are short, some aren't. Short has advantages and disadvantages..the primary disadvantage is baffling, additional baffling may be required. Single rings have issues of their own.

A UTA has to have a least a minimum size if the secondary rides with the UTA. The UTA either protects the secondary, or the secondary is disassembled and rides in a box.

 

I found no such limitations in the size of the primary mirror box. These can be made quite small indeed.



#15 a__l

a__l

    Gemini

  • -----
  • Posts: 3,293
  • Joined: 24 Nov 2007

Posted 23 October 2021 - 07:25 PM

The UTA either protects the secondary, or the secondary is disassembled and rides in a box.

 

Or a box is made for UTA.
But it is better that UTA protects the secondary mirror during transport.


Edited by a__l, 23 October 2021 - 07:28 PM.

  • 25585 likes this

#16 Bill Weir

Bill Weir

    Gemini

  • *****
  • Posts: 3,017
  • Joined: 01 Jun 2004
  • Loc: Metchosin (Victoria), Canada

Posted 23 October 2021 - 11:51 PM

Or you can make a rigid secondary protector. This one is made out of an old peanut butter container. This is on my 12.5” where the upper cage is already long enough but I like this better than a cloth bag.

 

Bill

Attached Thumbnails

  • C054EE8F-F085-4697-A276-7830586D2A20.jpeg
  • EECC5F63-F7F5-4AA9-AB4E-3D9C9AA647C2.jpeg
  • 1BFB28A2-AFD1-4C20-B496-4F66E6842909.jpeg

  • paul, George N, Pierre Lemay and 6 others like this

#17 25585

25585

    Voyager 1

  • *****
  • topic starter
  • Posts: 14,793
  • Joined: 29 Aug 2017
  • Loc: In a valley, in the SW UK. 51°N

Posted 24 October 2021 - 08:19 AM

If you were having a bespoke scope made, would you stipulate a certain depth or % length of the OTA, for your UTA? 



#18 wrvond

wrvond

    Soyuz

  • *****
  • Posts: 3,731
  • Joined: 25 Sep 2014
  • Loc: West Virginia

Posted 24 October 2021 - 09:59 AM

If you were having a bespoke scope made, would you stipulate a certain depth or % length of the OTA, for your UTA? 

A certain depth. It has to be deep enough to hold the spider/secondary and still have enough room to mount the focuser (which, in my case, rotates). Nothing can project from the top of the UTA - this would prevent the use of a full aperture solar filter. The mounting hardware for the truss tubes will provide "feet" for the UTA to sit on, but again, the focuser must have room to operate without the knobs coming in contact with the ring.

The mirror box needs to be deep enough to hold the mirror cell, anything more than that is simply to provide weight to counterbalance the UTA or other considerations - such as fans or UTA storage.

The rocker box needs to be deep enough to allow the mirror box to rotate to zenith, yet shallow enough to allow the scope to rotate to the horizon. It also must have room to accommodate and electronics, such as drive motors.


Edited by wrvond, 24 October 2021 - 10:04 AM.


#19 CrazyPanda

CrazyPanda

    Gemini

  • *****
  • Posts: 3,273
  • Joined: 30 Sep 2012

Posted 24 October 2021 - 10:16 AM

If you were having a bespoke scope made, would you stipulate a certain depth or % length of the OTA, for your UTA? 

I would want it to have sufficient room to hold the spider and focuser, but be very stiff, which means making it shallow.

 

To provide shielding, you can use lighter weight materials.

 

I'm currently working on a re-design of my 12" Lightbridge that will feature a very narrow UTA (basically just wide enough for the focuser), but then feature a light weight frame that extends up past the UTA. It will be a 6 or 8 ply drum shell glued and screwed between two rings. Whole thing will be about 5-6" deep.

 

Then unlike most dobs where the light shroud starts at the lower ring to cover the truss, the shroud would be long and start at a light weight frame that extends up past the UTA. That frame has no structural significance, so it can be very, very light. All the structural stuff is in the UTA itself, where it counts.

 

So now you have the best of both worlds:

 

Your UTA is stiff and light weight, but you also get dew and light protection from the shroud.


Edited by CrazyPanda, 24 October 2021 - 10:16 AM.

  • wrvond likes this

#20 wrvond

wrvond

    Soyuz

  • *****
  • Posts: 3,731
  • Joined: 25 Sep 2014
  • Loc: West Virginia

Posted 24 October 2021 - 11:16 AM

Ultimately I decided to go with an Astrosystems spider and holder. Tip to tail it is right at 9 inches. My 5 ply drum shell is 10 inches tall. I'll be making the top and bottom rings out of 1/2" thick plywood. The rings will be 1.75" wide with a groove inset from the inside edge 1/4" for the drum shell to fit down inside. I'm also using 1" aluminum tubes to fasten the two rings to each other (trapping the drum shell between) and supporting the spider. The completed UTA should end up being right at 10" tall, not counting the Aurora Precision brackets mounted to the bottom of the lower ring.

The UTA will end up being 13 1/8" inside diameter on a 12.5" scope.

I could make the UTA lighter by using Kydex or similar, but then I'd have to use thicker plywood, so I think that's a good trade for much more stiffness.

The focuser board, while 1/2" thick on the side edges will be radiused on the inside to match the drum shell. This removes some weight from the board and moves the focuser closer to the secondary. Move it in far enough and I may end up using a 2.6" secondary instead of a 3.1". which translates to a bit less weight and (more importantly) less leverage on the spider at lower altitudes.



#21 tommm

tommm

    Apollo

  • *****
  • Posts: 1,044
  • Joined: 16 Dec 2015

Posted 24 October 2021 - 01:08 PM

Why shallower UTA?  So it will fit in your car:

scope and ramp boards loaded in the Bolt.JPG

 

The secondary is still protected, nested in between the spider vanes:

 

spider & holder from side.JPG

 

I do use a plastic light baffle that velcros in place.

 

Another reason is it is nice to keep the weight of the UTA low so it is easier to lift on/off the truss poles.  More a consideration for scopes about 24" and larger.  The UTA can be any length you want, just depends on your constraints.  My biggest ones were the car hatch opening height, and UTA weight - the altitude bearings are already quite large and one end had to cut off to fit in the car, limiting me to 18 deg altitude min.

 

 


  • Jon Isaacs likes this

#22 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 52,824
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 24 October 2021 - 01:57 PM

Ultimately I decided to go with an Astrosystems spider and holder. Tip to tail it is right at 9 inches. My 5 ply drum shell is 10 inches tall. I'll be making the top and bottom rings out of 1/2" thick plywood. The rings will be 1.75" wide with a groove inset from the inside edge 1/4" for the drum shell to fit down inside. I'm also using 1" aluminum tubes to fasten the two rings to each other (trapping the drum shell between) and supporting the spider. The completed UTA should end up being right at 10" tall, not counting the Aurora Precision brackets mounted to the bottom of the lower ring.

The UTA will end up being 13 1/8" inside diameter on a 12.5" scope.

I could make the UTA lighter by using Kydex or similar, but then I'd have to use thicker plywood, so I think that's a good trade for much more stiffness.

The focuser board, while 1/2" thick on the side edges will be radiused on the inside to match the drum shell. This removes some weight from the board and moves the focuser closer to the secondary. Move it in far enough and I may end up using a 2.6" secondary instead of a 3.1". which translates to a bit less weight and (more importantly) less leverage on the spider at lower altitudes.

Careful, or the focuser drawtube will stick inside the UTA I.D., which causes two additional diffraction spikes in the star images.

If you use a 13.125" I.D. on the UTA and any normal focuser height, a 2.6" secondary is more than large enough.

My own 12.5" has a 14.5"I.D. on the UTA and an un-radiused 1/2" focuser board and a 10.75" secondary-to-focal plane distance, yet a 2.6" secondary is fine.

And not a bit undersized, as 2.45" is adequate to provide a 70% illumination at the edge of my lowest power 30mm eyepiece.

 

BTW, simple math shows that if you don't want the UTA I.D. to vignette the field in any way, it's minimum I.D. should be the

primary mirror diameter PLUS the field stop diameter of your lowest power eyepiece.  My UTA was a tad oversized, but the focuser drawtube never intrudes, even with a Paracorr 2 in the focuser.

If the low power eyepiece is a 31mm Nagler, the minimum UTA I.D. is 14.15"  If the low power eyepiece is a 30mm APM UFF, it can be 13.93"

But even if the scope is designed to only use 1.25" eyepieces, it should be about 13.57".

13.125" will vignette just about all eyepieces longer than 12mm.


  • wrvond likes this

#23 wrvond

wrvond

    Soyuz

  • *****
  • Posts: 3,731
  • Joined: 25 Sep 2014
  • Loc: West Virginia

Posted 24 October 2021 - 02:28 PM

Careful, or the focuser drawtube will stick inside the UTA I.D., which causes two additional diffraction spikes in the star images.

If you use a 13.125" I.D. on the UTA and any normal focuser height, a 2.6" secondary is more than large enough.

My own 12.5" has a 14.5"I.D. on the UTA and an un-radiused 1/2" focuser board and a 10.75" secondary-to-focal plane distance, yet a 2.6" secondary is fine.

And not a bit undersized, as 2.45" is adequate to provide a 70% illumination at the edge of my lowest power 30mm eyepiece.

 

BTW, simple math shows that if you don't want the UTA I.D. to vignette the field in any way, it's minimum I.D. should be the

primary mirror diameter PLUS the field stop diameter of your lowest power eyepiece.  My UTA was a tad oversized, but the focuser drawtube never intrudes, even with a Paracorr 2 in the focuser.

If the low power eyepiece is a 31mm Nagler, the minimum UTA I.D. is 14.15"  If the low power eyepiece is a 30mm APM UFF, it can be 13.93"

But even if the scope is designed to only use 1.25" eyepieces, it should be about 13.57".

13.125" will vignette just about all eyepieces longer than 12mm.

Both my 55 Plossl and 41 Pan have 46mm field stops. This means I actually need a 15.5" drum shell. Guess I better call Precision Drum in the morning.

Thanks for catching that, Don. I would really hate to mess it up that badly.



#24 George N

George N

    Fly Me to the Moon

  • *****
  • Posts: 6,579
  • Joined: 19 May 2006
  • Loc: Binghamton & Indian Lake NY

Posted 24 October 2021 - 03:32 PM

Or you can make a rigid secondary protector. This one is made out of an old peanut butter container. This is on my 12.5” where the upper cage is already long enough but I like this better than a cloth bag.

 

Bill

The fellow who offered his review here on CN of his NMT 20" F/3.3 with single-ring UTA -- had NMT make a large case to transport the UTA & secondary - but found it took up too much space in his vehicle. He took a large plastic painting bucket about a foot in diameter and cut 4 slots in it - puts it over the spider and bunji cords it on tightly. He then just puts in on top of 'soft stuff' like his packed tent. With several years of camping trips to the Negev under his belt - he has had no issues as of yet. This is a sort of 'king size' version of your solution.


  • wrvond likes this

#25 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 26,043
  • Joined: 22 Feb 2010

Posted 24 October 2021 - 03:48 PM

 

While I understand ultra light OTAs need to be that, standard truss scopes look to need more top solid tube.

somthing like this is what drives designers to design... you don't like what others are doing so you have to design/make your own.

 

lets see what you got.


  • Pingu and wrvond like this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics