32 replies to this topic

[Julia Cecchetti]

Hey CN!
First time posting, and I'm excited to share my first telescope build! Currently in progress. I got a weee bit of aperture fever, so I got a 20" F4.2 sandwich mirror on sale from Hubble optics. I basically got the biggest mirror I could afford/transport and designed the scope around that. I figure, I'm 5'1" and already need a ladder to view through any large scope. Might as well make it a big ladder!
I'm an engineer and love woodworking projects, so a dob build seemed like the perfect balance of woodworking and fun engineering (without tooooo much time needed staring at CAD like I already do all day at work). Plus I'll end up with an awesome toy at the end!  
I wanted something that would be easy to store and would fit into my small car, and be light enough for my tiny self to move around.
I made an 18 point mirror support designed in PLOP, and went with front collimation.
The truss pole attachment to the primary mirror box is a solid 3D printed block, but for now is just a temporary prototype until I figure out what works best. The plan is to either make one out of wood or 3D print a much higher quality version.
The truss poles are 26mmx1mm carbon fiber. They break in half and screw together with a tight fitting  joint. The inserts holding the threads are 3d printed and epoxied together.
The alt and az bearings are FRP and Teflon. They turned out as smooth as butter with no sticktion.
Just finished mounting the bearings last weekend. A lot of the bolts in these pictures will be replaced with thumb screws once I get around to ordering them.

Picture in the transport configuration. I'm planning on making a cover out of 1/4" plywood to act as the top of the box for storage and transport.

Here is a picture with all of the parts temporarily clamped together to see what the final scope will look like!

The plan is to go for first light out in a bortle 2 area of the Mojave a few hours away in late September! If it's cloudy during that time you'll know who to blame. 

Expert ATM'ers please post feedback 

[Julia Cecchetti]

Here is a closeup of the truss pole collapsible joint:

[YeloSub]

I'm no real ATM'r myself... yet. Been drawing up plans myself to convert my old 16" Discovery tube.

Looks great! Can't wait to see the final result.

[CharLakeAstro]

Awesome build!! Please keep us posted as it finalizes!!

[havasman]

 first light out in a bortle 2 area of the Mojave a few hours away in late September!

     That is a fantastic plan!

 

Very fine looking scope too. And a great response to the ladder matter.

[tommm]

Really nice work! Clearly a lot of learning applied to that build. I think we will have a hard time suggesting improvements. Here's hoping for clear nights with good seeing during your trip.

[HenkSB]

Beautiful product!  I am impressed with how you did the carbon fiber poles and their attachments.  You will enjoy that scope and the dark sites and star parties where it will take you.

[Achernar]

Very nice design and workmanship. You'll want to make a shroud for the truss, and only to use Maine or exterior grade polyurethane on the wood. Anything else will not survive long without degrading rapidly due to the temperature changes telescopes are exposed to. Once you finish it, you could see everything in the NGC and IC catalogs with you fantastic telescope.

 

Taras

 

[mconnelley]

Hello:

  Looks like it's coming together nicely.  I'd like to make two suggestions:

 

1) Add some cross bracing between the 'front' parts of the elevation bearings where they stick out from the mirror box.  When I built a scope like that, the side-to-side flexure in the elevation bearings caused the telescope to be wobbly when pointed low in the sky. 

 

2) I don't see a finder anywhere.  It also looks like balancing will be a challenge as the elevation bearings aren't super big.  I really wanted a proper finder scope for my 20" rather than relying entirely on a tetrad.  To solve these problems, I mount my finder on the truss pole below the eyepiece.  This helps balance since the finders aren't as high up on the tube, and I can use the finders while standing on the ground.  I use a red dot finder and a 50 mm RACI mounted together.  The geometry is a bit weird, but I find that it works really well.

 

Mike

[Diego]

Hi Julia! Congratulations on the new project. You must be a genius being this your first time building a telescope! The work is impressive with very good finish.

I have been going on for 2 or 3 years building a 16" truss dob. Granted I've had to build everything from scratch except the optics...it has been quite of a challenge since I have no previous experience with truss tube telescopes. Finding the right truss lengths, trunnions diameters, bearing materials, friction, balance points,etc was very tedious.

Good job! Keep us posted with the progress!

[Julia Cecchetti]

Thanks for the kind words everyone! Very encouraging to hear. I've been working on it since June, and got a ton of inspiration for the design from CN.

Hello:

  Looks like it's coming together nicely.  I'd like to make two suggestions:

 

1) Add some cross bracing between the 'front' parts of the elevation bearings where they stick out from the mirror box.  When I built a scope like that, the side-to-side flexure in the elevation bearings caused the telescope to be wobbly when pointed low in the sky. 

 

2) I don't see a finder anywhere.  It also looks like balancing will be a challenge as the elevation bearings aren't super big.  I really wanted a proper finder scope for my 20" rather than relying entirely on a tetrad.  To solve these problems, I mount my finder on the truss pole below the eyepiece.  This helps balance since the finders aren't as high up on the tube, and I can use the finders while standing on the ground.  I use a red dot finder and a 50 mm RACI mounted together.  The geometry is a bit weird, but I find that it works really well.

 

Mike

Thanks for the suggestions, Mike! Great suggestion on the cross bracing. Should be easy enough using the same threaded insert method as the truss poles. The secondary structure is still a work in progress. You'll notice you also don't see a focuser anywhere  I was planning on just using a telrad. I weighed all the parts and calculated the balance point in a spreadsheet, so fingers crossed my math was right! 

I am not a fan of needing to add and remove weights from the primary mirror box, so the plan is a movable weight that slides along one of the truss poles, or potentially a separate pole on the side to double as a handle for aiming/holding goto controls. My math says 3.5 lbs should be enough to correct for everything from my 4mm plossl to my 31mm Nagler with the whole stack up. 

 

Very nice design and workmanship. You'll want to make a shroud for the truss, and only to use Maine or exterior grade polyurethane on the wood. Anything else will not survive long without degrading rapidly due to the temperature changes telescopes are exposed to. Once you finish it, you could see everything in the NGC and IC catalogs with you fantastic telescope.

 

Taras

Definitely planning on sewing up a nice shroud! Thanks for the polyurethane suggestion. That would be so sad if it got ruined from moisture 

[tommm]

Expandiing on mconnelley's point, you are likely aware there are two torques acting on the OTA. One due to gravity acting through the CoM of the OTA and one acting through the altitude bearings or trunnions due to the frictional force of the bearings.  The latter is the product of the trunnion radius of curvature, normal force on the bearings, and the coefficient of friction, and is affected by changing any of these.  The trunnions on my 22" scope are a bit over 20" radius, so I would guess maybe 70% larger than yours, so about 70% more torque for a given friction force.

 

Weights are one way to balance with changing eyepiece weight, another is to use a bungie cord to supply another torque that increases with angle of the OTA from vertical (due to increasing torque due to gravity with angle). There are a few threads on here showing that. Roel used that approach on his travel dobs.

[Psalm19One]

I am not a fan of needing to add and remove weights from the primary mirror box, so the plan is a movable weight that slides along one of the truss poles, or potentially a separate pole on the side to double as a handle for aiming/holding goto controls. My math says 3.5 lbs should be enough to correct for everything from my 4mm plossl to my 31mm Nagler with the whole stack up. 

 

Definitely planning on sewing up a nice shroud! Thanks for the polyurethane suggestion. That would be so sad if it got ruined from moisture 

Really impressive build Julia.

 

I hate counterweights too, so I use the heavy glass counterweights that I always seem to have with me.  I designed my dob to be tail heavy so that I can easily adjust balance with a couple of eyepieces:

 

Hard to beat Shrouds by Heather if you don't like sewing.

https://www.scopeshrouds.com/

 

If you are looking for a club site, GMARS in Landers is very welcoming. Lots of flat open areas and concrete pads to set up on but more in the Bortle 3-4 range.

 

http://www.rivastro.org/gmars.php

[Julia Cecchetti]

Expandiing on mconnelley's point, you are likely aware there are two torques acting on the OTA. One due to gravity acting through the CoM of the OTA and one acting through the altitude bearings or trunnions due to the frictional force of the bearings.  The latter is the product of the trunnion radius of curvature, normal force on the bearings, and the coefficient of friction, and is affected by changing any of these.  The trunnions on my 22" scope are a bit over 20" radius, so I would guess maybe 70% larger than yours, so about 70% more torque for a given friction force.

 

Weights are one way to balance with changing eyepiece weight, another is to use a bungie cord to supply another torque that increases with angle of the OTA from vertical (due to increasing torque due to gravity with angle). There are a few threads on here showing that. Roel used that approach on his travel dobs.

Got it. I imagine the main downside of slightly small altitude bearings is that the scope needs to be more accurately balanced because there is a lower max friction torque to make up the difference before it slips? Mine are 26" in diameter. 

I'll find out soon if it's an issue and remake them if necessary. I'm going to bet I'll remake every part of this telescope at some point. Hey! Maybe I'll remake so many parts I'll end up with two whole telescopes! Is that how baby telescopes are made? 

[Waynosworld]

Welcome Julia

 

I made a 16" F/4.42 dob with a Hubble Optics Sandwich Mirror from scratch(first version) using plywood like you, in my opinion it was too heavy so I did it over again and got rid of all the plywood a couple years later, this is the latest version, I bought aluminum side bearings from Teeter Telescopes, it weighs 65lbs.

 

 

Here is my Hubble Optics 20" F/3.7, it is highly modified from the original Hubble Optics structure, it is also on wheels to make it easy to use as you can see in this photo, it has an early plywood UTA in this photo.

 

 

And here it is with the latest UTA version(aluminum mountain bike rims with 3/32" ABS sheeting), it weighs less than the original Hubble Optics UTA and is very solid, my collimation does not change even with the 2 focusers and 2" eyepiece(one focuser is my Coma Corrector, the CC is not light), the dob weighs less than a 100lbs including the wheels/axle.

 

 

I posted this mostly for you to see my UTAs, they are very solid structures and hold collimation no matter where it is pointed even with the TV 31mm T5 eyepiece, and these UTAs are very light weight, I use a 5" secondary also.

 

 

[Waynosworld]

Here are a couple photos of my 16" F/4.42 UTA so you can see how simple it is, this can be done with the rims I used in my 20" F/3.7 DOB.

 

 

 

My 20" UTA.

 

[Achernar]

Thanks for the kind words everyone! Very encouraging to hear. I've been working on it since June, and got a ton of inspiration for the design from CN.

Thanks for the suggestions, Mike! Great suggestion on the cross bracing. Should be easy enough using the same threaded insert method as the truss poles. The secondary structure is still a work in progress. You'll notice you also don't see a focuser anywhere  I was planning on just using a telrad. I weighed all the parts and calculated the balance point in a spreadsheet, so fingers crossed my math was right! 

I am not a fan of needing to add and remove weights from the primary mirror box, so the plan is a movable weight that slides along one of the truss poles, or potentially a separate pole on the side to double as a handle for aiming/holding goto controls. My math says 3.5 lbs should be enough to correct for everything from my 4mm plossl to my 31mm Nagler with the whole stack up. 

 

Definitely planning on sewing up a nice shroud! Thanks for the polyurethane suggestion. That would be so sad if it got ruined from moisture 

Which reminds me, have you ever worked with varnish before? If you have not, there's an art to applying it. It is possible to build up a high gloss finish with exterior or marine grade polyurethane that'll look like a layer of ice is covering the wood. Since your wood working looks excellent, it would be fitting to spare no effort in finishing the wood like fine exterior furniture or exterior woodwork on your home or boat. The best method to use is a high volume, low-pressure sprayer, and to thin the polyurethane with mineral spirits. First sand the plywood by hand with #220 grit or finer sandpaper and make sure no glue is anywhere on the wood, as varnish will make it glaringly apparent, especially if you stain the wood first. Apply the first coat, and allow it to dry for a day, then sand with #400 grit or even #600 grit sandpaper to smooth the varnish. Remove all dust with a rag dampened with mineral spirits, then once the mineral spirits evaporate spray another coat, and repeat the process again. It takes three or four coats to build up the polyurethane sufficiently without sags or runs. This takes time and you want to leave the telescope in a dust free area so nothing gets into the polyurethane, because it will as it dries act like fly paper until it hardens on the wood. The effort will be worthwhile, because not only will the telescope look great, but the finish will also be durable and long lasting through the repeated handling as you observe with it. When you buy the varnish, look for exterior or spar varnish on the can. Interior varnish, even interior grade oil-based polyurethane, is not suitable for outdoor use. The exterior or spar varnish is. Use good ventilation, gloves and do the work away from anything that might ignite vapors or spray mist. Also use old, ratty clothes while doing the work, this work can ruin clothing too. Varnish the outside first, then paint the interior with oil-based flat black paint. That way, if you get any on the outside, you can wipe it off with a rag dampened with mineral spirits.

 

As for the shroud, black Ripstop Nylon fabric is the material of choice, it does not tear easily, does not fade, is very color fast, and available at well stocked craft and sewing supply stores such as Jo Ann Fabrics. I am no tailor, but I managed to make three shrouds despite the fact I can otherwise at best, sew a button back on.

 

Taras

Edited by Achernar, 26 August 2024 - 06:48 PM.

[Scott E]

I spotted something I didn't see anyone else mention. Your upper truss mount locations are in the mid-span of each quarter of ring, between struts mounting the top ring. This means the bottom ring is not taking advantage of the rigidity provided by the upper ring. I recommend a thin sheet of carbon fiber or similar to tie the two rings together (with lots of screws all the way around). This will reduce any flexure of the lower ring at the truss mount points.

 

Impressive build!

[Julia Cecchetti]

Taras, I have indeed not applied varnish before. It will be an adventure! I'm planning on using a nice dark stain to the outside and applying some optical black foil to blacken the inside. 

Was rip stop nylon hard to tailor to fit perfectly? I was originally thinking of using a more stretchy material. I make my own kites and camping gear out of rip stop nylon though, so lots of experience with the material!

 

Really impressive build Julia.

 

I hate counterweights too, so I use the heavy glass counterweights that I always seem to have with me.  I designed my dob to be tail heavy so that I can easily adjust balance with a couple of eyepieces:

Counterweight1.jpg

Counterweight2.jpg

 

Hard to beat Shrouds by Heather if you don't like sewing.

https://www.scopeshrouds.com/

 

If you are looking for a club site, GMARS in Landers is very welcoming. Lots of flat open areas and concrete pads to set up on but more in the Bortle 3-4 range.

 

http://www.rivastro.org/gmars.php

Thanks! This is a great idea! Those might be the most expensive counterweights I have ever seen! Probably won't work for my purposes as I will need to share my eyepieces with my other scope while out observing with friends (my current scope is a 16" explore scientific). 

I will definitely need to check out GMARS now! I had not heard of it before.

 

I spotted something I didn't see anyone else mention. Your upper truss mount locations are in the mid-span of each quarter of ring, between struts mounting the top ring. This means the bottom ring is not taking advantage of the rigidity provided by the upper ring. I recommend a thin sheet of carbon fiber or similar to tie the two rings together (with lots of screws all the way around). This will reduce any flexure of the lower ring at the truss mount points.

 

Impressive build!

Yep! Definitely needs some more beefing up. It's still in progress. I'll be adding struts at the truss pole mount locations as well. I didn't consider thin CF to tie them together. I have been on the lookout for thin plastic that will work, but I'll consider it!

Edited by Julia Cecchetti, 27 August 2024 - 11:05 AM.

[Dan Watt]

This is a great design! Love how compact it can stow without significantly sacrificing rigidity or setup speed. And front collimation too! Very impressive. 

[ChrisLansdale]

Fantastic build!!

 

I'd like to know a bit more about the connection between the two halves of each truss pole. You mentioned that the threads were heat-set into 3D printed inserts, which makes sense to me. But it seems like there's a smaller carbon-fiber tube glued inside one of the pole halves, is that what's going on to keep everything stiff? Did you just choose a tube diameter one size down, if so how are the tolerances?

[Julia Cecchetti]

Fantastic build!!
 
I'd like to know a bit more about the connection between the two halves of each truss pole. You mentioned that the threads were heat-set into 3D printed inserts, which makes sense to me. But it seems like there's a smaller carbon-fiber tube glued inside one of the pole halves, is that what's going on to keep everything stiff? Did you just choose a tube diameter one size down, if so how are the tolerances?

The trusses need to be just under 2000mm long, but CF in long sections is quite expensive. I got 32x sections of 26x24x500mm CF tubing and 4x pieces of 24x22x500mm. I chopped up the 24mm OD tubes into 24x ~3" pieces to serve as nice strong joints.
I was hoping they would all fit nicely together, but it was not to be... So I spent a whole day sanding the 3" sections down on a Jerry rigged holder on a drill press. I wish I had taken pictures. 
 
After I had 16x with a nice tight fit, I used weldbond 2 part epoxy to glue the sections together. To make 16x 1000mm long poles. The remaining 8x 3" pieces I sanded a bit more to make a perfect slip fit. These ones will be used for the threaded joints. 

 

I epoxied the slip fit sections into just one side, being careful to clean all the squeeze out from the joint. I now had all the poles without inserts.

 

I then 3D printed inserts to hold bolts and threaded inserts and epoxied those in with the same weldbond. I will post the CAD files here and some screenshots when I get a minute for anyone that wants to replicate this!

 

I ended up with perfect joints that are super stiff and make a very satisfying pop when pulled apart 

[Achernar]

Taras, I have indeed not applied varnish before. It will be an adventure! I'm planning on using a nice dark stain to the outside and applying some optical black foil to blacken the inside. 

Was rip stop nylon hard to tailor to fit perfectly? I was originally thinking of using a more stretchy material. I make my own kites and camping gear out of rip stop nylon though, so lots of experience with the material!

 

Thanks! This is a great idea! Those might be the most expensive counterweights I have ever seen! Probably won't work for my purposes as I will need to share my eyepieces with my other scope while out observing with friends (my current scope is a 16" explore scientific). 

I will definitely need to check out GMARS now! I had not heard of it before.

 

Yep! Definitely needs some more beefing up. It's still in progress. I'll be adding struts at the truss pole mount locations as well. I didn't consider thin CF to tie them together. I have been on the lookout for thin plastic that will work, but I'll consider it!

It's not any harder to achieve a good fit with RipStop than any other fabric, but it does tends to tighten in the heat of the Sun. Therefore a tight fit is not required, and foam insulation the truss poles also helps the shroud stay tight and is also kinder to your hands when the nights are cold. You want it to be somewhat loose until the draw cords are drawn tight, Nylon slide locks also available at any fabric outlet are perfect for this, the kind you see in sweat shirts and hoodies. They let you adjust as needed. However, a cover for your telescope is also highly recommended not only to keep it from baking in the Sun, but also to protect it from the surprise thunderstorms or rain showers that can come out of nowhere. Astro Systems sells excellent covers that are very good and both wards off the Sun's heat and rain. You can leave the telescope covered with one and have peace of mind while you're getting some

sleep at a star party.

 

Taras

Edited by Achernar, 27 August 2024 - 01:20 PM.

[Scott E]

 

Yep! Definitely needs some more beefing up. It's still in progress. I'll be adding struts at the truss pole mount locations as well. I didn't consider thin CF to tie them together. I have been on the lookout for thin plastic that will work, but I'll consider it!

I mention CF because I've been down the road with plastic. One hot day in the hot car in the hot sun... Now it's thin 2-layer carbon fiber. If you do use plastic, be sure you're familiar with its thermal limits...and yours.

[symbiosis]

Here is a closeup of the truss pole collapsible joint:

iMarkup_20240825_235225.jpg

Very similar to what I did on my 12" f/5 UC.