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24.4" f/3.2 truss dobsonian project


planetman83
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Hello, 

my name is Manos Tsikalas. A few days ago, it came to my hands a 24.4" f/3.2 primary mirror and a 5.3" secondary, made by John Nichol optical , so my duty is to make a very beautiful, functional and well made truss dobsonian telescope. 

My friend Dimitris Manousos designed a very clever sketch of a secondary holder that we already printed. They are 2 parts that hold the secondary together like a sandwich. It is a little larger than the secondary in every dimension so it does not deforms it.

I personally made the spider and the system that aligns the secondary. I made the spider vanes at 80mm offset to each other. At this geometry the spider vanes are mostly loaded in tension when you try to rotate the holder. A very important thing here is for the height of the upper cage to be as small as possible. I have calculated that it will be 28cm total.

I am going to use carbon fiber truss poles for the trusses and for the struts of the upper cage. Using carbon fiber stiffens the construction and the weight stays low. I will kydex or 2mm birch plywood inside the cage also. The goal for the upper cage is to be no more than 6-6.5kg, moonlite 2 speed focuser included. 

The height of the mirror box is going to be also 28cm total. 

I made a draft sketch of the upper cage and of the mirror box. I haven't decided yet about the truss connections to the mirror box and to the upper cage. There are many solutions, but I am trying to find the best, easier and more durable. Do you have any recommendations?

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upper cage.jpg

mirror box.jpg

Edited by planetman83
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I always wonder why there is so much wood used in a dobson. Expensive newtonians in a chipboard frame.   Even the most durable waterproof plywood can change shape or worse.  
 

What do you think are the costs of not only carbon fibre trusses, but also a carbon fibre box/base? Or pvc/plastics? 

or is there a specific reason why wood is used for the mirrorbox?

 

 

edit: a nice start btw for a big enough dobson😍

Edited by Robindonne
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I think that the main reason is that you can easily cut and edit wood. Who can do the same with carbon fiber? Birch plywood is also very light and durable. It is also very well looking, like a furniture. 

Many dobsonian telescopes are made of aluminum but it is not for everyone who wants to build his own scope. You need the right tools to bend it, glue it each other etc.

Edited by planetman83
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Reasons for plywood in particular are: relatively low cost, good availability , good strength to weight ratio, and ease of working.   In general, these are the most common reasons for any material specification in any project.  Remember that the classic Dobsonian mount as advanced by John Dobson was intended as a low cost option that would be within the reach and capability of amateur backyard astronomers. 

https://stellafane.org/ 

For the mirror box you are looking for a material that is dimensionally stable and lightweight.   Quite a few minimalist designs (commercial and diy) use a space frame solution where the mirror cell is held in a minimum structure space frame; great solution for portability. 

Jim 

Edited by saac
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On 26/02/2021 at 09:04, planetman83 said:

Hello, 

my name is Manos Tsikalas. I am the owner of the company PLANETARIUM CRETE in Greece. We provide starry nights and planetarium services in the beautiful island of Crete.

 

A few days ago, it came to our hands a 24.4" f/3.2 primary mirror and a 5.3" secondary, made by John Nichol optical , so our duty is to make a very beautiful, functional and well made truss dobsonian telescope. 

My friend Dimitris Manousos designed a very clever sketch of a secondary holder that we already printed. They are 2 parts that hold the secondary together like a sandwich. It is a little larger than the secondary in every dimension so it does not deforms it.

I personally made the spider and the system that aligns the secondary. I made the spider vanes at 80mm offset to each other. At this geometry the spider vanes are mostly loaded in tension when you try to rotate the holder. A very important thing here is for the height of the upper cage to be as small as possible. I have calculated that it will be 28cm total.

I am going to use carbon fiber truss poles for the trusses and for the struts of the upper cage. Using carbon fiber stiffens the construction and the weight stays low. I will kydex or 2mm birch plywood inside the cage also. The goal for the upper cage is to be no more than 6-6.5kg, moonlite 2 speed focuser included. 

The height of the mirror box is going to be also 28cm total. 

I made a draft sketch of the upper cage and of the mirror box. I haven't decided yet about the truss connections to the mirror box and to the upper cage. There are many solutions, but I am trying to find the best, easier and more durable. Do you have any recommendations?

 

Hi Manos , I like your secondary holder , It will be good to hear how it functions once you get the secondary mounted on it.  I also like your spider, I made something similar for a 16 inch build a few years ago, if you would be able to show more photographs of your spider at the hub and how it carries the secondary I'd be grateful.  As for truss connectors,  with my build I went for the standard moonlight drelin connectors - I'm not sure that they still make these components anymore. It's a shame because they work really well. 

Jim

 

large.5946f0ec5ed93_MirrorBoxUpperOTAandCradle.jpg.ea7f7e3ce9274a32c33037847c01a654.jpglarge.593f133861c0e_MoonliteTrussTubeConnectors.jpg.bc7c1e55dbb6369426c9dbfe0c792bb3.jpg

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Jim, here are some more detailed photos of the spider. I need to paint it again. The spider vanes are not 4 pieces but only 2. 

I printed the holder again with PETG and with an improved design. I am now waiting for the cnc cutter to finish the upper cage and mirror box wood parts.

 

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Edited by planetman83
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On 03/03/2021 at 18:11, Robindonne said:

I always wonder why there is so much wood used in a dobson. Expensive newtonians in a chipboard frame.   Even the most durable waterproof plywood can change shape or worse.  
 

What do you think are the costs of not only carbon fibre trusses, but also a carbon fibre box/base? Or pvc/plastics? 

or is there a specific reason why wood is used for the mirrorbox?

 

 

edit: a nice start btw for a big enough dobson😍

plywood especially good quality dampens the vibration quickly. i used birch plywood on mine

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9 hours ago, planetman83 said:

Jim, here are some more detailed photos of the spider. I need to paint it again. The spider vanes are not 4 pieces but only 2. 

I printed the holder again with PETG and with an improved design. I am now waiting for the cnc cutter to finish the upper cage and mirror box wood parts.

 

DSC_1906.JPG

DSC_1907.JPG

DSC_1908.JPG

DSC_1909.JPG

you will probably get double deffraction spikes with your spider offset like that. nicely made holder

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12 hours ago, planetman83 said:

Jim, here are some more detailed photos of the spider. I need to paint it again. The spider vanes are not 4 pieces but only 2. 

I printed the holder again with PETG and with an improved design. I am now waiting for the cnc cutter to finish the upper cage and mirror box wood parts.

 

 

 

 

 

Manos thanks for the photographs showing more detail of the design.  I've never been entirely satisfied with mine so I may take some design leads from yours, it looks more substantial and up to the job. 

Jim 

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Just to echo what faulsky said above regarding good quality ply. If you do decide to go  for  Birch ply try to source the top quality stuff, here in UK I think BB grade.  The lower grade ply that typically sells in DIY chain stores should be avoided as the finished layers are never as good and the sheets may also contain a high number of voids.   The sheets that I sourced from my local timber specialist were supplied in 1.6 by 2.2 m I think .  I can confirm the grade if it is any use to you but off the top of my head I think it was BB. 

https://www.woodworkerssource.com/blog/woodworking-101/tips-tricks/your-ultimate-guide-to-baltic-birch-plywood-why-its-better-when-to-use-it/

Jim

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3 hours ago, faulksy said:

you will probably get double deffraction spikes with your spider offset like that. nicely made holder

I have read that if the opposite vanes are parallel and 90 degrees with the other 2 vanes, then the spikes are single and just a cross. I own a 16" with the same design and the spikes are just a cross.

Edited by planetman83
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35 minutes ago, planetman83 said:

I have read that if the opposite vanes are parallel and 90 degrees with the other 2 vanes, then the spikes are single and just a cross. I own a 16" with the same design and the spikes are just a cross.

thats good news then

lovely design

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The David Kriege/Richard Berry  book have for a 25 inch mirror -  side bearing 32 inch diameter and 1 5/8 th thickness (41 mm),  rocker box  1 1/2 inch ( 38mm).   So that agrees well with faulksy's dims.   

For my 16 inch  Dob the bearing on the rocker box  is effectively the thickness of the sheet so 28 mm.  The actual side bearings are formed by a 28 mm sheet sandwiched between two other sheets - so I guess the actual bearing is again 28 mm thick .  My bearing and rocker box was a bit unconventional inspired by  Gordon Waite's design (http://www.waiteresearch.com/index.php/about/)   so my bearings dimensions were very much  "if it looked right , then it should be ok" :) 

 

 

Jim 

Edited by saac
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The tube assembly is going to be something like the following sketches. I cannot attach the truss tubes on the upper cage in solidworks.

I have added the weight of everything I will use on the upper cage. The bearings' diameter are 1.4x the mirror diameter's and you can see that the center of mass is a little lower than the bearings. So I think I go around 1.3-1.5x at the final design.

I am not designing a classic design. My design will be lighter, with as light upper cage as possible, carbon trusses etc. This is why I don't follow the old classic recipe.

For the alt and az movement I will not use ebony star and teflon, but teflon, ball bearings and insted of ebony star I will use thin steel. 

tube1.jpg

tube2.jpg

tube3.jpg

Edited by planetman83
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  • planetman83 changed the title to 24.4" f/3.2 truss dobsonian project
On 03/03/2021 at 18:11, Robindonne said:

always wonder why there is so much wood used in a dobson. Expensive newtonians in a chipboard frame.   Even the most durable waterproof plywood can change shape or worse

Someone should tell that to this 20 year old plywood dob still wearing its original varnish and working as well as the day it was made 😉

 

20210130_190009.jpg

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22 minutes ago, faulksy said:

i take it your just doing a standard mirror cell ?

What do you mean standard? The truth is that I am rebuilding a used 24.4" telescope that is too big for my taste. The mirror cell and the mirrors are the only things that are going to stay the same. So the mirror cell is like the one in the sketches. 18 point made of stainless steel. I will upload photos tomorrow.

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1 hour ago, CraigT82 said:

Someone should tell that to this 20 year old plywood dob still wearing its original varnish and working as well as the day it was made 😉

And still looking nice. You’re right, not all are made from bad quality plywood.  But the chipboard modern dobs....😖  

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1 hour ago, planetman83 said:

What do you mean standard? The truth is that I am rebuilding a used 24.4" telescope that is too big for my taste. The mirror cell and the mirrors are the only things that are going to stay the same. So the mirror cell is like the one in the sketches. 18 point made of stainless steel. I will upload photos tomorrow.

i ment edge support

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Sounds like a fantastic build and well done on choosing a Nichol mirror set for this.

Nice to see new technology making parts for astronomy. I made a similar holder from carbon fibre for a secondary the same size. It weighs a fraction of steel 

20160206_175811.thumb.jpg.42c8618ed6cd2adf3b5712caa239b533.jpg

The front lip gives excellent support to the secondary the same as your printed version. 

Watching with interest as it seems a while since we've had a flurry of build projects 

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mapstar, I have seen your built, it is world class, congratulations! And you holder made of carbon fiber is what  I would like to do but I cannot work this material and it is very expensive to find someone to build something like this.

I attach some new photos. The CNC cutter finished the 1st batch, mirror box and upper cage wood. I don't like the 24 number. maybe I will make it again with laser.

 

 

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