24.4" f/3.2 truss dobsonian project

[mdstuart]

I just got a bigger ramp!

Mark

[Michele Scotti]

On 30/04/2021 at 10:52, planetman83 said:

It works only with iphone? I don't own an iphone.

Fount it! I tried on a old-ish Android I have and it works.

https://play.google.com/store/apps/details?id=de.bremen.btm.movalyzer&hl=it&gl=US

Sorry to pester but if you could do that it's would be of great help

[planetman83]

9 hours ago, Michele Scotti said:

Fount it! I tried on a old-ish Android I have and it works.

https://play.google.com/store/apps/details?id=de.bremen.btm.movalyzer&hl=it&gl=US

Sorry to pester but if you could do that it's would be of great help

I can see here your post but I cannot find the youtube video in order to understand exactly how it works. If I understand right, we can see how rigid and well built is our construction in terms of vibrations? I am very interested in doing it. Please help me doing this experiment.

 

[planetman83]

I took some better photos with my DSLR and I would like to introduce the upper cage assembly in this post.

Total height is 25cm. The inner diameter is 636mm, the outside diameter is 716mm. There are 7 carbon fiber tubes that connect the rings. The spider vanes are made of 1mm thick stainless steel with 4.5cm width. There is 1.5mm aircraft plywood to fill the space and connect the rings also. There is laser crafted the number 24 next to the focuser.

The final total weight is 5.8kg mirror included.

[Mick J]

Lovely looking scope, good work, nice read thanks.

[Michele Scotti]

5 hours ago, planetman83 said:

I can see here your post but I cannot find the youtube video in order to understand exactly how it works. If I understand right, we can see how rigid and well built is our construction in terms of vibrations? I am very interested in doing it. Please help me doing this experiment.

 

Yeah somehow the link is not working. Here it is again:

Yes, it's a quantitative measurement to assess the structural soundness of the telescope - this is a major design parameter when engineering big telescopes like VLT, E-ELT etc...

It's not self-explaining but you can measure the wobbling of the structure which is basically what you are experiencing at the eyepiece when touching the focuser or moving the scope or if there's a gust of wind. The reaction of the telescope it's embedded in its intimate construction parameters.

You can place the phone vertically where you have the "24" engraved with double-side tape or even with a clamp -just firm enough not to let it move. The idea being that scope and phone moves together. As an external

The X axis for the app will be left-right so hit the structure from a side - like the side of the focuser board- with an hammer. Of course doesn't need to be hard at all - you'll see with the sensitivity of the app.

Glad that you're interested - speak later

[planetman83]

After 3 times I tried the telescope, I think I can see astigmatism and I think it is because of how I glued the secondary on the holder. So, I made a new holder design that I will attach it to the aluminum triangle, I 3D printed it and I will try it again tonight. I hope this time the stars will be better.

[mdstuart]

Interesting. I have fought Astigmatism at times. You can rotate the main mirror to see if the Astigmatism rotates to isolate which mirror is causing the issue.

Good luck.

Mark

 

[planetman83]

I had another night after the new holder and it seems now ok. No astigmatism seen. I also sewed a light shroud. I still need to fix the 2 piece bearing problem because the rollers bump there.

[mdstuart]

Great news on the Astigmatism.

Love the Shroud.

What have you put on the bearings for the rollers to run on? How is the alt motion now apart from the bumps. Are you able to raise and lower the alt with just one finger etc.

Mark

[planetman83]

I have not put anything yet, I am in the process of trying different materials. I ordered t-molding yesterday in order to try this material. I added a 3rd set of rollers on alt and now the motion is softer. I will soft the az also. The motion and bearing thing is the last thing I want to improve (I hope). I haven't finish with this matter yet.

Edited May 17, 2021 by planetman83

[planetman83]

Hello again guys,

in order to solve the bearing gap problem, I just have concluded to the solution you can see in the photos. It is a pvc abs strip with 2mm thickness. It is very flexible and also seems very durable. I also added a 3rd group of roller bearings to alt that is adjustable.  As long as I turn the screw to the right, the rollers are getting closer to the bearings so the motion gets softer. On azimuth, I also added a 3rd group of rollers and tightened the center bolt so the motion there is quite soft and smooth.

[mdstuart]

Very interesting re the PVC ABS strip.

I am going to trial a lazy susan approach to my scope AZ motion this summer to see how that compares to the traditional teflon laminate approach.

I have also bought some sash window rollers which I will trial on my ALT motion.

I wish you smooth alt and az motions

Mark

[Swoop1]

A bit late to be asking this Planetman but, did you overcome the star nut issue? I ask because the thought of star nuts in carbon fiber tubing makes certain parts of me twitch uncomfortably!

In the cycle industry, star nuts are used in steel and aluminium steerer tubes whilst expanding wedges/ plugs are used in carbon fiber. Like these (sorry for the Amazon link). Star nuts can cause carbon fiber to crack/ split because they are quite harsh on the internal surfaces.

[planetman83]

3 hours ago, Swoop1 said:

A bit late to be asking this Planetman but, did you overcome the star nut issue? I ask because the thought of star nuts in carbon fiber tubing makes certain parts of me twitch uncomfortably!

In the cycle industry, star nuts are used in steel and aluminium steerer tubes whilst expanding wedges/ plugs are used in carbon fiber. Like these (sorry for the Amazon link). Star nuts can cause carbon fiber to crack/ split because they are quite harsh on the internal surfaces.

I added (glued) 0.7mm thickness aluminum foil inside the carbon tubes with epoxy and then I added the star nuts.

[Swoop1]

Nice solve to that potential problem.

[astrosathya]

What an excellent thread I must say. I recently completed an 8.5" f/8 Mirror and since I plan to use it for outreach (needs to be portable), I do not want to make a full tube DOB for it but a truss design. I have used The Dobsonian Telescope Book and found that with a 9" tall mirror box and a single ring secondary cage (0.75kg total weight including secondary mirror, focuser etc) and 70 inch long trusses of 3/4th inch OD, I get the torque in the front aseembly as 5.4kgm and the bottom section a mere 1kgm. Will this play a role in the balancing of the scope? How big would the bearings need to be? I hope I am not hijacking this thread, just thought my question would fit in with the very experienced folks here.

TIA.

[planetman83]

Hello again I made some touches to the telescope.

I made new bearings in order to balance perfectly without any counterweights. U made them 1 piece this time but the drawback is that the scope can point from 15 degrees and not lower. I made them like this in order to fit in my car.

I also painted the rocker and the mirror box black.

[planetman83]

On 23/12/2021 at 19:24, astrosathya said:

What an excellent thread I must say. I recently completed an 8.5" f/8 Mirror and since I plan to use it for outreach (needs to be portable), I do not want to make a full tube DOB for it but a truss design. I have used The Dobsonian Telescope Book and found that with a 9" tall mirror box and a single ring secondary cage (0.75kg total weight including secondary mirror, focuser etc) and 70 inch long trusses of 3/4th inch OD, I get the torque in the front aseembly as 5.4kgm and the bottom section a mere 1kgm. Will this play a role in the balancing of the scope? How big would the bearings need to be? I hope I am not hijacking this thread, just thought my question would fit in with the very experienced folks here.

TIA.

I don't know. You have to balance the OTA to see where the balance point is.