24.4" f/3.2 truss dobsonian project

[mapstar]

Many thanks for the compliments.

It's looking good keep up the good work. That mirror looks really nice. 

I'd like a cnc router for the next builds but don't have the room for one so it'll be done completely manual. 

The holder was a lot of trial and error but the next should be easier to make now. 

[mdstuart]

Will follow with interest. I put a 20 inch together recently. With hindsight I would have gone for thicker bearings to allow for rollers. I would go for different top connectors as I had to brace mine.

However my dob now works well even though its not perfect.

Mark

 

[planetman83]

10 hours ago, mdstuart said:

Will follow with interest. I put a 20 inch together recently. With hindsight I would have gone for thicker bearings to allow for rollers. I would go for different top connectors as I had to brace mine.

However my dob now works well even though its not perfect.

Mark

 

How thick are your bearings?

[mdstuart]

Planetman

18mm. It all works fine and is strong enough etc but if they were wider it would make it easier to embed rollers rather than just using the normal Teflon pads.

At some point I might make a new rocker box and bearings with thicker walls.

Mark

 

 

[oldfruit]

On 08/03/2021 at 19:35, planetman83 said:

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. 

Beautiful looking scope, watching this thread with interest!

[oldfruit]

20 minutes ago, mdstuart said:

Planetman

18mm. It all works fine and is strong enough etc but if they were wider it would make it easier to embed rollers rather than just using the normal Teflon pads.

At some point I might make a new rocker box and bearings with thicker walls.

Mark

 

 

Just out of interest, is there a weight thresh hold where rollers would be required rather than teflon? 

Edited March 12, 2021 by oldfruit

[mdstuart]

Old fruit

My understanding is Teflon works fine for any size as long as you get the angles, size of pads and bearing surfaces right but once you get to 20 inches plus some designs start to include bearing rollers.

There are lots of views on the subject. I ended up with Teflon on the front alt bearings as some rollers at the back alt bearing which sort of works.

Mark

[planetman83]

I assembled the mirror box today. I used glue for birch plywood and over 30 wood screws.

[mdstuart]

Very low profile

[planetman83]

1 hour ago, mdstuart said:

Very low profile

This was my goal, to make the mirror box and upper cage as small as possible in order to fit easier in the car. They are 28cm each.

The truss tubes will be carbon fiber so they are stiff holding them together.

Edited March 13, 2021 by planetman83

[mdstuart]

Make sure the top cage is big enough. Has to be a bit bigger than the mirror diameter to avoid vignetting. Mine is slightly smaller than recommended but to be fair it has not impacted the performance in terms of the faintest stars I can see.

Mark

[planetman83]

14 hours ago, mdstuart said:

Make sure the top cage is big enough. Has to be a bit bigger than the mirror diameter to avoid vignetting. Mine is slightly smaller than recommended but to be fair it has not impacted the performance in terms of the faintest stars I can see.

Mark

The mirror is 620mm and the inside diameter of the upper cage is 636mm.

[mdstuart]

 

Just fir fun have a look at this..

https://www.cloudynights.com/topic/56129-inside-diameter-of-secondary-cage/

Mark

[mdstuart]

My mirror is 51cm and the inside diameter is 53cm. I have not noticed any vignetting etc but then I rarely use any eyepiece with a focal length above 16mm.

Mark

[faulksy]

16 hours ago, planetman83 said:

I assembled the mirror box today. I used glue for birch plywood and over 30 wood screws.

looks brilliant. what pads are you using for your mirror ?

[planetman83]

3 hours ago, mdstuart said:

My mirror is 51cm and the inside diameter is 53cm. I have not noticed any vignetting etc but then I rarely use any eyepiece with a focal length above 16mm.

Mark

I did not know about this to tell you the truth. Is it so serious? I am 636 to 620 only.

3 hours ago, faulksy said:

looks brilliant. what pads are you using for your mirror ?

They are feet for small furniture, something like this.

[mdstuart]

One thing is certain. You will learn so much building a scope like this. Also you will get lots of help/views on this forum..

Mark

[faulksy]

3 hours ago, planetman83 said:

I did not know about this to tell you the truth. Is it so serious? I am 636 to 620 only.

They are feet for small furniture, something like this.

thats ok then, dont be tempted with cork/felt or ptfe because they can cause astigmatism especially with your lovely nichol mirror

[planetman83]

On 13/03/2021 at 21:08, mdstuart said:

Make sure the top cage is big enough. Has to be a bit bigger than the mirror diameter to avoid vignetting. Mine is slightly smaller than recommended but to be fair it has not impacted the performance in terms of the faintest stars I can see.

Mark

The truth is that I am rebuilding a used 24.4" telescope making it smaller and lighter. The builder of the previous one just told me that he took into consideration all these factors and that the dimension of the ring is the correct one.

[mdstuart]

Planetman

Sounds like you have someone with experience helping you which is good to hear.

Looking forward to more pictures as you progress.

Mark

[planetman83]

Yes, there is a friend of mine that is experienced helping me out.

I have a question: I just tried a 1 1/8" star nut to a 28mm inside diameter carbon fiber tube and the star nut is too small. I though these star nuts are for 28mm tubes. Am I wrong?  How can I use them to a 28mm inside tube?

[Astrobits]

Surely, it is simple trigonometry to determine the size of the top cage needed. Just use the max field of view you and the distance from mirror to the top of the cage.

Nigel

[Piero]

10 hours ago, planetman83 said:

Yes, there is a friend of mine that is experienced helping me out.

I have a question: I just tried a 1 1/8" star nut to a 28mm inside diameter carbon fiber tube and the star nut is too small. I though these star nuts are for 28mm tubes. Am I wrong?  How can I use them to a 28mm inside tube?

Not sure how you tried to insert it, but there are tools in the bike industry which can make the process very straightforward.

[planetman83]

10 hours ago, Piero said:

Not sure how you tried to insert it, but there are tools in the bike industry which can make the process very straightforward.

The 1 1/8" star nut is too small for the 28mm inside dimeter tube! It just goes in and gets out from the other side.

[Piero]

1 hour ago, planetman83 said:

The 1 1/8" star nut is too small for the 28mm inside dimeter tube! It just goes in and gets out from the other side.

Are those star nuts new or used? What is the diameter of your pole using a caliper?