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24.4" f/3.2 truss dobsonian project


planetman83

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Guys, I am having second thoughts about the 3D printed secondary holder. I am afraid of its stability and rigidity. I will glue the 5.3" secondary mirror to a 12mm birch plywood with silicone and then I attach it with screws to a piece o aluminum. The mirror will touch only to the plywood, will it be ok like this? 1) Will I have astigmatism? 2) Is there any chance the mirror will detach and fall from the silicone?

 

 

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Edited by planetman83
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On 22/03/2021 at 17:02, planetman83 said:

Guys, I am having second thoughts about the 3D printed secondary holder. I am afraid of its stability and rigidity. I will glue the 5.3" secondary mirror to a 12mm birch plywood with silicone and then I attach it with screws to a piece o aluminum. The mirror will touch only to the plywood, will it be ok like this? 1) Will I have astigmatism? 2) Is there any chance the mirror will detach and fall from the silicone?

 

Firstly, silicone will not fail at a ( grease free ) glass boundary. If it is going to fail it will be at the wood boundary or if you have prepared that correctly then within the wood itself. I cannot vouch for all silicones, I use ones that specify that they are suitable for glass applications ( i.e. greenhouses )

I once sent a 200mm Newtonian telescope ( solid tube design ) by courier. They dropped it so heavily that the mirror, which was bonded to the MDF backing plate with silicone, and part of the MDF went down the tube to hit and break the secondary. The failure was within the 19mm thick MDF board and not at any of the (3 x approx 25mm dia.) adhesive joints that were holding the mirror to it.

To prepare the MDF I smear some epoxy over the bonding areas and when set just rough it up with a bit of sandpaper. I would suggest that you do the same with the ply that you intend to use.

Secondly, DO NOT USE MORE THAN THREE points of adhesion or you WILL get astigmatism.  Also make sure that the silicone is about 3mm thick between the mirror and support. This will further prevent astigmatism by giving the mirror a softer "cushion". Personally, I use only one central silicone blob on my secondaries, about 20mm on a 70mm M/A mirror. My current 16" scope has lasted 20 years so far without any adhesive problem. By allowing about 3mm space between mirror and backing it will be easy to remove the mirror when the time comes for re-coating it. I had another time when a customer failed to heed the three blobs x 3mm thick rule and that resulted in astigmatism ( in 19mm thick glass ) and a very difficult time to remove the mirror causing some damage to the backing plate.

Nigel

 

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47 minutes ago, Astrobits said:

Firstly, silicone will not fail at a ( grease free ) glass boundary. If it is going to fail it will be at the wood boundary or if you have prepared that correctly then within the wood itself. I cannot vouch for all silicones, I use ones that specify that they are suitable for glass applications ( i.e. greenhouses )

I once sent a 200mm Newtonian telescope ( solid tube design ) by courier. They dropped it so heavily that the mirror, which was bonded to the MDF backing plate with silicone, and part of the MDF went down the tube to hit and break the secondary. The failure was within the 19mm thick MDF board and not at any of the (3 x approx 25mm dia.) adhesive joints that were holding the mirror to it.

To prepare the MDF I smear some epoxy over the bonding areas and when set just rough it up with a bit of sandpaper. I would suggest that you do the same with the ply that you intend to use.

Secondly, DO NOT USE MORE THAN THREE points of adhesion or you WILL get astigmatism.  Also make sure that the silicone is about 3mm thick between the mirror and support. This will further prevent astigmatism by giving the mirror a softer "cushion". Personally, I use only one central silicone blob on my secondaries, about 20mm on a 70mm M/A mirror. My current 16" scope has lasted 20 years so far without any adhesive problem. By allowing about 3mm space between mirror and backing it will be easy to remove the mirror when the time comes for re-coating it. I had another time when a customer failed to heed the three blobs x 3mm thick rule and that resulted in astigmatism ( in 19mm thick glass ) and a very difficult time to remove the mirror causing some damage to the backing plate.

Nigel

 

Very useful information. I am also wondering: Do I have to glue them at the temperature close to what I will usually stargaze? so both materials be at their dimensions they will be when I stargaze?

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Not necessary. The silicone is flexible enough to absorb slight changes in temperature. Anyway, I'm sure that your night time temps vary throughout the year so which night time temps would you match?

Just make sure that your blobs are not too large. For your 5.3" secondary I would aim for one central blob about 1" dia OR three about 1/2" dia.

Nigel

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I made the secondary holder designed before. I glued the mirror on a piece of birch plywood with silicone for glass/aquariums. I also used a second glue (3M) for safety with a cord. I don't know if I will have astigmatism. I guess I will find out with the first light. If this is a problem, I will think something else.

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I managed to build the secondary cage. I changed the secondary holder, so the total height of the upper cage  went from 28cm previously designed to 25cm. I hope it will not produce astigmatism this way. The weight of the upper cage is about 5.3kg. I will add 1.5mm birch plywood inside instead of kydex so the weight will go to around 5.8kg. I also added the same fan panel on the mirror cell that existed in the previous telescope.

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Edited by planetman83
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Guys, I managed to finish the tube assembly. I made the weighing and the result is that I need a bearing of 1.4x mirror's diameter if I place it 1 centimeter above the lowest mirror box's point like in the sketch. What do you suggest? Making 1.4x bearings and placing them like in the sketch? Or is it better to make smaller bearings placing them higher or bigger bearings placing them lower? There is a small problem using 1.4x bearing like in the sketch. I cannot connect the 2 bearings in the higher end with a tube because the 2 trusses are in the way.

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telescope 1.4x bearing.jpg

Edited by planetman83
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Guys, this is the last simulation up to date. The first sketch is the bottom of the rocker box with the az ball bearings. I am planning to replace 1 group of bearings with a piece of teflon pad. The 2nd sketch is the rocker up wood with the ring made of stainless steel on where the bearings are going to roll. I still want to design the alt ball bearing system to complete the rocker box.

az ball bearings.jpg

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telescope.jpg

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And this will be the alt bearing system. The right bearing will be adjustable and I will add only if needed. So in 2 points I will use roller bearings and on the other 2 points, formica.

alt bearings.jpg

Edited by planetman83
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19 minutes ago, mdstuart said:

If you make the base and arcs wide enough you could use embedded rollers like on your az movement.

Mark 

If I put the rollers in the rocker side wood, the wood need to be at least 32mm wide. And the bearings 36mm or so. And the weight goes up. Also, the weight of the tube assembly will be around 70kg eyepieces included, so will I feel more comfortable if the rollers are on stainless steel parts, also I can drill the steel at very close to the edge required. My rocker side wood is now 24mm wide.

Edited by planetman83
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Also, because of the mirror box inside, if something is inside of the rocker sides will be a problem because the mirror box will hit on it. So I chose to make this stainless steel roller system in order to be more rigid and small.

Edited by planetman83
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Aluminium is probably too soft for that application. The load on the bearing contact will be very high. It is a narrow line  of contact and there is the possibility of the Al deforming under that load. Also Aluminium will slowly rub off on the PTFE and leave fine black dust.

The reason we use textured Melamine laminate/PTFE combination when making conventional Dobsonians is that using smooth melamine gives rise to stick-slip motion. I don't know if using smooth steel and PTFE will do the same.

Nigel

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The reason I want to use metal is because the roller bearings motion on formica is not so smooth. So is it better to use thinner stainless steel? For example 0.8 or 1mm thickness? And glue it on the wood?

The roller bearings will be in groups of 2.

alt bearings.jpg

Edited by planetman83
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I use the same method, a 1mm stainless steel strip running on stainless steel rollers that are themselves in bearings.  The free movement is controlled by an adjustable PTFE pad.       🙂 

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Something else I want to do is to cut the bearings. The half bearing will be permanently on the mirror box and I am going to put the other half when setting up the telescope. The reason is to fit in the car. My concern is where the 2 bearings are going to join each other, will I have jerky motion? I need to put the stainless steel pieces perfectly side to side.

Edited by planetman83
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