800mm Telescope Project

[michael.h.f.wilkinson]

1 minute ago, Peter Drew said:

The outcome of this will be of interest to me. I have two secondary mirrors for my 30" Dobsonian, one as a spare. Both are 150mm minor axis and 12mm thick. I have mostly "glued" secondary mirrors to backplates with success so did the same with these. The telescope then showed astigmatism  and knowing that the primary was ok I suspected the secondary, and the spare one when fitted for a comparison. The secondaries were independently tested and found to be good when unmounted so I subsequently remounted one in a "hung" arrangement. The astigmatism disappeared.  At least it's a simple thing to experiment with.   🙂

I have had similar issues with a secondary that had astigmatism when glued and none when hung from the perimeter (this on a much smaller scope), so I guess the gluing can put some strain on the glass

[mapstar]

I created a holder from carbon fibre below. It's not a work of art but took me 3 attempts to get it to work right. The next one will be much better made after the experience.

 

In the background is the aluminium top with spherical bearing that the holder pivots on for collimation.  

 

[mapstar]

5 hours ago, Michele Scotti said:

I put the density in the model and ran the analysis - I actually didn't know the wieght till you asked. However with some rough calculation your secondary should be around 850g whereas mine that has almost double the area but 12mm thickness id is ca. 1.2Kg. It's going to be 'glued' on its back rather than hung from the perimeter.

It was just over 900g being pyrex. I was thinking your secondary would be 20mm thick so will be a lot lighter at 12mm which is a good move if it has enough support. 

[Michele Scotti]

6 hours ago, michael.h.f.wilkinson said:

I have had similar issues with a secondary that had astigmatism when glued and none when hung from the perimeter (this on a much smaller scope), so I guess the gluing can put some strain on the glass

Peter, Michael, thanks for the info. To be honest this is a part of the project that I haven't really paid too much of details into.  I know glueing was tricky though but also the desing of the holder can be modified to hook the mirror in a more traditional way.

May I ask what glue did you use?

[Michele Scotti]

2 hours ago, mapstar said:

I created a holder from carbon fibre below. It's not a work of art but took me 3 attempts to get it to work right. The next one will be much better made after the experience.

 

In the background is the aluminium top with spherical bearing that the holder pivots on for collimation.  

 

I have a similar design - I'll post it soon. You used a spherical joint didn't you?

[Peter Drew]

On 12/11/2019 at 21:39, Michele Scotti said:

Peter, Michael, thanks for the info. To be honest this is a part of the project that I haven't really paid too much of details into.  I know glueing was tricky though but also the desing of the holder can be modified to hook the mirror in a more traditional way.

May I ask what glue did you use?

I used Dow Corning silicone on the successful mountings which were maximum 90mm minor axis.

It has been suggested that an annulus of silicone between the rear of the mirror and its backplate with a small hole in the silicone to release air might be a better method than the commonly used silicone blobs.      🙂

[Michele Scotti]

Placing the 4 brackets (those clamped parts) for the altitude bearings on the azimuth table using the mirror box as a jig.

Here below a close-up of the bracket

[Robert72]

One of those aluminium soldering kits would have been ideal for this project.  I have no experience of them, but they look pretty good.

[Michele Scotti]

6 hours ago, Robert72 said:

One of those aluminium soldering kits would have been ideal for this project.  I have no experience of them, but they look pretty good.

I tried to stay away from soldering for few reasons although I grazed the idea of using brazing for the H frame.

Anyway, reasons in random order are:

1 - welding may introduce distortions with the subsequent need to maching surfaces

2 - welding is not ready available to all ATMers, requires skills and - this project has to be as easy and affordable as it can be

3 - most of the construction is not involving highly stressed parts

4 - modern bonding epoxies can be as strong as a good weld  

 

Here below some details of the Alt bearing bracket before bonding:

[Michele Scotti]

Bit by bit....so here is the crossing bar in the back of the lower assembly.
From a construction point of view it’s a 40x80mm with a 1.5mm wall thickness. Not been able to procure such a section we opted to join two 20x80 section with the benefit of having a stiffer 3mm wall running across the bar. All the holes are to keep it together during bonding with some bolts.

The bracket is made of CF once again just because we had a suitable section ready available. Connection is via rivnuts hence the reliefs on the bottom of the bracket.

Modal analysis suggested a residual weakness that makes the bar sag - as a consequence 2 supporting bars will bee added to help transferring the load to the pac-men

[Robert72]

Low heat aluminium welding. (Unfortunately I have no affiliation whatsoever with this company):

https://www.durafixonline.com/gb/

[Astrobits]

On 13/11/2019 at 22:50, Peter Drew said:

I used Dow Corning silicone on the successful mountings which were maximum 90mm minor axis.

It has been suggested that an annulus of silicone between the rear of the mirror and its backplate with a small hole in the silicone to release air might be a better method than the commonly used silicone blobs.      🙂

There is no way you should use more than three blobs to hold a mirror in place. Any more than three, and that includes extended areas, will almost certainly lead to astigmatism. An anulus of adhesive will act as many points and should not be used on anything where distortion will be unwanted. 

My 16" mirror has been held by 3 blobs for the last 15 years or so and the 70mm ma diagonal with just one blob for a similar length of time without any problems. 

Nigel 

Edited November 24, 2019 by Astrobits

[Michele Scotti]

2 hours ago, Robert72 said:

Low heat aluminium welding. (Unfortunately I have no affiliation whatsoever with this company):

https://www.durafixonline.com/gb/

Yep, exactly. 

And that's what is in my mind every time I was thinkig about giving it a try.... 😁

 

 

[Peter Drew]

2 hours ago, Astrobits said:

There is no way you should use more than three blobs to hold a mirror in place. Any more than three, and that includes extended areas, will almost certainly lead to astigmatism. An anulus of adhesive will act as many points and should not be used on anything where distortion will be unwanted. 

My 16" mirror has been held by 3 blobs for the last 15 years or so and the 70mm ma diagonal with just one blob for a similar length of time without any problems. 

Nigel 

I've never tried an annular silicone secondary attachment, the suggestion came from a very respected optical expert.     🙂 

[Robert72]

Try this one.  She makes it look so simple.  I think in that video you posted the guy was perhaps a bit old school and aiming for failure.

Or this.

 

[Astrobits]

I have done some aluminium soldering on small parts for years. I saw a demonstration at a hobby show over 10 years ago and purchased a kit from the stall. The technique that was shown to me at the time was to tin the joint components first by heating the separate bits up to melt the rod and then use the stainless steel brush to push the molten solder through the oxide coating to get the key to the base metal over the area to be joined. Then do the normal soldering of the bits together. With a background as a research chemist, I know that aluminium will start oxidising immediately on exposure* thus the technique shown in the one video from the above post that I looked at, where brushing preceded the assembly and soldering, will possibly produce weaker joints than the pre-tinning that I do as there will inevitably be a layer of oxide between the solder and aluminium.

*  Many years ago in one of my chemistry classes we were tinning pennies with mercury just by rubbing them ( no elf 'n safety in them days). One of my classmates decided to tin his medallion which was made of aluminium. the result was that the mercury prevented the protective oxide layer from building up on the surface and the oxidation of the aluminium went ahead unhindered. The medallion got too hot to hold within seconds and we watched, fascinated, as the medallion turned into a heap of white powder in less than a few minutes.

Nigel

[Michele Scotti]

1 hour ago, Astrobits said:

With a background as a research chemist, I know that aluminium will start oxidising immediately on exposure*

Hi Nigel, thanks for the contribution. Would you be able to elaborate on that? Whilst is clear that thre's no reason for oxide to wait conversely I wonder how long does it take to reach its final layer thickness.

I think I rememebr that to complete the oxidation it's a matter or hours although I can;t rememebr that paper and it might not have been linear.

If you think about rust on iron that takes time and it's dependant on other factors on top of O2 in the air. 

Althuogh I prefer bonding - for personal reason- the oxide layer in very relevant

[Astrobits]

I have not done any definitive work just that the reaction on that aluminium medallion started  while the unfortunate owner was still rubbing the mercury on to it, i.e the reaction started immediately the bare surface was available. It got so hot that he had to put it down quickly before he dropped it.

The oxidation on the surface of aluminium will slow down as soon as the first reaction provides that protective layer ( provided it is allowed to of course unlike my mates experience ) and will probably continue for years as long as there is aluminium available. Unfortunately, rust provides no protection to Iron.

Nigel

[Michele Scotti]

In retrospect - the altitube bearing bracket. Maybe not the most exiting part (?).

Main thing here was to avoid soldering which would have most likely involved a following machining of the piece. Hence the interlocking and bonding approach which concentrate most of the work in machining different sections of a flat aluminium bar.

Here's some details:

  

 

 

As the aim of the bracket is to carry the load but also to place with (some) accuracy the altitube rolling bars, we machined the side of the bearings to a specific lenght. You can see the ends being trimmed on the milling machine - this involved placing them with the bar. A ground pin is placed on the bracket.

Bearings are standard heavy duty pillow block bearing - pretty inexpensive at les than £10 each. The ground bars are 20mm OD - here in the picture a smaller section used to align the bearings for "trimming"

  

 

The set ready for bonding (upside-down and withouth the dowel pin):

 

[Michele Scotti]

Central hub secondary mirror colimation mechanism

The secondary hub mechanism is nested inside the spider central hub. The idea here is to provide a movable platform that offers smooth collimating operation as well as a wide contact area for the secondary mirror collimation screws.

I have the feeling we have a bit of over engineering here.....

 

Best thing we came up with was to re-purpose a piston and its cylinder. They are Aluminium bits duly gutted and lightneded.

 

 

To complete the system 2 flanges connect the cylinder to the spider hub whilst the top one accomodates 3 knobs for collimation and ease of use - potentially they could be motorized if we'll fill the need for that.

The target for the whole system including the mirror holder but without the mirror itself is 725g.

[markse68]

Nice bit of up-cycling! Looking forward to discovering how the adjustment works. This, the primary cell and the focuser are probably the best parts of a dobsonian  to over-engineer 😉

Edited December 16, 2019 by markse68

[Michele Scotti]

5 hours ago, markse68 said:

Nice bit of up-cycling! Looking forward to discovering how the adjustment works. This, the primary cell and the focuser are probably the best parts of a dobsonian  to over-engineer 😉

In the meantime a hint of the sec mirror holder. Mold is soft wood not meant to be permanent...

It looks shi....unrefined as of now.

 

I'm collecting some sketches fo rthe whole system as it is not detailed very much on CAD - need to do that at a later stage

[Michele Scotti]

Some progress on the secondary holder. It's still pretty rough and I overused sandpaper in some spots. Needs some finessing...

The small fan to help the thermal stabilization is loosely housed in the upper part - I still have to figure out the best pattern for the outlet holes.

You can also see the tube inserted inside the secondary. The idea here is to have the pivot as close as possible to the centre of the sec so that adjustments are not throwing the light path too much around during alignment. Still not sure if it's worthwhile though.... 

I attach a rudimental sketch of the whole system - pinch of salt needed as it's nor an engineering drawing or a fully functional schematics. Just to give a gist of it....

[Michele Scotti]

Here is a quick and dirt first assembly of the UTA - this is more to have a glimpse of the assembly method and whether everythign was falling in place.

Pretty happy witht the weight as it's in line with the projected 15Kg target for the whole UTA

Say hi to Marco! Fellow astro club member and 33% of the workforce on this project. 

 

[Michele Scotti]

UTA columns connect the hexagons and provide anchoring to the spider's vanes. Their weight is crucial and the FEA assumed being Aluminum with a 50x50mm cross-section and a 1.5mm wall thickness.

System overview:

 

For our application we wanted to toy with CF so why not vacuum bagging5 layers around a 'mandrel' - represented by an Aluminium tubing?!?  Sleeving it out was quite a painful process. though...

 

 

Coloumns are internally reinforced in the area where spider's screws poke through them to create tension on the vanes - this is achieved by sleeving 2 pieces of small tube - 20mm in diameter- to help to transfer the compression forces across.

You can see them in this CAD here below, running horizontally – you can see also that the ends are closed with 2mm plates to evenly distribute the pressure and help with the placement durinf the assembly.

  

 

A 400mm long M4 rod is used to clamp the hexagons and the  together. An M4 can withstand up to 250kg of pull but if used at yielding it climbs up to 430kg - we reckon this is enough clamping force.

Each M4 rod weighs in at 60g and helps to load the structure with compression forces in a uniform way. It also represents a weight opportunity compared to few M6 bolts I would have chosen to use instead. Let alone that is dead easy to implement.