Carbon Serrurier truss Newtonian 10" f4 build

[orly_andico]

Hi Raymond,

How did you attach the L brackets to the ends of the carbon tube? I assume the carbon tubes are off the shelf items?

[RAC]

Hi Raymond,

How did you attach the L brackets to the ends of the carbon tube? I assume the carbon tubes are off the shelf items?

There is a piece of dowel 15mm long glued inside each end with a nut on the inside of it. Yes the carbon tubes come from China for only $5.60us each.

[Astroscot2]

Raymond, when you complete your 18" can  you box up the 10" and send it to me .

[RAC]

Raymond, when you complete your 18" can  you box up the 10" and send it to me .

Mmmmmm not too sure about that. I'm sure i can use two at once.

[Dave_D]

Mmmmmm not too sure about that. I'm sure i can use two at once.

And who said men can't multitask?

[orly_andico]

Hi Raymond, what's the diameter of the carbon tubes?

Is the dowel an off-the-shelf size or did you need to turn it down on a lathe (or similar) so that it would fit in the carbon tube?

Also the nut on the dowel.. is it an actual nut that you glued in there or is it some form of threaded insert?

I'm thinking of making a carbon truss newt with a rotating upper cage and I've been advised that I need for my trusses to be a Stewart platform for centering the upper cage so that collimation is not lost when I rotate the cage.

[Ajohn]

My carbon truss poles came from Hobby king in china at $5.60us each. I'm going to make another scope similar but it's going to be for an 18" f3.3 mirror that I am making and that's only if i can get the figure nice on it and that's proving hard!

I'll be using alloy for the trusses this time though. More like a Royce Dall-Kirkham

http://www.rfroyce.com/DK%20OTAs%20struts.htm

I've been aiming at sub F3 for a cassegrain. All sorts of problems including doubts if  can actually make a corrector I have designed for it. I came across a book by a Scottish optician who did some work for NASA and others on fast mirrors. He does his sums and works out how much glass needs to be removed for figuring and if more than 0.001 inch he grinds it off.  This might involve more than two tools. I feel that this could only be done on a grinding machine with some sort of stroke arm. He basically reckons he's describing the old way of doing this sort of thing in case some one needs the techniques in the future. Big mirrors by amateur standards. I understand he upset NASA by making an IR mirror way more accurate than it needed to be in case some one wanted to use it for something else later.  Not sure on that point.

He did his tests with Foucault tester mounted on a lathe cross slide and pins on the mirror but he also checked for smoothness using a method he puts down to Dall. An achromat, moved around to null the mirror to spot lack of smoothness  and ripples etc.  I have used the wire form of the Hartmann test and reckon that is more fool proof than judging shadows in masks or via pins but I don't do much of this sort of thing. I have been pretty active on cloudy nights at times and there are all sorts of ideas about measuring mirrors but when they are ray traced they don't look so good as the set up needs to be rather accurate. Depends what some one wants to finish up with really.

He also talks about mirror grinders  I get a vision of one that follows a template at times surrounded by polythene curtains, water jetted with some form of lubricant. He points out that glass dust is good at blocking drains so needs a settling tank etc.

Never tried his method but it does make sense but  pass really. If I ever do one again I'm pretty sure it will be a Dall Kirkam as per some on CN. Put all of the problems together and it makes a lot of sense.

Whoops - meant to add that is a pretty amazing piece of work.

John

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[skybadger]

Interested in knowing more about the book please.

[Ajohn]

Optics of the Cassegrain Telescope, Robert L Waland ISBN 0-8062-3393-1. Carlton Press

I happened on it by accident on holiday in the Scotish boarders. The seller reckoned it came from his estate sale so was his own copy.  Pass on that aspect. It's bulked out a bit with calculations and the readings taken on a couple of mirrors. There is a chapter on figuring a 60in F2 for NASA and  another on a 60in F4 precision parabaloid. There are some plates in the book but very poor black and white printing. Info on the design of all cassegrain types.

The way he did it is pretty simple. Grind and polish to the curve needed at the margins and then grind and polish the central curve using a spherometer for the measurements. It's obviously going to be easier on larger mirrors. This could be done with tiled tools  by removing the outer edges once the 1st rad is done. The central area is polished with full sized lap and was ground with a tool of about 40in diameter for a 60 in mirror. After all of this the usual small laps are used. It makes sense to me but I doubt if the central curve could be kept more or less where it aught to be by hand.

As some one asked I've added some detail as I would guess that the book is going to be hard to find. There is a bit about him here

http://www.futuremuseum.co.uk/collections/people/key-people/science-invention/robert-waland.aspx

I don't want to hijack the thread but thought it might be of interest to some one trying to make an F3.3 mirror which is going to be tough as he says.

I should add that though the forum might show me on line I may not be. I tend to retain a lot of tabs on my browser.

John

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