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I figured it out! Secondary mirror placement...

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I had always wondered why the secondary is supposed to be a little over towards the far side of the scope from the focuser. I was mentally designing a scope, and the answer came to me in a flash, and I confirmed it by doing a diagram. If you draw a Newtonian schematic, and draw the cone of light coming from the mirror to the focus, you then want to put a secondary in the system, at 45 degrees, to divert the light cone into the focuser tube. If you draw that diagonal on the diagram, then locate the centre of the diagonal, you will find that the centre of the diagonal must be to the side of the tube away from the focuser. This is because the edge of the cone is farther from the centre of the tube at the lower edge, and closer at the upper edge.

I was quite excited about this, as I thought it was because of some exotic reason involving quantum mechanics or something. It's just geometry!

This also means that if you are making a truss tube design with a rotatable focuser end, you don't have to worry about the location of the secondary - just set it up as you would for any tube, and it must work.

See what happens to my brain on a night with no stars...

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I would love to say I know exactly what you mean....but, I can't.... :lol: Well done WH, I think... :lol:

Don't worry Caz it will all come to you in time.

Hmmmm...I hope so... :? :lol:

It's the old car wheels thing Caz. If a car is driving towards a 90 degree line drawn across the road then the front right hand wheel is going to hit the line before the left one. Same with light. This puts the right hand part of the light "beam" a little ahead of the left as it had to start turning sooner to "bounce" off the line (secondary mirror).

I think.


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