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Re shelling a 8" newtonian


MikeMS

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I have a home made 8" newtonian and I have put on it a 2" focuser. It is 7 cms deep when racked in and the canon 300 D needs to go a bit further in to achive focus. The 26mm 2" eyepiece I have does not come into focus when it is racked all the way out, it needs a little more.

I would say it is 50 inches (sorry for converting from cms to inches) from the mirror to the end of the focuser when racked in.

The OTA is old and I am thinking of making a different tube (or box wood with struts). Now, can I shorten the tube to get the canon to focus? Could I move it 40mm down and have the canon focus at full extension and the 2" eyepiece focus with a 80mm extension tube when the focuser is racked in?

And where does this leave the secondary mirror which is about 35mm wide?

Thanks in advance for any advice.

Kind regards

Mike.

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Hi Mike, I'd say measure, as best you can, how much more in travel you need to bring the 300D to focus, then shift the primary & cell (shorten the tube) closer to the secondary by that amount, plus a bit more to be certain of focus. Then use whatever extension tube you need to focus your eyepieces.

I would leave the secondary as is, because you won't be shifting anything far enough from it's original position to warrant a different size (assuming it was the correct size in the first place)

Alternatively, if the 300D is only just short of focus, could you modify the focuser for more in travel ?

When you are happy with it, you could think of making a new tube, and will have learnt enough to get it right first time. Hope this helps & good luck, Ed.

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I think Ed is saying very sensible things.

Don't worry about the secondary size -- if you move the mirror up the tube, the secondary will end up slightly oversized, which is absolutely fine (it will give you a larger unvignetted field).

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Hi Mike,

Download - Dale Keller's Newt Software page

Donate about an hour of your time to get to grips with what is a very simple programme. Play about with a few settings and all will become clear.

Then enter your own measurements. The facts you learn from this are worth more than a thousand theories.

I hope it helps,

Dave.

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I think Ed is saying very sensible things.

Don't worry about the secondary size -- if you move the mirror up the tube

I am thinking of moving the secondary down, not up.

Yes, I meant move the PRIMARY mirror up the tube, which is optically equivalent to moving the secondary mirror down; but probably a lot easier mechanically, as the others say.

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