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Walking on the Moon

Crayford focuser - recognise the brand?

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I have a classic Prinz 660 3" long tube refractor and I'm considering replacing the (all metal) R&P with a crayford. However, it has a skinny tube c. 76mm diameter and all the after market ones I can see are made for fatter ones.

What about this one, any ideas?



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Internal diameter of the one you are condsidering, is 73.3mm which is too small to
fit the outside diameter of your tube, which you say is 75mm. 
Unless you have a friend who can reamer out 1.7mm from that focuser connection tube, It isn't going to fit.
If that job can be done, then it will need to be done very carefully. It will make the tube connection portion 
rather thin walled. It isn't very thick to start with.
Is your refractor tube definitely 75mm OD?
You could get a light engineering workshop to insert a reducer at the focuser end of your refractor
The original length will need to be shortened to accommodate the insert.   The overall length of the frac. has to be maintained.
The reduced diameter shouldn't affect the light path, such as vignetting.
Of course that will need examination before proceeding with that remedy.
You will be reducing the classical value of the telescope doing this.

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