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Silly question...really!!

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I know this is a silly question and i fully expect to be laughed at for asking it but......

What is to stop me taking my astromaster 130 and cutting it in half and then adding 215mm of 130 diameter steel or aluminium tubing to the centre thus making it 875mm long and f6.5??

Edited by marcdearn
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The mirror at the back of the scope is made in such a way that the focus will be where you sit the eye piece. My extending the tube, you will move the location of the secondary mirror, and the eyepiece. This will mean you can no longer focus the telescope, making it useless.


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The primary mirror is shaped to bring light to a point at a specific focal length. On a newtonian this point is usually somewhere outside the focus tube. Only if the shape of the mirror is altered will the focal length/focal ratio change.

A fast (small focal ratio - <f/6) newt will have a mirror which is more steeply curved than a slow scope (>f/6)



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