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60mm focus extender?


perdrix

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OK here's a quandry.  The 50mm focus extender that allows me to use eyepieces with my MN190 is fine for all my 1.25" EPs (though it could be a smidge longer without harm).

It's too short for my 2" 40mm EP.  So I thought I'd just get an 80mm extender.  Which I did.

Of course that's to darn long :sad: !  It would seem that a 60mm long extender would do very nicely, but my Google foo seems to have deserted me, and I can't find one (also looked for 10mm ones).

Does anyone know where I might find a good solution?

PS I may have an brand new (checked with EP above once) 80mm extender FS.

Thanks
Dave

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Well,

The problem is I do need to pull the 2" EP out quite way and I'm not overly happy with the amount it sticks out from the tube.  A par focal ring might make it easier to put in consistently, but it would still be almost hanging out in the breeze...   Maybe 60mm is too short and perhaps I need a smidge more ...  Hmmm - I wonder is there such thing as a very long helical 2" focuser?

Thanks
Dave

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I guess it could - it is after all just a regular Newt with a front corrector plate.  Not sure I'm up for the necessary tube surgery to achieve this tho' - call me a coward!!!  I don't think a regular hacksaw is a good tool for accurately cutting 10-15mm off the (steel) tube (plus I'd need to re-drill the mounting holes ...).  Is this sort of thing done regularly?

Cheers, Dave

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If you move the mirror up the tube the light cone will be larger where it's intercepted by the diagonal and some of the light at the edges might get lost. You might get away with some movement of the primary if the secondary is oversized, which most are on newtonians but I'm not sure about mak-newtonians.

Also remember with a mak-newtonian that the corrector lens / meniscus at the front end of the tube is part of the optical system - I'm not sure what the effect would be of moving the primary closer to it :undecided:

Another option would be to put a Baader Fine Tuning Ring, say the 14mm, on the bottom end of the 2" eyepiece. This will leave the eyepiece sticking out of the extension tube but the Fine Tuning Ring will make sure that there is something all the way down inside the extension tube.

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A fine tuning ring is eyepiece tube diameter (IOW == 2").  That just extends the bit of the 50mm extender the goes inside the focuser.  BUT adding to that a parfocalising ring WILL work to make it all go out further.

I'll see how I get on with the mirror pushed up the tube and if that causes a problem I can always do this.

Thanks again to all.

Dave

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