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Originally Posted by xrekcor
Ok, I have a 8" f/6 newt. I can make an off axis aperture mask of 80mm
and still clear any obstructions (vanes and secondary). This I understand
now turns my 8" f/6 newt into and un-obstructed 80mm f/15 reflector, right?
Would this now perform like or similar to a 80mm f/15 refractor now that
the central obstruction is gone?
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Rob,
Yes you are basically turning your scope into an 80mm/F15 APO with a couple of differences. Being a reflective optic no fasle colour at all, the downside is about 80% light transmission compared to high 90's for a refractor. so maybe a 75mm APO? Can't be bothered doing the numbers.
I occasionally use a 93mm off axis mask on my own 10"/F5, which turns it into a 93mm/F13.4 refractor. Invariably full aperture is a better way to go because in the fleeting moments of good seeing full aperture shows a lot more detail and provides a lot brighter image. However the off -axis mask does make the views "refractor like" in that the stars become tight pinpoints and the effects of diffraction are eliminated, although lunar/planetary images are dimmed considerably. Andrew Murrell occasionally uses a 7" off axis mask on Hector (20" dob) for planetary observation as the larger aperture is sometimes severely affected by poor seeing yet 7" is still plenty of aperture to extract good detail
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Originally Posted by xrekcor
Also why cant you use a focal reducer on a newt like you can on a SCT.
Apart from the corrector plate and the fact a SCT fires the photons back
though the centre of it's primary as appose to a newt who fires it sideways.
There isn't a big difference in the basic concept of each. Can you use
a FR on a Mak-Newt? is there any kind of focal reduction you can do on
a newt?
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Optically there is no reason you can't use a focal reducer on a Newt or Mak Newt, if you can get it to reach focus.
A SCT/MCT focuses by moving the primary mirror axially along the optical tube, therefore it has an almost infinite amount of focuser travel in comparison to a Newt or a Mak Newt, or even a refractor for that matter.
CS-John B