Expert: Tom Whiting - 3/24/2006

Question
How does a SNT work? What does it consist of?. george phelps

Answer
Hi George,
Boy, I had to look that one up on Google...I thought initially
you meant SNR (Supernova Remnants), but then I saw your
title.  {I've always heard them called Catadioptric Newts}.

But no matter, to your question....it appears from the diagram
I see on
http://www.skywatchertelescope.net/EducationTBOD.html
and halfway down that page,

that all they are doing is taking a standard short focal length
Newtonian, and adding a 2X Barlow lens, fixed, in the bottom
of the tube of the eyepiece holder, effectively doubling the
focal length without the inconvenience of a longer length
tube to carry around.  A neat idea, BUT.......

Personally, I wouldn't want one...why not just purchase
a standard short focal length Newt, and your own  2X
barlow lens, and you'd get the same thing...a doubling of
the focal length, without the inconvenience of a factory
aligned fixed lens in the bottom of your eyehole tube.

The problem I have with the fixed- in- position Barlow lens is...what if that scope gets bumped, (out of collimation)?
That always happens when you travel to our observing
sites for darker skies.   then you'd have to send the scope back to the manufacturer for re-collimating that one lens....which is ridiculous.

We know all scopes eventually, with traveling and bumping,
eventually need some touching up of the collimation, so
I would never want a scope that you can't fully collimate
yourself.  And that fixed lens in the catadioptric-Newts.
would be a real problem using self-collimation.  So that's why
I would not buy one.  You can achieve the very same thing
as I suggest above, AND still be able to collimate your
scope without the worry of an in-position fixed lens.

In fact, I double check my collimation
using a laser collimator on EVERY setup, and even again
halfway through the observing session as even slight temperature changes can affect your perfect collimation.

Then there's the problem of cleaning that in-position fixed
lens (all optics need cleaning occasionally, perhaps once
per year).....You want to take the chance of reaching in there
and cleaning that lens without changing it's tilt in it mount?
And if it gets just even slightly tilted, you are....well, politely,
speaking, out of luck.  There's two good reasons why I would
not purchase one.
Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA