Re: ATM 18 inch telescope

[Jerry Hudson <hudsonjk@pacbell.net>]

Hi -

This is I hope a satisfactory reply to Amelia Thomas' note:

<<
I haven't seen any comments on this email so I'd like to ask, besides a
larger central obstruction and more difficulty collimating are there any
drawbacks to this "folded telescope" design. How difficult would collimating
be? Are there any websites dedicated to a scope built this way? Again sorry
for the repeated message. If I accidentally hit enter it sends the email.
This feature is very annoying to us all. Amelia
>>

1.  If you are to fold the telescope to bring the focus back
behind or near the primary mirror, then the obstruction reaches
50%.  Light blockage isn't really the problem with this, it's the
nasty diffraction image* that results: Slightly less than 1/2 of the
light goes into the central Airy disk of a star, 1/3 into the
first bright ring, and the remainder distributed among the rest
of the diffraction image.  Very poor for looking at planets, but
probably acceptable for photography of nebulae and the like.

2.  You can cut back on the blockage by moving the flat mirror
upward towards the focus, letting it thereby be smaller.  Then,
you need to bring the light out to the side using a tertiary
(but small) flat mirror.  I guess the original suggestion was
to tilt the first flat, but a second one is needed for observing
comfort.  The collimation difficulties associated
with 3 mirrors can be a problem, particularly if this telescope
is to be portable (and be driven over back roads).

3.  Even if you reduce the blockage from 50% diameter to, say,
30% (thereby only reducing the tube length by 30% or so), the
diffraction image isn't very wonderful.  About 68% of the light
goes into the central disk and 22% into the first bright ring.
That's also unacceptable for planetary viewing, but probably OK
for low power viewing and wide-field photography.

4.  The flat folding mirror, if not tilted, doesn't have to be
accurately flat; it could have a few fringes convexity or concavity,
and be otherwise a good sphere.  This could be made easily by
amateurs, and tested by placing at a 45% angle to a good sphere.
Bring the astigmatism down to maybe 0.1 inch and otherwise shoot for
a sharp null.  A flat mirror 30% of 18 inches is pretty expensive,
so I suggest it be home-made.

5.  Baffling, Michael Lindner suggests, could be a problem if
the first mirror is tilted, and the eyepiece made to look at
sky.  This is similar to the baffling needed for a Cassegrain,
and would require a long tube extending inward from the eyepiece.
And this tube would cut into the light path badly (causing all
sorts of mayhem to the diffraction image) for a tilted folding
mirror.  If the first mirror isn't tilted, then the problem is
similar to Cassegrain systems with a "Nasmyth" tertiary focus,
and solvable using a suitably long baffling tube.

So... I conclude you are better off with a sturdy ladder up to
the Newtonian focus!

- Jerry Hudson

*See Texereau, _How to Make a Telescope_, Wilmann-Bell, 1984,
for a discussion of the effects of obstruction on the diffraction
image (Ch. 6.)