Re: [ATM] Why and how theMersenne-Schmidt-Paul-Baker-Willstrop-Stevick telescope works

[Hugues.Laroche@ses-astra.com]

Hello Mike,

You said:
>>>>>>>>>>>>>>>>>>>>>>>>>
It's residual 5th
order aberrations that account for the slight departures from theoretical
perfection in the ray-traced designs.
There's a good explanation by Willstrop of the optical principles of the
design here: <http://www.ast.cam.ac.uk/history/mmmt/>. Basically, the
primary and secondary are confocal, which makes the combination an afocal
beam compressor (i.e. Mersenne telescope). The tertiary is separated from
the secondary by an amount equal to it's radius of curvature. In effect it
is seeing an aperture stop at it's center of curvature, which is the basis
for a Schmidt camera. The primary and secondary together have an amount of
spherical aberration exactly equal (to 4th order) to a Schmidt corrector,
so combined with the spherical tertiary you get an all reflective Schmidt
telescope.
<<<<<<<<<<<<<<<<<<<<<<<<<

You are totally right that it is a very interesting design. I already
considered it carefully. It is doable by an ATM since there is a large
parabolic primary, and two smaller spherical mirrors. However I found
the 3 miror version is unfortunately not very practical in that you have
to insert your viewing device (argentic or large CCD array, not even
speaking
of an eyepiece or diagonal) at half-hight on the optical axis of the
telescope and cause a big obstruction, especially regarding the beam
coming back from the secondary to the tertiary. Which
would point that the design is preferable for large apertures.

Stevick-Paul variant cleverly avoids this.

More classically, a folded gregorian idea gave me a clue to improve
the practicality of the design at the price of more reflective
surfaces.
1) you mount your primary as a newtonian.
2) you insert a tiny tertiary flat near the focal plane
   of the primary system, sending the light back to the sky.
3) you add the system described in the original design
   PARALLELY to the primary light path, the
   secondary becoming a quaternary and so on.

The practicality is improved since now the system focus
is accessible with a relatively small obstruction. In total
I would bet that the obstruction would be hardly more than
the corresponding newt.

OK, you have two flat mirrors in addition to the original system.
BUT: you can baffle the light path extremely well (much better
than the original design, actually) beyond the
first flat mirror (from the tertiary, the whole light path can
fit in a nearly closed tube, of small diameter w.r.t. the
primary diameter.

The general "look" of the scope would be that of a newtonian with
a long, lean external "socks" parallel to its main axis,
all that could even be mounted as a dobsonian. When
do we start?

Hugues




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