Off Axis Maksutov

Hi,
Has anyone attempted to make an off-axis Mak. All the elements are
spherical, which should make such a scope much easier to construct
than an off-axis paraboloid. Combining a long f/ratio Mak set in a
manner where the corrector are at angle which would simulate the side
of a larger aperture Mak might do it.
Just imagine a Mak with a sub-aperture mask.

I would imagine that a MakNewt setup may be easier with secondary
reflecting the image sideways. The corrector and primary cells would
be more difficult and require very precise alignment.

Al M

The corrector will be a lens with a wedge in it which may make things a bit
more difficult.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works evevery time it is tried!

Yes, D.M. Maksutov sketched such a system in one of his notebooks,
calling it the Meniscus Herschelian.
And a post to the ATM list from 2000 discusses it:


15 Feb 2000 From: Roger Ceragioli
Subject: ATM Off-Axis Maksutov: perfect planetary scope?
I was doing some reading tonight and discovered a design that I had
never seen before, or understood: the off-axis Maksutov-Newtonian.
Invented by Franklin Wright and discussed in ATM III, Rutten and van
Venrooij mention it on p. 140 of their book.
The design consists of a tilted Maksutov corrector and a tilted
spherical mirror. The corrector can be thought of as an off-axis
segment of a larger full-sized ordinary corrector plate and the mirror
as the corresponding off-axis segment of a larger ordinary primary
Maksutov mirror.
The surfaces are all spherical, and the only somewhat peculiar feature
of the optics is that the corrector must be thicker on one side than the
other. In other words, the corrector is a wedged lens. Contrary to
what R. and vV. say, it should not be difficult to make such a lens.
Lenses normally all start out wedged, i.e. with their two curves
decentered relatively, and you have to carefully dewedge them. In this
case, you do the opposite: carefully maintain their wedge.
The performance of such a system could be very good: essentially zero
chromatic aberration, zero spherical, coma small, unabstructed system,
closed tube, all spherical optics, moderately long focal ratios.
But then I realized (maybe someone else has realized the same?) that you
could build an unabstructed Maksutov-Cassegrain. If the secondary
mirror were built as a separate unit and not made as an aluminized spot
on the back of the primary, you could correct coma too (like the Rumak).
It should be possible to build something like a 10" unobstructed
version.
Here is a sample idea: imagine a 28" f/2 spherical mirror aimed at an
8.25" convex Cassegrain secondary. The focal length of the primary is
56". Using ordinary Cassegrain formulas, you can add 10" back focus
giving 66". Divide this by 4 giving 14, which you subtract from 56,
giving 42. 42" is the separation of the primary from the secondary
mirror. Now imagine taking a 10" off-axis unobstructed segment of the
primary mirror and corrector plate. The image of this segment will
cover a circular area of the secondary mirror that is (10"/28")x8.25" in
diameter: 2.95".
Thus the off-axis primary will be 10" dia. and spherical. Focal length
equals 56", for f/ratio of 5.6. Very easy to make. Back focus is 42 +
10" = 52". Secondary mirror is spherical and 2.95" diameter. Effective
f/ratio is 17.6. ROC of seconary is 49.5"; f/ratio is 8.4. Again, very
easy to make.
In other words, you could make an unobstructed, apochromatic, aplanatic
(or nearly) reflector of 10" aperture, f/17.6, and thus still fast
enough to encompass of field of view slightly larger than 1/2 degree
with a 55mm Plossl, in a tube of only about 4ft. long. The tube will be
closed, and of a relatively small cross section (15"x12" or so). The
surfaces will be all spherical, relatively easy to test, and the field
will not be tilted or show image anamorphism (since the theoretical
parent optics are just an ordinary 28" f/6.3 Maksutov).
Roger Ceragioli



(Al M) wrote in news:bc2d008a.0311170715.4a3f9245
@posting.google.com:

Hi,
Has anyone attempted to make an off-axis Mak. All the elements are
spherical, which should make such a scope much easier to construct
than an off-axis paraboloid. Combining a long f/ratio Mak set in a
manner where the corrector are at angle which would simulate the side
of a larger aperture Mak might do it.
Just imagine a Mak with a sub-aperture mask.

I would imagine that a MakNewt setup may be easier with secondary
reflecting the image sideways. The corrector and primary cells would
be more difficult and require very precise alignment.

Al M

"Al M" wrote in message
om...
Hi,
Has anyone attempted to make an off-axis Mak. All the elements are
spherical, which should make such a scope much easier to construct
than an off-axis paraboloid. Combining a long f/ratio Mak set in a
manner where the corrector are at angle which would simulate the side
of a larger aperture Mak might do it.
Just imagine a Mak with a sub-aperture mask.
[SNIP]

Al,

See "The Maksutov Lens Applied to Herschelian and Newtonian Telescopes," by
Franklin B. Wright, in Amateur Telescope Making, Book Three. For one that
was actually made, see Sky & Telescope, January, 1972, pages 47 - 52.

Clear skies, Alan

Thanks Guys!
Now why hasn't any manufacturer made one of these scopes?

Al M


"Alan French" wrote in message ...
"Al M" wrote in message
om...
Hi,
Has anyone attempted to make an off-axis Mak. All the elements are
spherical, which should make such a scope much easier to construct
than an off-axis paraboloid. Combining a long f/ratio Mak set in a
manner where the corrector are at angle which would simulate the side
of a larger aperture Mak might do it.
Just imagine a Mak with a sub-aperture mask.
[SNIP]

Al,

See "The Maksutov Lens Applied to Herschelian and Newtonian Telescopes," by
Franklin B. Wright, in Amateur Telescope Making, Book Three. For one that
was actually made, see Sky & Telescope, January, 1972, pages 47 - 52.

Clear skies, Alan

Thanks Guys!
Now why hasn't any manufacturer made one of these scopes?

Why bother. A full sized Mak with small central obstruction will have higher
light grasp and better resolution. It will fit into a similar sized tube.

Roland Christen