Simple telescope design question

Robert Maxwell Robinson wrote:

Hi, I'm new to the group. I have been learning about telescope
designs for a month or so, and have a question that I haven't been
able to find the answer to; I thought one of you might like to answer
it.

Would it be a Bad Idea to reverse the order of the two mirrors? The
flat elliptical mirror would have to grow to have the same diameter
(along it's _shorter_ axis) as the parabolic mirror, and would be
similar in position to what I think is called a Steering Mirror.
Light would hit the steering mirror, then the parabolic mirror, then
pass through a hole in the steering mirror and go directly into the
eyepiece, like this (only longer):

pppp......................S
ppp ... S
pp ... S
pp ... S
p ...
p.....................|= Eyepiece
p ...
pp ... S
pp ... S
ppp ... S
pppp...........S

I can't believe noone has considered this simple variant on a
Newtonian before; so does anyone know the name of this design? Also
I've never heard of one being constructed, so there must be some
significant problem with it. Can anyone tell me what it is?

You have described (very well) the telescope Charles Fundingsland
invented, built and patented in the 90's, called the "Fundyscope". Mr.
Fundingsland published his 6" aperture design in S&T, but I don't recall
the year and month - maybe someone can look that up. The George B. Wren
II Supernova Search Telescope (SNST) at McDonald Observatory is the
largest Fundyscope in the world, with a Galaxy Optics 18" f/4.5 primary
mirror and 24.25" diameter steering flat made by Mike Marcario at High
Lonesome Optics. I derived the tracking equations and algorithms for
SNST, and Wayne Rosing (also a VP at Google) implemented the tracking
equations and made the thing work, and it worked very well. Bill Wren
used it to discover several supernova. See
http://hej3.as.utexas.edu/~www/SN/.

The Fundyscope steering flat has to be VERY flat to prevent image
astigmatism, on the order of 1/20 wave peak-to-valley, and must be
supported by an edge/back flotation system that can maintain that flatness
over the full angular pointing range. Achieving 1/20 wave P-V precision
on a 24" flat right up to the edge requires a truly skilled optician such
as Mike Marcario. The hole in the flat must also be a tapered 45º cone to
prevent vignetting the field at maximum mirror tilt. Making the steering
flat is the main drawback to Fundyscopes.
Mike

Robert Maxwell Robinson wrote in message . washington.edu...
Hi, I'm new to the group. I have been learning about telescope
designs for a month or so, and have a question that I haven't been
able to find the answer to; I thought one of you might like to answer
it.

My question is about a variant of a Newtonian reflector. A Newtonian
reflector has a parabolic primary and a flat secondary that is placed
on the optical axis some distance shy of the focal point. The light
reflected off the secondary goes to the eyepiece.

Would it be a Bad Idea to reverse the order of the two mirrors? The
flat elliptical mirror would have to grow to have the same diameter
(along it's _shorter_ axis) as the parabolic mirror, and would be
similar in position to what I think is called a Steering Mirror.
Light would hit the steering mirror, then the parabolic mirror, then
pass through a hole in the steering mirror and go directly into the
eyepiece, like this (only longer):

There is a very good example of this idea at Stellafane: The Porter Turret
Telescope:
http://www.stellafane.com/public_rel...elations1.html
(and subsequent web pages)
Nothing wrong with the idea at all, except for the difficulty of making
a big flat or the expense of buying one. Porter was willing to spend
the effort because he was able to achieve a comfortable, enclosed,
relatively fixed observing position with the design. Think of it, you walk
into a cozy little building a bit bigger than an outhouse, close the door
behind you and look out through the wall of the turret through the eyepiece
and you can see the entire sky out of the wind and the 10 below Vermont
winter weather. Definitely worth the added effort and expense. But
if what you want is a portable telescope or one for use in a conventional
observatory, forget it. The telescope just gets bigger and heavier and
more expensive.
Clif Ashcraft

Robert Maxwell Robinson wrote in message . washington.edu...
Hi, I'm new to the group. I have been learning about telescope
designs for a month or so, and have a question that I haven't been
able to find the answer to; I thought one of you might like to answer
it.

My question is about a variant of a Newtonian reflector. A Newtonian
reflector has a parabolic primary and a flat secondary that is placed
on the optical axis some distance shy of the focal point. The light
reflected off the secondary goes to the eyepiece.

Would it be a Bad Idea to reverse the order of the two mirrors? The
flat elliptical mirror would have to grow to have the same diameter
(along it's _shorter_ axis) as the parabolic mirror, and would be
similar in position to what I think is called a Steering Mirror.
Light would hit the steering mirror, then the parabolic mirror, then
pass through a hole in the steering mirror and go directly into the
eyepiece, like this (only longer):

There is a very good example of this idea at Stellafane: The Porter Turret
Telescope:
http://www.stellafane.com/public_rel...elations1.html
(and subsequent web pages)
Nothing wrong with the idea at all, except for the difficulty of making
a big flat or the expense of buying one. Porter was willing to spend
the effort because he was able to achieve a comfortable, enclosed,
relatively fixed observing position with the design. Think of it, you walk
into a cozy little building a bit bigger than an outhouse, close the door
behind you and look out through the wall of the turret through the eyepiece
and you can see the entire sky out of the wind and the 10 below Vermont
winter weather. Definitely worth the added effort and expense. But
if what you want is a portable telescope or one for use in a conventional
observatory, forget it. The telescope just gets bigger and heavier and
more expensive.
Clif Ashcraft

Robert Maxwell Robinson wrote in message . washington.edu...

pppp......................S
ppp ... S
pp ... S
pp ... S
p ...
p.....................|= Eyepiece
p ...
pp ... S
pp ... S
ppp ... S
pppp...........S


Three issues: 1) the primary mirror is in the worst position while
observing zenith (potato chipping=astig) 2) the flat might cost
more than the primary by around 2X rather than the 10% of the cost
of the primary for a regular NEWT, 3) when it comes time to recoat
the mirrors, the cost doubles again.

Robert Maxwell Robinson wrote in message . washington.edu...

pppp......................S
ppp ... S
pp ... S
pp ... S
p ...
p.....................|= Eyepiece
p ...
pp ... S
pp ... S
ppp ... S
pppp...........S


Three issues: 1) the primary mirror is in the worst position while
observing zenith (potato chipping=astig) 2) the flat might cost
more than the primary by around 2X rather than the 10% of the cost
of the primary for a regular NEWT, 3) when it comes time to recoat
the mirrors, the cost doubles again.

Robert Maxwell Robinson wrote in message . washington.edu...


My question is about a variant of a Newtonian reflector. A Newtonian
reflector has a parabolic primary and a flat secondary that is placed
on the optical axis some distance shy of the focal point. The light
reflected off the secondary goes to the eyepiece.

Would it be a Bad Idea to reverse the order of the two mirrors? The
flat elliptical mirror would have to grow to have the same diameter
(along it's _shorter_ axis) as the parabolic mirror, and would be
similar in position to what I think is called a Steering Mirror.
Light would hit the steering mirror, then the parabolic mirror, then
pass through a hole in the steering mirror and go directly into the
eyepiece, like this (only longer):

More viable arrangement is one with the first surface as a round flat,
tilted at a small angle (less than 10 degrees) reflecting incoming
light to a concave mirror, which focuses through a hole in the tilted
flat. Such arrangement has refractor-like configuration, in having the
eyepiece placed at the tube end opposite to the sky. It is also
spider-free, although requires additional small diagonal for
comfortable observing. Good part is that flat doesn't have to be
really flat - a very mild sphere would be much easier to make, and
would induce entirely negligible amount of astigmatism. Collimation is
nearly as simple as in the arrangement with large diagonal, because it
can be done without the small diagonal at the eyepiecen location. In
other words, only the primary needs to be adjusted (of course, star
diagonal itself needs to be well collimeted).

Vlad

Robert Maxwell Robinson wrote in message . washington.edu...


My question is about a variant of a Newtonian reflector. A Newtonian
reflector has a parabolic primary and a flat secondary that is placed
on the optical axis some distance shy of the focal point. The light
reflected off the secondary goes to the eyepiece.

Would it be a Bad Idea to reverse the order of the two mirrors? The
flat elliptical mirror would have to grow to have the same diameter
(along it's _shorter_ axis) as the parabolic mirror, and would be
similar in position to what I think is called a Steering Mirror.
Light would hit the steering mirror, then the parabolic mirror, then
pass through a hole in the steering mirror and go directly into the
eyepiece, like this (only longer):

More viable arrangement is one with the first surface as a round flat,
tilted at a small angle (less than 10 degrees) reflecting incoming
light to a concave mirror, which focuses through a hole in the tilted
flat. Such arrangement has refractor-like configuration, in having the
eyepiece placed at the tube end opposite to the sky. It is also
spider-free, although requires additional small diagonal for
comfortable observing. Good part is that flat doesn't have to be
really flat - a very mild sphere would be much easier to make, and
would induce entirely negligible amount of astigmatism. Collimation is
nearly as simple as in the arrangement with large diagonal, because it
can be done without the small diagonal at the eyepiecen location. In
other words, only the primary needs to be adjusted (of course, star
diagonal itself needs to be well collimeted).

Vlad