Simple telescope design question

Greetings, I'm new to the group. I am sure my question has a simple
answer, but I have looked on the Web and couldn't find one easily...so
here goes.

The standard Newtonian design has a parabolic primary mirror and a
flat secondary mirror tilted at 45 degrees that obstructs part of the
view. Does anyone make a telescope of the opposite design, with a
large flat steering mirror tilted at 45 degrees that directs light to
an equal aperture parabolic mirror? The focused image would then go
through a hole in the steering mirror to an eyepiece, or maybe to an
erecting lens and then the eyepiece.

The "bad" things about this design would seem to be obvious: a big,
heavy steering mirror to lug around, and a telescope a little longer
than the focal length of the primary mirror. But what about some
advantages?

1. It seems to me that the weight of the steering mirror, being near
the other end of the scope, would essentially act as a
counterbalance weight. I'd rather have a counterbalance weight
that is doing something for me optically than one that doesn't.

2. While the telescope might be long, the 90 degree turn the steering
mirror would provide would make it a lot easier to put the
eyepiece at eye level, irrespective of the length of the
telescope. People use steering mirrors for this purpose with
large binoculars already, don't they, even at the cost of loss of
photons?

3. Since the "obstruction" is a hole in the steering mirror, the
photons that are "lost" could be reclaimed rather easily. A
standard Newtonian would need another small diagonal mirror to
reclaim these photons, which would then need to be collimated with
the telescope axis (as would whatever was used to reclaim them).
In this design, the photons remain on-axis for free.

One could reclaim these photons for your eyes with an on-axis
sighting scope (which would be attached to and turn with the
steering mirror's casing), or digital astrophotography some
enterprising young soul could find a way to add the photons that
get "obstructed" back in to the digital image, e.g. with an
equivalent focal length lens and a second CCD.

4. A Maksutov-Newtonian applies a correction to the Newtonian optics
before the parabolic primary. Perhaps the steering mirror could
serve the same purpose by being slightly curved? It seems to me
that such a mirror would be cheaper to construct than the deep
Maksutov corrector lens. Perhaps you could get Mak-Newtonian
optics in a two-mirror system, rather than a two-mirror, one-lens
system? Or at least the Mak corrector could be placed after the
light passes through the steering mirror, it could be made out of
a *much* smaller piece of glass--like some websites show in the
Klevtsov Cassegrain design.

I am so new to thinking about telescope design, I'm sure there must be
a major problem that I'm missing here. So what am I missing?

Thanks,

Max Robinson
Seattle

The closes thing to your design would be the SCT's, but even they have a
small % of blocked light, BUT it is No Big Deal, when the light falls apon a
normal Newt's mirror, it is reflected back in a CONE, the secondary mirror
reflects that cone upward to the EP which if the scope is set up right is at
the very foucal plane, the small amount of light blocked by the secondary
would not be seen anyway as it would run into the cone of light coming back
from the main mirror and SCT's not only have the secondary, they have a
corrector plate too. A flat mirror reflecting to a good parabolic mirror
would lose a lot of the light as the flat one would not reflect all the
light down to the other mirror.

The closest type of scope I've seen in S&T that did away with the blocked
secondary light is one that has 2 paarabolic mirrors, ground off center and
placed offcenter in the scope. But this design from what I have read, seems
to be limited to small 6inch scopes, I've never heard of a big 12inch or
bigger one being made.

Check out the newsgroup Sci.Astro.Am.

"Robert Maxwell Robinson" wrote in message
ashington.edu...

Greetings, I'm new to the group. I am sure my question has a simple
answer, but I have looked on the Web and couldn't find one easily...so
here goes.

The standard Newtonian design has a parabolic primary mirror and a
flat secondary mirror tilted at 45 degrees that obstructs part of the
view. Does anyone make a telescope of the opposite design, with a
large flat steering mirror tilted at 45 degrees that directs light to
an equal aperture parabolic mirror? The focused image would then go
through a hole in the steering mirror to an eyepiece, or maybe to an
erecting lens and then the eyepiece.

The "bad" things about this design would seem to be obvious: a big,
heavy steering mirror to lug around, and a telescope a little longer
than the focal length of the primary mirror. But what about some
advantages?

1. It seems to me that the weight of the steering mirror, being near
the other end of the scope, would essentially act as a
counterbalance weight. I'd rather have a counterbalance weight
that is doing something for me optically than one that doesn't.

2. While the telescope might be long, the 90 degree turn the steering
mirror would provide would make it a lot easier to put the
eyepiece at eye level, irrespective of the length of the
telescope. People use steering mirrors for this purpose with
large binoculars already, don't they, even at the cost of loss of
photons?

3. Since the "obstruction" is a hole in the steering mirror, the
photons that are "lost" could be reclaimed rather easily. A
standard Newtonian would need another small diagonal mirror to
reclaim these photons, which would then need to be collimated with
the telescope axis (as would whatever was used to reclaim them).
In this design, the photons remain on-axis for free.

One could reclaim these photons for your eyes with an on-axis
sighting scope (which would be attached to and turn with the
steering mirror's casing), or digital astrophotography some
enterprising young soul could find a way to add the photons that
get "obstructed" back in to the digital image, e.g. with an
equivalent focal length lens and a second CCD.

4. A Maksutov-Newtonian applies a correction to the Newtonian optics
before the parabolic primary. Perhaps the steering mirror could
serve the same purpose by being slightly curved? It seems to me
that such a mirror would be cheaper to construct than the deep
Maksutov corrector lens. Perhaps you could get Mak-Newtonian
optics in a two-mirror system, rather than a two-mirror, one-lens
system? Or at least the Mak corrector could be placed after the
light passes through the steering mirror, it could be made out of
a *much* smaller piece of glass--like some websites show in the
Klevtsov Cassegrain design.

I am so new to thinking about telescope design, I'm sure there must be
a major problem that I'm missing here. So what am I missing?

Thanks,

Max Robinson
Seattle

The closes thing to your design would be the SCT's, but even they have a
small % of blocked light, BUT it is No Big Deal, when the light falls apon a
normal Newt's mirror, it is reflected back in a CONE, the secondary mirror
reflects that cone upward to the EP which if the scope is set up right is at
the very foucal plane, the small amount of light blocked by the secondary
would not be seen anyway as it would run into the cone of light coming back
from the main mirror and SCT's not only have the secondary, they have a
corrector plate too. A flat mirror reflecting to a good parabolic mirror
would lose a lot of the light as the flat one would not reflect all the
light down to the other mirror.

The closest type of scope I've seen in S&T that did away with the blocked
secondary light is one that has 2 paarabolic mirrors, ground off center and
placed offcenter in the scope. But this design from what I have read, seems
to be limited to small 6inch scopes, I've never heard of a big 12inch or
bigger one being made.

Check out the newsgroup Sci.Astro.Am.

"Robert Maxwell Robinson" wrote in message
ashington.edu...

Greetings, I'm new to the group. I am sure my question has a simple
answer, but I have looked on the Web and couldn't find one easily...so
here goes.

The standard Newtonian design has a parabolic primary mirror and a
flat secondary mirror tilted at 45 degrees that obstructs part of the
view. Does anyone make a telescope of the opposite design, with a
large flat steering mirror tilted at 45 degrees that directs light to
an equal aperture parabolic mirror? The focused image would then go
through a hole in the steering mirror to an eyepiece, or maybe to an
erecting lens and then the eyepiece.

The "bad" things about this design would seem to be obvious: a big,
heavy steering mirror to lug around, and a telescope a little longer
than the focal length of the primary mirror. But what about some
advantages?

1. It seems to me that the weight of the steering mirror, being near
the other end of the scope, would essentially act as a
counterbalance weight. I'd rather have a counterbalance weight
that is doing something for me optically than one that doesn't.

2. While the telescope might be long, the 90 degree turn the steering
mirror would provide would make it a lot easier to put the
eyepiece at eye level, irrespective of the length of the
telescope. People use steering mirrors for this purpose with
large binoculars already, don't they, even at the cost of loss of
photons?

3. Since the "obstruction" is a hole in the steering mirror, the
photons that are "lost" could be reclaimed rather easily. A
standard Newtonian would need another small diagonal mirror to
reclaim these photons, which would then need to be collimated with
the telescope axis (as would whatever was used to reclaim them).
In this design, the photons remain on-axis for free.

One could reclaim these photons for your eyes with an on-axis
sighting scope (which would be attached to and turn with the
steering mirror's casing), or digital astrophotography some
enterprising young soul could find a way to add the photons that
get "obstructed" back in to the digital image, e.g. with an
equivalent focal length lens and a second CCD.

4. A Maksutov-Newtonian applies a correction to the Newtonian optics
before the parabolic primary. Perhaps the steering mirror could
serve the same purpose by being slightly curved? It seems to me
that such a mirror would be cheaper to construct than the deep
Maksutov corrector lens. Perhaps you could get Mak-Newtonian
optics in a two-mirror system, rather than a two-mirror, one-lens
system? Or at least the Mak corrector could be placed after the
light passes through the steering mirror, it could be made out of
a *much* smaller piece of glass--like some websites show in the
Klevtsov Cassegrain design.

I am so new to thinking about telescope design, I'm sure there must be
a major problem that I'm missing here. So what am I missing?

Thanks,

Max Robinson
Seattle

Does anyone make a telescope of the opposite design, with a
large flat steering mirror tilted at 45 degrees that directs light to
an equal aperture parabolic mirror? The focused image would then go
through a hole in the steering mirror to an eyepiece, or maybe to an
erecting lens and then the eyepiece.

Check out the "Springfield mount" in ATM volume 1. I don't know of a
current manufacturer though.
jc


--
http://mysite.wanadoo-members.co.uk/jc_atm/



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Does anyone make a telescope of the opposite design, with a
large flat steering mirror tilted at 45 degrees that directs light to
an equal aperture parabolic mirror? The focused image would then go
through a hole in the steering mirror to an eyepiece, or maybe to an
erecting lens and then the eyepiece.

Check out the "Springfield mount" in ATM volume 1. I don't know of a
current manufacturer though.
jc


--
http://mysite.wanadoo-members.co.uk/jc_atm/



---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.712 / Virus Database: 468 - Release Date: 27/06/2004