Simple telescope design question

On Mon, 28 Jun 2004 16:29:55 -0700, lal_truckee wrote:

if you are trying to make a flat mirror, there is only one solution,
radius=0, and no easy natural physical processes to get you there.

Sure there is - you get a sphere grinding two surfaces together (works
for any surface, not just glass) and you get a flat grinding three
surfaces together, rotating through the combinations. Only problems is
you need to do 50% more work (three surfaces to shape, not two)

If you want to call that an "easy" process, fine. Next time I need a big flat
ground I'll send the job to you g.

In any case, you still have the fundamental difficulty that there is only one
solution. One of the things that increases the cost of lenses over mirrors is
the usual requirement of reaching a specific radius very precisely- a constraint
not usually placed on mirrors.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Chris L Peterson wrote:

CLIP
if you are trying to make a flat mirror, there is only one solution,
radius=0, and no easy natural physical processes to get you there.

Sure there is - you get a sphere grinding two surfaces together (works
for any surface, not just glass) and you get a flat grinding three
surfaces together, rotating through the combinations. Only problems is
you need to do 50% more work (three surfaces to shape, not two)

And if you set the telescope up with the mirror tracking for a fixed
telescope, the optical flat needs to be much bigger than the aperature
of the telescope, a correspondingly greater task.

On Mon, 28 Jun 2004 15:42:00 -0700, Robert Maxwell Robinson
wrote:


Thanks for your help; I'll look at websites on solar scopes. The idea
that large, flat mirrors are harder to make than large parabolic
mirrors sounds *way* strange to me; I thought you practically started
with the one to make the other!

You start with a flat piece of glass, but not flat in an optical sense (that is,
to a fraction of a wavelength).

When you make a Newtonian primary mirror, you start by making it spherical. That
turns out to be very easy, because when you grind two pieces of glass against
each other that is the natural shape that results. It is a low energy solution.
But you don't worry about the exact degree of sphericity- that would make things
very difficult. You just get close. Nobody usually cares if their mirror that
was intended to have a 2m focal length ends up a few cm on either side of that.
But if you are trying to make a flat mirror, there is only one solution,
radius=0, and no easy natural physical processes to get you there. Optical flats
are much more expensive on an area basis than spherical mirrors. (Converting a
sphere to a paraboloid is a more manual operation, but again, there are an
infinite number of solutions, so the cost isn't very high.)

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Robert Maxwell Robinson wrote:

One comment I was going to make was that I don't think the large
"flat" mirror would need to be nearly as flat as the flat secondary of
a standard Newtonian, since the flat secondary is put at a place where
the image is already highly magnified.

Actually, it's far worse. The secondary in a Newtonian only needs to be
accurate over an area as large as a point in the final image appears on
it. So, for instance, my 2" diagonal on my 10" f/6.5 Dob needs to be 1/10
wave (or whatever you're going for) accurate over each 1.25" area of its
surface.

A pre-primary flat has to be that accurate over the *entire* surface - or
over 10" in my case. And do it after a hole has been put in it, with the
change in stresses that yields. And do it at the front, unprotected from
dew and thermal changes.

It does eliminate spider (secondary mount) diffraction though.

Best to you!

Jim Horn

Thanks, James! This puts the nail in the coffin.

To sum up the responses I've received, a pre-primary flat surface
isn't a good replacement for a small flat secondary. The flat would
need to be bigger than the primary (the problem I already knew about).
It would need to be quite flat over its entire surface, and there is
no known process for grinding something optically flat that approaches
the ease with which a parabolic mirror can be ground; the best idea
involves grinding three blanks against each other, which takes half
again as much work.

All of this leaves the idea of using a curved mirror instead of a flat
mirror, and that puts the question firmly into a different category.
If I keep going on this idea, I am certain I'll end up reinventing the
classical Cassegrain design, or something else that was discarded in
favor of the classical Cassegrain.

I was sure there was a good reason I haven't seen that design; turns
out there is.

--Max

On Tue, 29 Jun 2004, James Horn wrote:

|Robert Maxwell Robinson wrote:
|
| One comment I was going to make was that I don't think the large
| "flat" mirror would need to be nearly as flat as the flat secondary of
| a standard Newtonian, since the flat secondary is put at a place where
| the image is already highly magnified.
|
|Actually, it's far worse. The secondary in a Newtonian only needs to be
|accurate over an area as large as a point in the final image appears on
|it. So, for instance, my 2" diagonal on my 10" f/6.5 Dob needs to be 1/10
|wave (or whatever you're going for) accurate over each 1.25" area of its
|surface.
|
|A pre-primary flat has to be that accurate over the *entire* surface - or
|over 10" in my case. And do it after a hole has been put in it, with the
|change in stresses that yields. And do it at the front, unprotected from
|dew and thermal changes.
|
|It does eliminate spider (secondary mount) diffraction though.
|
|Best to you!
|
|Jim Horn
|

Your idea has been used to build binocular mounts. The first surface
mirrors found in photocopy machines are sufficiently flat for low power
viewing.

http://www.backyard-astro.com/equipment/skywindow.html




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

Thanks, James! This puts the nail in the coffin.

To sum up the responses I've received, a pre-primary flat surface
isn't a good replacement for a small flat secondary. The flat would
need to be bigger than the primary (the problem I already knew about).
It would need to be quite flat over its entire surface, and there is
no known process for grinding something optically flat that approaches
the ease with which a parabolic mirror can be ground; the best idea
involves grinding three blanks against each other, which takes half
again as much work.

All of this leaves the idea of using a curved mirror instead of a flat
mirror, and that puts the question firmly into a different category.
If I keep going on this idea, I am certain I'll end up reinventing the
classical Cassegrain design, or something else that was discarded in
favor of the classical Cassegrain.

I was sure there was a good reason I haven't seen that design; turns
out there is.

--Max

On Tue, 29 Jun 2004, James Horn wrote:

|Robert Maxwell Robinson wrote:
|
| One comment I was going to make was that I don't think the large
| "flat" mirror would need to be nearly as flat as the flat secondary of
| a standard Newtonian, since the flat secondary is put at a place where
| the image is already highly magnified.
|
|Actually, it's far worse. The secondary in a Newtonian only needs to be
|accurate over an area as large as a point in the final image appears on
|it. So, for instance, my 2" diagonal on my 10" f/6.5 Dob needs to be
1/10
|wave (or whatever you're going for) accurate over each 1.25" area of its
|surface.
|
|A pre-primary flat has to be that accurate over the *entire* surface - or
|over 10" in my case. And do it after a hole has been put in it, with the
|change in stresses that yields. And do it at the front, unprotected from
|dew and thermal changes.
|
|It does eliminate spider (secondary mount) diffraction though.
|
|Best to you!
|
|Jim Horn
|

Your idea has been used to build binocular mounts. The first surface
mirrors found in photocopy machines are sufficiently flat for low power
viewing.

http://www.backyard-astro.com/equipment/skywindow.html




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

Thanks, James! This puts the nail in the coffin.

To sum up the responses I've received, a pre-primary flat surface
isn't a good replacement for a small flat secondary. The flat would
need to be bigger than the primary (the problem I already knew about).
It would need to be quite flat over its entire surface, and there is
no known process for grinding something optically flat that approaches
the ease with which a parabolic mirror can be ground; the best idea
involves grinding three blanks against each other, which takes half
again as much work.

All of this leaves the idea of using a curved mirror instead of a flat
mirror, and that puts the question firmly into a different category.
If I keep going on this idea, I am certain I'll end up reinventing the
classical Cassegrain design, or something else that was discarded in
favor of the classical Cassegrain.

I was sure there was a good reason I haven't seen that design; turns
out there is.

--Max

On Tue, 29 Jun 2004, James Horn wrote:

|Robert Maxwell Robinson wrote:
|
| One comment I was going to make was that I don't think the large
| "flat" mirror would need to be nearly as flat as the flat secondary of
| a standard Newtonian, since the flat secondary is put at a place where
| the image is already highly magnified.
|
|Actually, it's far worse. The secondary in a Newtonian only needs to be
|accurate over an area as large as a point in the final image appears on
|it. So, for instance, my 2" diagonal on my 10" f/6.5 Dob needs to be
1/10
|wave (or whatever you're going for) accurate over each 1.25" area of its
|surface.
|
|A pre-primary flat has to be that accurate over the *entire* surface - or
|over 10" in my case. And do it after a hole has been put in it, with the
|change in stresses that yields. And do it at the front, unprotected from
|dew and thermal changes.
|
|It does eliminate spider (secondary mount) diffraction though.
|
|Best to you!
|
|Jim Horn
|

Robert Maxwell Robinson wrote:

I was sure there was a good reason I haven't seen that design; turns
out there is.

Actually, similar designs are fairly common, for specialized purposes:
These types are called "Siderostats" or "Ceolostats" if the employ two
mirrors.(And "Heliostats" when used for solar observations - the more
common usage for design.)

http://mthamilton.ucolick.org/techdocs/telescopes/CAT/CAT_descr.html
provides a diagram of the Sidereostat on Mt Hamilton.

Here's a link to photos of the Lick instrument:
http://www.haruko.ca/pictures/Lick%20Observatory/005_4A.jpg/

Here's a historic photo of Draper viewing through his Sidereostat:
http://buhlplanetarium3.tripod.com/Siderostat-Draper2.JPG

Ceolostats use two mirrors - here are some designs:
http://spectrohelioscope.net/ceolostats.htm

Robert Maxwell Robinson wrote:

I was sure there was a good reason I haven't seen that design; turns
out there is.

Actually, similar designs are fairly common, for specialized purposes:
These types are called "Siderostats" or "Ceolostats" if the employ two
mirrors.(And "Heliostats" when used for solar observations - the more
common usage for design.)

http://mthamilton.ucolick.org/techdocs/telescopes/CAT/CAT_descr.html
provides a diagram of the Sidereostat on Mt Hamilton.

Here's a link to photos of the Lick instrument:
http://www.haruko.ca/pictures/Lick%20Observatory/005_4A.jpg/

Here's a historic photo of Draper viewing through his Sidereostat:
http://buhlplanetarium3.tripod.com/Siderostat-Draper2.JPG

Ceolostats use two mirrors - here are some designs:
http://spectrohelioscope.net/ceolostats.htm

On Mon, 28 Jun 2004 20:07:17 -0700, Robert Maxwell Robinson
wrote:


Thanks, James! This puts the nail in the coffin.

To sum up the responses I've received, a pre-primary flat surface
isn't a good replacement for a small flat secondary. The flat would
need to be bigger than the primary (the problem I already knew about).
It would need to be quite flat over its entire surface, and there is
no known process for grinding something optically flat that approaches
the ease with which a parabolic mirror can be ground; the best idea
involves grinding three blanks against each other, which takes half
again as much work.

All of this leaves the idea of using a curved mirror instead of a flat
mirror, and that puts the question firmly into a different category.
If I keep going on this idea, I am certain I'll end up reinventing the
classical Cassegrain design, or something else that was discarded in
favor of the classical Cassegrain.

There are things called Tilted Component Telescopes (TCT), aka
Trischifspieger, Yolo and so on that have three curved, tilted
surfaces where the abberations produced by the tilts cancel out. A
Google search on some of these terms will come up with some really
strange boxes containing scopes. They avoid the problems with the
spider and central obstruction but at a cost in size and complexity.

Chris