Optics question

I'm reading "Telescope Optics" by Rutten & van Venrooij and have
some questions.

First the observation that the authors appear wildly inconsistent
about what they think their target audience knows. I get the impression
the book most often assumes a knowledge of optics in general and
its goal is to flesh out telescope optics in particular with perhaps a brief
review.

In 4.2.2 they discuss coma and attribute it to "the intersection of
rays not being symmetrical." Shouldn't "off axis light" come into the
telescope in a symmetrical fashion when confronting an evenly distributed
light source? If one were to rotate the lens or mirror, would the coma
rotate as well?

WRT diffraction spikes, if the secondary mirror and struts are not in the
plane of focus, why would the diffraction effect occur where the mirror
itself is not visible? And if it's an "edge effect," way doesn't the edge of
the telescope tube diffract the light as well in a way that is visible?

Also, (and this may sound silly to those more knowledgeable than I)
isn't there a compound glass that has the property of being neutral to light
in one direction and reflect it 90 degrees from the other? It would be like
a two-way mirror with the mirror embedded into the prism at an angle.
It's such a simple solution to having an obstruction that this type of
prism must present insurmountable problems.

--
Craig Franck

Cortland, NY

On Thu, 17 Jun 2004 20:09:29 GMT, "Craig Franck"
wrote:

In 4.2.2 they discuss coma and attribute it to "the intersection of
rays not being symmetrical." Shouldn't "off axis light" come into the
telescope in a symmetrical fashion when confronting an evenly distributed
light source? If one were to rotate the lens or mirror, would the coma
rotate as well?

In most cases, the optics in a telescope are rotationally symmetric. That means
that rotating elements does not affect the image. The direction of the coma is a
function of the position of the off-axis source.


WRT diffraction spikes, if the secondary mirror and struts are not in the
plane of focus, why would the diffraction effect occur where the mirror
itself is not visible? And if it's an "edge effect," way doesn't the edge of
the telescope tube diffract the light as well in a way that is visible?

It does. In a Newtonian telescope, there is diffraction from three sources: the
vanes or stalk, the central mirror, and the outside of the aperture. The latter
two are visible in the form of diffraction rings around a point source- what is
commonly referred to at the Airy disk. They are perfectly visible, but may go
largely unnoticed because they are rotationally symmetric with the star. You see
them in an unobstructed telescope from the aperture alone. The diffraction
spikes from the stalk or spider are much more obvious because they are oriented.


Also, (and this may sound silly to those more knowledgeable than I)
isn't there a compound glass that has the property of being neutral to light
in one direction and reflect it 90 degrees from the other? It would be like
a two-way mirror with the mirror embedded into the prism at an angle.
It's such a simple solution to having an obstruction that this type of
prism must present insurmountable problems.

I'm not sure what you are thinking of here- a material that passes light in one
direction but reflects it in the other? It is worthwhile to remember that
outside the quantum domain, optical materials are very symmetric. That means
that you can trace a ray through a system in either direction and the results
will be the same. Another way of thinking of this is that if you look at a ray
trace, there is no way of determining which way the light was traveling.

In reality, the "problem" of an obstruction is greatly overstated. In most
cases, the effects of an obstruction are nearly impossible to detect. The few
cases where an unobstructed design provides better results correspond rather
nicely to cases where size of aperture isn't too important, and unobstructed
designs are practical.

_________________________________________________

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

"Chris L Peterson" wrote

"Craig Franck" wrote:

In 4.2.2 they discuss coma and attribute it to "the intersection of
rays not being symmetrical." Shouldn't "off axis light" come into the
telescope in a symmetrical fashion when confronting an evenly distributed
light source? If one were to rotate the lens or mirror, would the coma
rotate as well?

In most cases, the optics in a telescope are rotationally symmetric. That means
that rotating elements does not affect the image. The direction of the coma is a
function of the position of the off-axis source.

I understand much of the light is "oblique" as the book states; you have
the entire lens or much of the mirror gathering light. But I don't grasp why
it's not a symmetrical blob instead of a tail along one axis. If the lens if free
from defect, and the star is a symmetrical point, what causes the rays to
bunch up on one side of one axis?

Also, (and this may sound silly to those more knowledgeable than I)
isn't there a compound glass that has the property of being neutral to light
in one direction and reflect it 90 degrees from the other? It would be like
a two-way mirror with the mirror embedded into the prism at an angle.
It's such a simple solution to having an obstruction that this type of
prism must present insurmountable problems.

I'm not sure what you are thinking of here- a material that passes light in one
direction but reflects it in the other? It is worthwhile to remember that
outside the quantum domain, optical materials are very symmetric. That means
that you can trace a ray through a system in either direction and the results
will be the same. Another way of thinking of this is that if you look at a ray
trace, there is no way of determining which way the light was traveling.

What about two-way mirrors? One side sees a window, the other a mirror.
I thought there was a large class of objects that what happens to the light
depends strongly on the angle of the ray. It could be an optical diode: light
passes one way freely but gets reflected back when coming from the
opposite direction.

--
Craig Franck

Cortland, NY

On Thu, 17 Jun 2004 23:54:00 GMT, "Craig Franck"
wrote:

I understand much of the light is "oblique" as the book states; you have
the entire lens or much of the mirror gathering light. But I don't grasp why
it's not a symmetrical blob instead of a tail along one axis. If the lens if free
from defect, and the star is a symmetrical point, what causes the rays to
bunch up on one side of one axis?

I'm not sure how to explain that clearly in words. To me the simple ray trace
and spot diagrams on page 28 of R & vV are very clear. Basically, the focal
"plane" isn't a plane at all, but a curved surface. Depending on where on the
face of the optic a pair of close off-axis rays pass, their point of common
focus is either in front of or behind the nominal focal "plane", resulting in an
axial smear of the planar image. This is just a basic geometrical problem with
spherical (and paraboloidal) optics. There are optical designs that are
coma-free, and there are other aberrations that can produce symmetric blurred
off-axis spots.

Something that might not be clear if you aren't used to looking at ray trace
diagrams: you are seeing the rays that are meridional- that is, they are
coplanar with the optical axis. But the spot diagrams are generated from skew
rays, which are not. The ray trace is derived from a 2-dimensional model of the
system; the spot diagrams are based on a 3-dimensional model.



What about two-way mirrors? One side sees a window, the other a mirror.
I thought there was a large class of objects that what happens to the light
depends strongly on the angle of the ray. It could be an optical diode: light
passes one way freely but gets reflected back when coming from the
opposite direction.

There is no such thing as a "two-way mirror". This is just a piece of glass that
is partly aluminized. Light passes through it equally well from either side. The
effect is possible only because the observer on the "transparent" side is
sitting in the dark. If he turns on the lights and the guy on the other side
turns his off, the situation will be reversed. If you used a piece of material
like this for a secondary, you'd still get some diffraction (and refractive and
scatter effects from the light passing through) and you'd lose half your total
light when it failed to reflect on the way to the EP. In short, it would be a
disaster.

Again, outside of the realm of quantum optics there is no such thing as an
optical diode.

_________________________________________________

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

On Thu, 17 Jun 2004 23:54:00 GMT, "Craig Franck"
wrote:

I understand much of the light is "oblique" as the book states; you have
the entire lens or much of the mirror gathering light. But I don't grasp why
it's not a symmetrical blob instead of a tail along one axis. If the lens if free
from defect, and the star is a symmetrical point, what causes the rays to
bunch up on one side of one axis?

I'm not sure how to explain that clearly in words. To me the simple ray trace
and spot diagrams on page 28 of R & vV are very clear. Basically, the focal
"plane" isn't a plane at all, but a curved surface. Depending on where on the
face of the optic a pair of close off-axis rays pass, their point of common
focus is either in front of or behind the nominal focal "plane", resulting in an
axial smear of the planar image. This is just a basic geometrical problem with
spherical (and paraboloidal) optics. There are optical designs that are
coma-free, and there are other aberrations that can produce symmetric blurred
off-axis spots.

Something that might not be clear if you aren't used to looking at ray trace
diagrams: you are seeing the rays that are meridional- that is, they are
coplanar with the optical axis. But the spot diagrams are generated from skew
rays, which are not. The ray trace is derived from a 2-dimensional model of the
system; the spot diagrams are based on a 3-dimensional model.



What about two-way mirrors? One side sees a window, the other a mirror.
I thought there was a large class of objects that what happens to the light
depends strongly on the angle of the ray. It could be an optical diode: light
passes one way freely but gets reflected back when coming from the
opposite direction.

There is no such thing as a "two-way mirror". This is just a piece of glass that
is partly aluminized. Light passes through it equally well from either side. The
effect is possible only because the observer on the "transparent" side is
sitting in the dark. If he turns on the lights and the guy on the other side
turns his off, the situation will be reversed. If you used a piece of material
like this for a secondary, you'd still get some diffraction (and refractive and
scatter effects from the light passing through) and you'd lose half your total
light when it failed to reflect on the way to the EP. In short, it would be a
disaster.

Again, outside of the realm of quantum optics there is no such thing as an
optical diode.

_________________________________________________

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

"Chris L Peterson" wrote

"Craig Franck" wrote:

In 4.2.2 they discuss coma and attribute it to "the intersection of
rays not being symmetrical." Shouldn't "off axis light" come into the
telescope in a symmetrical fashion when confronting an evenly distributed
light source? If one were to rotate the lens or mirror, would the coma
rotate as well?

In most cases, the optics in a telescope are rotationally symmetric. That means
that rotating elements does not affect the image. The direction of the coma is a
function of the position of the off-axis source.

I understand much of the light is "oblique" as the book states; you have
the entire lens or much of the mirror gathering light. But I don't grasp why
it's not a symmetrical blob instead of a tail along one axis. If the lens if free
from defect, and the star is a symmetrical point, what causes the rays to
bunch up on one side of one axis?

Also, (and this may sound silly to those more knowledgeable than I)
isn't there a compound glass that has the property of being neutral to light
in one direction and reflect it 90 degrees from the other? It would be like
a two-way mirror with the mirror embedded into the prism at an angle.
It's such a simple solution to having an obstruction that this type of
prism must present insurmountable problems.

I'm not sure what you are thinking of here- a material that passes light in one
direction but reflects it in the other? It is worthwhile to remember that
outside the quantum domain, optical materials are very symmetric. That means
that you can trace a ray through a system in either direction and the results
will be the same. Another way of thinking of this is that if you look at a ray
trace, there is no way of determining which way the light was traveling.

What about two-way mirrors? One side sees a window, the other a mirror.
I thought there was a large class of objects that what happens to the light
depends strongly on the angle of the ray. It could be an optical diode: light
passes one way freely but gets reflected back when coming from the
opposite direction.

--
Craig Franck

Cortland, NY

On Thu, 17 Jun 2004 20:09:29 GMT, "Craig Franck"
wrote:

In 4.2.2 they discuss coma and attribute it to "the intersection of
rays not being symmetrical." Shouldn't "off axis light" come into the
telescope in a symmetrical fashion when confronting an evenly distributed
light source? If one were to rotate the lens or mirror, would the coma
rotate as well?

In most cases, the optics in a telescope are rotationally symmetric. That means
that rotating elements does not affect the image. The direction of the coma is a
function of the position of the off-axis source.


WRT diffraction spikes, if the secondary mirror and struts are not in the
plane of focus, why would the diffraction effect occur where the mirror
itself is not visible? And if it's an "edge effect," way doesn't the edge of
the telescope tube diffract the light as well in a way that is visible?

It does. In a Newtonian telescope, there is diffraction from three sources: the
vanes or stalk, the central mirror, and the outside of the aperture. The latter
two are visible in the form of diffraction rings around a point source- what is
commonly referred to at the Airy disk. They are perfectly visible, but may go
largely unnoticed because they are rotationally symmetric with the star. You see
them in an unobstructed telescope from the aperture alone. The diffraction
spikes from the stalk or spider are much more obvious because they are oriented.


Also, (and this may sound silly to those more knowledgeable than I)
isn't there a compound glass that has the property of being neutral to light
in one direction and reflect it 90 degrees from the other? It would be like
a two-way mirror with the mirror embedded into the prism at an angle.
It's such a simple solution to having an obstruction that this type of
prism must present insurmountable problems.

I'm not sure what you are thinking of here- a material that passes light in one
direction but reflects it in the other? It is worthwhile to remember that
outside the quantum domain, optical materials are very symmetric. That means
that you can trace a ray through a system in either direction and the results
will be the same. Another way of thinking of this is that if you look at a ray
trace, there is no way of determining which way the light was traveling.

In reality, the "problem" of an obstruction is greatly overstated. In most
cases, the effects of an obstruction are nearly impossible to detect. The few
cases where an unobstructed design provides better results correspond rather
nicely to cases where size of aperture isn't too important, and unobstructed
designs are practical.

_________________________________________________

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

Craig Franck wrote:
In 4.2.2 they discuss coma and attribute it to "the intersection of
rays not being symmetrical." Shouldn't "off axis light" come into the
telescope in a symmetrical fashion when confronting an evenly distributed
light source?

It does not strike the mirror in a symmetrical fashion. It hits it
slanted, or "off axis." (I'm not sure this answer will help you, since
I'm not exactly sure what it is you're asking.)

If one were to rotate the lens or mirror, would the coma rotate as well?

No, generally not, unless there is something extra wrong with the lens
or mirror.

WRT diffraction spikes, if the secondary mirror and struts are not in the
plane of focus, why would the diffraction effect occur where the mirror
itself is not visible?

I'm not quite sure what you mean by "where the mirror itself is not
visible." Which mirror, the secondary mirror? Do you mean, why do you
see a diffraction effect even though you don't see the secondary mirror
itself in the eyepiece?

And if it's an "edge effect," way doesn't the edge of
the telescope tube diffract the light as well in a way that is visible?

It isn't, precisely speaking, an edge effect in the sense that it happens
*only* at the edge. And the edge of the telescope tube *does* diffract
the light. If it didn't, there would be no such thing as the Airy disc.
Light would focus down to an infinitesimal point, rather than the Airy
disc.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

"Brian Tung" wrote

Craig Franck wrote:
In 4.2.2 they discuss coma and attribute it to "the intersection of
rays not being symmetrical." Shouldn't "off axis light" come into the
telescope in a symmetrical fashion when confronting an evenly distributed
light source?

It does not strike the mirror in a symmetrical fashion. It hits it
slanted, or "off axis." (I'm not sure this answer will help you, since
I'm not exactly sure what it is you're asking.)

Is it off-axis because the lens or mirror has a curved surface? That makes
sense. But I don't understand why it favors a tail on one side.

WRT diffraction spikes, if the secondary mirror and struts are not in the
plane of focus, why would the diffraction effect occur where the mirror
itself is not visible?

I'm not quite sure what you mean by "where the mirror itself is not
visible." Which mirror, the secondary mirror? Do you mean, why do you
see a diffraction effect even though you don't see the secondary mirror
itself in the eyepiece?

Yes. You only see the secondary mirror if the eye piece is out of
focus, so it seems the diffraction effect should be in that off-focus
focal plane.

And if it's an "edge effect," way doesn't the edge of
the telescope tube diffract the light as well in a way that is visible?

It isn't, precisely speaking, an edge effect in the sense that it happens
*only* at the edge. And the edge of the telescope tube *does* diffract
the light. If it didn't, there would be no such thing as the Airy disc.
Light would focus down to an infinitesimal point, rather than the Airy
disc.

So with a lens or mirror it is the combination of waves of light being
gathered over the entire surface and combining that causes the airy disc.

It is interesting that the airy disk gets smaller with larger aperture. Is that
because the wave length of light gets smaller in comparison to the overall
area of the objective?

--
Craig Franck

Cortland, NY

Craig Franck wrote:
It does not strike the mirror in a symmetrical fashion. It hits it
slanted, or "off axis." (I'm not sure this answer will help you, since
I'm not exactly sure what it is you're asking.)

Is it off-axis because the lens or mirror has a curved surface? That makes
sense. But I don't understand why it favors a tail on one side.

That's right; the lens or mirror has an axis of symmetry, and the off-axis
light rays are tilted with respect to that axis. That means that light
rays don't get refracted in a symmetric way by the lens, so that they don't
come together to a point.

To see why it favors a tail, I think you would have to do the math, or see
a ray-trace diagram, or something like that. I'm not sure there's a good,
simple, first-order explanation in words alone.

Yes. You only see the secondary mirror if the eye piece is out of
focus, so it seems the diffraction effect should be in that off-focus
focal plane.

That's not the way that diffraction works. You see diffraction effects
because the wave front has a hole in it. The wave front does come to a
focus at the focal point, but because some parts of the wave front are
missing, the focus is disturbed. This disturbance can be seen in the
eyepiece as diffraction effects.

It isn't, precisely speaking, an edge effect in the sense that it happens
*only* at the edge. And the edge of the telescope tube *does* diffract
the light. If it didn't, there would be no such thing as the Airy disc.
Light would focus down to an infinitesimal point, rather than the Airy
disc.

So with a lens or mirror it is the combination of waves of light being
gathered over the entire surface and combining that causes the airy disc.

Yes, that's right.

It is interesting that the airy disk gets smaller with larger aperture.
Is that because the wave length of light gets smaller in comparison to
the overall area of the objective?

Hmm, the wavelength of light does get smaller in comparison to the size
of the objective (you can't really compare a length with an area), but I
hesitate to say that that is the *cause* of the trend. The math does work
out that way, though.

It's sort of like being better able to triangulate a position when you
have a longer baseline. I don't know if I can come up with a hard physical
analogue, however.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt