Prism Diagonal Anti Chromatic Aberration Effect?

I read that in a 90 degrees prism diagonal. Chromatic aberrations
are almost cancelled. I'd like to know to what extend it is true
and what is the rule. For the entering light cone, it encounters
flat surface via the air-glass interface, and since there is an angle
of the incident light, chromatic deviation occurs from the splitting
of the white light into the different wavelength inside the glass, But
when the light cone exits on the other flat surface of the prism diagonal,
the chromatic aberrations are cancelled from the opposite
glass-to-air interface and the light cone returns to its original
unchromatic aberrated form (this is assuming of course that the
objective lens of the telescope is an apo or sct where chromatic
aberrations are a nil compared to an achromat).

Now what is the rule, like does shorter focal ratio or steeper
light cone make the prism diagonal ineffective in cancelling
the chromatic aberrations inside the prism diagonal? In long focal
ratio scope or light cone entering and exiting a prism diagonal with
parallel entry and exit surface (remembering that there is no chromatic
aberrations from the internal reflections). How many percentage
approximately of the light cone returns to its original unchromatic
aberrated form after it exits the prism diagonal.

If anyone has any site or articles about this in details. Let me
know. Thanks. (Note: Some may say that a prism diagonal is
obsolete and just buy a mirror diagonal. Well, the above inquiry is
to understand better the behavior of chromatic aberrations in
parallel entering and exiting surfaces such as a prism diagonal
and novelty item like binoviewer (which has almost zero
chromatic aberration when I observe thru one) and also to
get some idea like how some products such as the chromacorr
(which removes spherical aberrations) work.

optidud

On 13 Jul 2003 07:53:49 -0700, (optidud) wrote:

I read that in a 90 degrees prism diagonal. Chromatic aberrations
are almost cancelled. I'd like to know to what extend it is true
and what is the rule.

The rays exit the prism at the same angle they enter, but they are
deviated laterally. The difference is pretty small at the focal
ratios of most of the telescopes you use star diagonals with.

optidud wrote:

When you say the rays exits the prism at the same angle they enter,
this means the red which has least dispersion when it enters the
prism exits the same angle as well as the violet with more dispersion.
This means that there is no secondary spectrum at the exit side as
compared to achromat because the angle are as before they enter the
prism. Yet the whole light cone is dispersed slightly... the same
effect when you move the focuser a little off the focal plane.
So why not just change the position of the focuser to bring the whole
dispersed light cone back to focus??


While the light is inside the prism, it is typically converging. This
implies that the change
in focal position of the red will differ slightly from the change for
blue. Simply refocussing
will not recover that.

Rick S.

What's the different between the
light cone in a defocused position and when it is deviated laterally?
Hope someone can clear this up as this puzzle bugs me day and night
for several days already. Thanks.

optidud

William Hamblen wrote in message . ..
On 13 Jul 2003 07:53:49 -0700, (optidud) wrote:

I read that in a 90 degrees prism diagonal. Chromatic aberrations
are almost cancelled. I'd like to know to what extend it is true
and what is the rule.

The rays exit the prism at the same angle they enter, but they are
deviated laterally. The difference is pretty small at the focal
ratios of most of the telescopes you use star diagonals with.

When you say the rays exits the prism at the same angle they enter,
this means the red which has least dispersion when it enters the
prism exits the same angle as well as the violet with more dispersion.
This means that there is no secondary spectrum at the exit side as
compared to achromat because the angle are as before they enter the
prism. Yet the whole light cone is dispersed slightly... the same
effect when you move the focuser a little off the focal plane.
So why not just change the position of the focuser to bring the whole
dispersed light cone back to focus?? What's the different between the
light cone in a defocused position and when it is deviated laterally?
Hope someone can clear this up as this puzzle bugs me day and night
for several days already. Thanks.

optidud

If the focal positions for red and blue is slight different inside
the prism as a result of passing thru the entry surface of the prism
(and the dispersion that goes with it). Then won't exiting from the
second surface bring back the focal positions to the original since it
is opposite in effect to air-glass in the 1st entry surface versus
glass-air in the 2nd entry surface??

No. Why? Because.

Roland Christen

In article , optidud wrote:

If the focal positions for red and blue is slight different inside
the prism as a result of passing thru the entry surface of the prism
(and the dispersion that goes with it). Then won't exiting from the
second surface bring back the focal positions to the original since it
is opposite in effect to air-glass in the 1st entry surface versus
glass-air in the 2nd entry surface??


Optidud, think about Snell's law and why dispersion happens a little bit.
Snell's law is sin(a1)*n1 = sin(a2)*n2 when a1 is the angle of the
incident ray, a2 is the angle of the refracted ray, n1 is the index
of refraction of the first medium and n2 is the index of refraction of
the second medium. The angles are measured from the line normal to the
surface between the two media. If the first medium is air, n1 is roughly
1 and if the second medium is glass n2 is roughly 1.5. This means than
when a ray of light strikes the surface of a thick glass plate at an
angle, it is refracted to make a slightly smaller angle in the glass.
When it is refracted at the second surface it comes out at a slightly
larger angle. Because you have air at both faces of the thick glass
plate and the faces are parallel, the rays come out at the same angle
they went in. You have that figured out already. The part you've missed
is that because of refraction in the glass and the thickness of the
plate, the rays are displaced a little to one side, compared to where
they would be if the plate wasn't there. Dispersion happens because
the index of refraction of glass isn't the same for every wavelength
of light - it is slightly more for blue light than it is for red light.
Imagine a ray of red light and a ray of blue light coming in at the same
angle and hitting the same spot on the first face of the glass plate.
The blue ray would be refracted differently from the red ray and the
spot where the blue ray exits the second face of the glass plate would
be displaced a little from the spot where the red ray exits. This causes
some false color to appear around your star image. It isn't much. For
slow telescopes where you usually use prism diagonals it is hardly
noticeable.

"William Hamblen" wrote in message
rthlink.net...

The blue ray would be refracted differently from the red ray and the
spot where the blue ray exits the second face of the glass plate would
be displaced a little from the spot where the red ray exits. This causes
some false color to appear around your star image.

Hey!, even _I_ understood that (I think). :-)

Good one Bill (if I can call you Bill).

-Stephen Paul

optidud wrote:

BTW.. The term for the above change of focal points for different
wavelengths is called Spherochromatism, right. So prism diagonal
and binoviewer introduces Spherochromatism, right?


No. It's called color, or chromatic aberration. Spherochromatism is
also affected
by the prism, though. The effect caused by the same principal. The
more steeply
inclined the rays are, the larger the impact (in absolute terms). This
causes the outer
rays to be moved in focus relative to the inner rays.

Rick S.

optidud

In article , optidud wrote:

BTW.. The term for the above change of focal points for different
wavelengths is called Spherochromatism, right. So prism diagonal
and binoviewer introduces Spherochromatism, right?

Different focal points for light of different wavelengths produces
chromatic aberration. You have longitudinal (measured along the optical
axis) and lateral (measured across the optical axis) aberration.

Spherochromatism is change in spherical aberration with wavelength.
An ordinary doublet is corrected for spherical aberration at one color.
For other colors the spherical aberration would be greater. I have a
Celestron/Vixen 102 mm f/9.8 refractor. I can just see some spherical
aberration with a deep blue filter. With a yellow or green filter I can't
see any spherical aberration. If it was 1500 mm focal length instead
of 1000 mm you probably couldn't see SA with the blue filter either.

Yes, I've realized the Spherochromatism is not the same as primary
color error and I've consulted many references about this a while ago.
But there is something that eludes me and I wonder if I'd figure it
out by simply sketching on paper. They say that when spherochromatism
is corrected for green light, it is undercorrected in red light and
overcorrected in blue light, Which means that in red light, the
outer rays focus closer to the lens than the paraxial rays, and
the opposite is true in the blue. Well. if the lens is corrected
for green light, that means the outer zones of the objective lens
are curved to get a perfect spherical aberration correction for
green light. Now how is that when red light passes thru the outer
zones of the lens, it focuses nearer the lens than in the center rays.
I'm still figuring out why this is the case. When manufacturers design
lenses. They all use green light, and not white light to grind the
lenses to correct for spherical aberrations even in cheap china
achromats, right?? This means the optical factories have a lot of
green laser or light source at the grinding site, has anyone seen
this themselves in optical factories such as Stellarvue's or Televue's
(I assume Astrophysics labs are restricted area)?

optidud

William Hamblen wrote in message arthlink.net...
In article , optidud wrote:

BTW.. The term for the above change of focal points for different
wavelengths is called Spherochromatism, right. So prism diagonal
and binoviewer introduces Spherochromatism, right?

Different focal points for light of different wavelengths produces
chromatic aberration. You have longitudinal (measured along the optical
axis) and lateral (measured across the optical axis) aberration.

Spherochromatism is change in spherical aberration with wavelength.
An ordinary doublet is corrected for spherical aberration at one color.
For other colors the spherical aberration would be greater. I have a
Celestron/Vixen 102 mm f/9.8 refractor. I can just see some spherical
aberration with a deep blue filter. With a yellow or green filter I can't
see any spherical aberration. If it was 1500 mm focal length instead
of 1000 mm you probably couldn't see SA with the blue filter either.