SCTs and chromatic error on double stars

Hi, Roland's point that an SCTs spherical mirror induces chromatic error or
imbalance leads to some questions. Is that also true in Mak/Cassegrains?
Why doesn't the corrector plate "correct" for this in an SCT. In about 5
years I'll be looking to buy that "retirement scope" in the mountains in
south eastern Colorado. Is a D&G 20" classical cassegrain a good choice vs.
a 20" Starmaster for DSOs? Thanks to all in advance ...

"Chuck" wrote in message
t...
Hi, Roland's point that an SCTs spherical mirror induces chromatic error
or
imbalance leads to some questions. Is that also true in Mak/Cassegrains?

It should be, because -- like an SCT -- the Maksutov is using a single layer
of glass to correct spherical aberration.

Why doesn't the corrector plate "correct" for this in an SCT.

It is only one layer of glass. In the process of correcting spherical
aberration (which is its purpose), it introduces slight chromatic aberration
(because all glass does this, unless you use multiple layers of glass to
cancel each other out). Glass bends different wavelengths of light
differently.

Frankly, in my experience the chromatic aberration of an SCT is very, very
small, much less than that of a refractor, and I have never been able to
observe it visually (even when doing sensitive star-tests). I have also not
heard of visible chromatic aberration in a Mak/Cass.

--
Clear skies,

Michael Covington -- www.covingtoninnovations.com
Author, Astrophotography for the Amateur
and (new) How to Use a Computerized Telescope

"Michael A. Covington" wrote:

(snip of a good post)
Frankly, in my experience the chromatic aberration of an SCT is very, very
small, much less than that of a refractor, and I have never been able to
observe it visually (even when doing sensitive star-tests). I have also not
heard of visible chromatic aberration in a Mak/Cass.

--


Hello, Michael,

This is a useful post, for the most part, but you miss one essential point.
It doesn't matter that you can't see the chromatic aberration in an SCT or a
MCT, an interferometer can see it, Even though you as a very experienced
observer can't see it, those SCT and MCT images with chromatic aberration are
going to you brain and affecting it and doing bad things there. That is why
interferometric testing of optics is so important, it protects you from this
kind of brain damage, and this is why it actually would be preferable if
interferometers, and not people, would do the actual posting to SAA. I don't
mean this in a bad way against you, after all you can't help being a person,
regrettable as that is, but you shouldn't arrogate to yourself things that
properly belong to machines, especially to interferometers, which cost a lot
more than you do and are a lot more sensitive and reliable..
Ciao,
Bill Meyers

Thanks Michael ...

"Michael A. Covington" wrote
in message ...

"Chuck" wrote in message
t...
Hi, Roland's point that an SCTs spherical mirror induces chromatic error
or
imbalance leads to some questions. Is that also true in
Mak/Cassegrains?

It should be, because -- like an SCT -- the Maksutov is using a single
layer
of glass to correct spherical aberration.

Why doesn't the corrector plate "correct" for this in an SCT.

It is only one layer of glass. In the process of correcting spherical
aberration (which is its purpose), it introduces slight chromatic
aberration
(because all glass does this, unless you use multiple layers of glass to
cancel each other out). Glass bends different wavelengths of light
differently.

Frankly, in my experience the chromatic aberration of an SCT is very, very
small, much less than that of a refractor, and I have never been able to
observe it visually (even when doing sensitive star-tests). I have also
not
heard of visible chromatic aberration in a Mak/Cass.

--
Clear skies,

Michael Covington -- www.covingtoninnovations.com
Author, Astrophotography for the Amateur
and (new) How to Use a Computerized Telescope

"Chuck" wrote in message
t...
Hi, Roland's point that an SCTs spherical mirror induces chromatic error
or
imbalance leads to some questions. Is that also true in Mak/Cassegrains?
Why doesn't the corrector plate "correct" for this in an SCT. In about 5
years I'll be looking to buy that "retirement scope" in the mountains in
south eastern Colorado. Is a D&G 20" classical cassegrain a good choice
vs.
a 20" Starmaster for DSOs? Thanks to all in advance ...
Yes.
The corrector, 'corrects' for spherical aberration. However as a 'downside',
it introduces the chromatic aberration. The corrector is a single lens, and
any lens of this sort, will refract different frequencies of light by
different amounts. If you 'ray trace' a typical SCT, or Mak, the error,
across most of the visible spectrum, is small. This is because the corrector
is a relatively 'gentle' lens (the steeper the angles at which light passes
through a lens, the larger the potential effects). However it is rising very
fast at the ends of the spectrum. This is a primary reason why a lot of CCD
images display 'bloated' stars, unless an IR rejection filter is used. On a
simple test, assuming a scope focussed on 'white' light, the green through
to red colours all put over 99% of their energy, inside the Airy disk. The
violet, showed the worst 'spread', but even here, 80% of the energy is
inside the Airy disk.
At the end of the day, SCT's, and Maksutov's, do show chromatic aberration,
but the effects are very small. It is worth in this 'context' realising,
that even APO's, which have multiple points across the spectrum, where they
are 'perfectly corrected', show errors between these points, and beyond
them. The only scope that does not show CA, is a simple reflector.
Unfortunately, in it's simplest form, this will then show coma off axis, and
if you add lenses to reduce this, you re-introduce CA!...
No optical system yet designed is 'perfect', and the SCT/Maksutov, designs,
show low CA, and pack a long focal length, into a short unit. Their
'downsides', are the central obstruction (which spreads light energy over
the visible spectrum, more than the chromatic error), and quite steeply
curved image field. CA, is low on the list of their faults...

Best Wishes

Hi Roger, I remember seeing a rather advanced ATM design for a 1 meter
"corrected" classical cassegrain. It was, I believe an f15 design with an
apochromatic corrector at the "back" of the telescope. Why are there so few
classical cassegrains being made commercially for amateurs? Is it tube
length? It seems that the design, excluding the secondary is more straight
forward than an SCT or MCT ....

"Roger Hamlett" wrote in message
...

"Chuck" wrote in message
t...
Hi, Roland's point that an SCTs spherical mirror induces chromatic error
or
imbalance leads to some questions. Is that also true in
Mak/Cassegrains?
Why doesn't the corrector plate "correct" for this in an SCT. In about
5
years I'll be looking to buy that "retirement scope" in the mountains in
south eastern Colorado. Is a D&G 20" classical cassegrain a good choice
vs.
a 20" Starmaster for DSOs? Thanks to all in advance ...
Yes.
The corrector, 'corrects' for spherical aberration. However as a
'downside',
it introduces the chromatic aberration. The corrector is a single lens,
and
any lens of this sort, will refract different frequencies of light by
different amounts. If you 'ray trace' a typical SCT, or Mak, the error,
across most of the visible spectrum, is small. This is because the
corrector
is a relatively 'gentle' lens (the steeper the angles at which light
passes
through a lens, the larger the potential effects). However it is rising
very
fast at the ends of the spectrum. This is a primary reason why a lot of
CCD
images display 'bloated' stars, unless an IR rejection filter is used. On
a
simple test, assuming a scope focussed on 'white' light, the green through
to red colours all put over 99% of their energy, inside the Airy disk. The
violet, showed the worst 'spread', but even here, 80% of the energy is
inside the Airy disk.
At the end of the day, SCT's, and Maksutov's, do show chromatic
aberration,
but the effects are very small. It is worth in this 'context' realising,
that even APO's, which have multiple points across the spectrum, where
they
are 'perfectly corrected', show errors between these points, and beyond
them. The only scope that does not show CA, is a simple reflector.
Unfortunately, in it's simplest form, this will then show coma off axis,
and
if you add lenses to reduce this, you re-introduce CA!...
No optical system yet designed is 'perfect', and the SCT/Maksutov,
designs,
show low CA, and pack a long focal length, into a short unit. Their
'downsides', are the central obstruction (which spreads light energy over
the visible spectrum, more than the chromatic error), and quite steeply
curved image field. CA, is low on the list of their faults...

Best Wishes

"Bill Meyers" wrote in message
...

It doesn't matter that you can't see the chromatic aberration in an SCT
or a
MCT, an interferometer can see it [snip].
That is why
interferometric testing of optics is so important, [snip]
it actually would be preferable if
interferometers, and not people, would do the actual posting to SAA.

Without experiencing the end result in terms of real performance, the
numbers would be pretty meaningless. However, it _would_ be great if there
were a larger quantity of technical explanations for the experiences we
share here on saa. But, in the absence of knowledge, ans seeing that this is
a hobby for most of us, it's fun to speculate, experiment, and discuss. As
an amateur, I guess thought that was science, and I thought this was
"sci".astro."amateur". g

Regards,
Stephen Paul

Hi, Roland's point that an SCTs spherical mirror induces chromatic error or
imbalance leads to some questions. Is that also true in Mak/Cassegrains?

It all depends on the design of the Mak. One can produce a Mak corrector that
nulls out the primary chromatic aberration. The result is zero color error over
a very wide spectrum. With the proper design the variation of spherical
aberration with wavelength (sphero-chromatism) can be reduced to a fraction of
the diffraction limit over the useful photo-visual wavelength range. The bent
corrector acts as a pure window with no dispersive power, only power to correct
for the various monochromatic errors introduced by the spherical primary. Such
a design normally requires some small amount of aspheric work to control the
higher order monochromatic aberrations, therefore it is not exactly easy to
make in high production.

One can purposely introduce chromatic aberration in order to correct for a
faster primary mirror without the penalty of deeper corrector curves and to
eliminate the need for aspheric correction. Such a system is cheaper to make
than the ideal Mak, and I have seen it used on some relatively cheap commercial
Maks.

Roland Christen

"Chuck" wrote in message
t...
Hi Roger, I remember seeing a rather advanced ATM design for a 1 meter
"corrected" classical cassegrain. It was, I believe an f15 design with
an
apochromatic corrector at the "back" of the telescope. Why are there so
few
classical cassegrains being made commercially for amateurs? Is it tube
length? It seems that the design, excluding the secondary is more
straight
forward than an SCT or MCT ....
It isn't...
The problem is that the simplest curve to make is a sphere. By default, this
is the shape you get when two surfaces are ground together. The modern 'mass
produced' SCT, is 99% done by grinding spheres. Basically, a spherical
primary, a spherical secondary, and then the corrector can be made by
distorting a piece of glass, and grinding this to a sphere again.
Technically, you can produce a 'better' SCT, by using an mildly ellipsoidal
secondary, but this is not normally done.
The classical Cassegrain, has a paraboloid primary (only a little more
difficult to make, but it then introduces the need to cut the hole in
exactly the right place, where the spherical mirror will work correctly,
with the hole anywhere). It then has an ellipsoidal secondary. Hence, like
the Ritchie Chretien, it is a much more expensive design to actually make.
It also 're-introduces' the problem that the optics are exposed to the
elements (the sealed nature of SCT's, is one reason their optics keep
working for so long), and you also need some way to support the secondary
(this is more critical than on a Newtonian, since the optical
'magnification' that takes place at the secondary, amplifies any movements
at this point. Adding a plain glass plate, re-introduces CA, for any light
that is not on-axis, while using a spider introduces diffraction spikes.
Going back to the Maksutov, it is worth realising, that the original designs
of the scope that is now so common, were largely quite high focal ratio
instruments (f/20 to f/35), and usually had seperate secondaries, rather
than the 'silvered spot' designs. The 'silvered spot' design, places a
serious constraint on the optics, that the rear curve of the corrector, has
to match the curve required for the secondary, this is cheap to make, but
does not give such good correction as is possible with seperate optics. With
seperate optics, it is possible to design an MCT, that has extremely low
levels of CA indeed. However this too, is a more expensive design to
actually make...

Best Wishes

"Roger Hamlett" wrote in message
...

"Chuck" wrote in message
t...
Hi, Roland's point that an SCTs spherical mirror induces chromatic
error
or
imbalance leads to some questions. Is that also true in
Mak/Cassegrains?
Why doesn't the corrector plate "correct" for this in an SCT. In
about
5
years I'll be looking to buy that "retirement scope" in the mountains
in
south eastern Colorado. Is a D&G 20" classical cassegrain a good
choice
vs.
a 20" Starmaster for DSOs? Thanks to all in advance ...
Yes.
The corrector, 'corrects' for spherical aberration. However as a
'downside',
it introduces the chromatic aberration. The corrector is a single lens,
and
any lens of this sort, will refract different frequencies of light by
different amounts. If you 'ray trace' a typical SCT, or Mak, the error,
across most of the visible spectrum, is small. This is because the
corrector
is a relatively 'gentle' lens (the steeper the angles at which light
passes
through a lens, the larger the potential effects). However it is rising
very
fast at the ends of the spectrum. This is a primary reason why a lot of
CCD
images display 'bloated' stars, unless an IR rejection filter is used.
On
a
simple test, assuming a scope focussed on 'white' light, the green
through
to red colours all put over 99% of their energy, inside the Airy disk.
The
violet, showed the worst 'spread', but even here, 80% of the energy is
inside the Airy disk.
At the end of the day, SCT's, and Maksutov's, do show chromatic
aberration,
but the effects are very small. It is worth in this 'context' realising,
that even APO's, which have multiple points across the spectrum, where
they
are 'perfectly corrected', show errors between these points, and beyond
them. The only scope that does not show CA, is a simple reflector.
Unfortunately, in it's simplest form, this will then show coma off axis,
and
if you add lenses to reduce this, you re-introduce CA!...
No optical system yet designed is 'perfect', and the SCT/Maksutov,
designs,
show low CA, and pack a long focal length, into a short unit. Their
'downsides', are the central obstruction (which spreads light energy
over
the visible spectrum, more than the chromatic error), and quite steeply
curved image field. CA, is low on the list of their faults...

Best Wishes

"Michael A. Covington" wrote in message ...
Is that also true in Mak/Cassegrains?

It should be, because -- like an SCT -- the Maksutov is using a single layer
of glass to correct spherical aberration.

No, it should not. It is not difficult to understand why a menisk can be made
to suffer far less from chromatic aberrations (spherochromatism), while Schmidt
plate can not. ("can" is a critical term - not all Maksutovs designs will
be free from spherochromatism)
It does require a bit deeper understanding of optics, maybe a step up from
"single layer of glass" analogy.

Why doesn't the corrector plate "correct" for this in an SCT.

It is only one layer of glass. In the process of correcting spherical
aberration (which is its purpose), it introduces slight chromatic aberration
(because all glass does this, unless you use multiple layers of glass to
cancel each other out). Glass bends different wavelengths of light
differently.

The last sentence is the only correct one. "Glass bends different wavelengths
differently".
Let's have a mental experiment.
Observe a ray of white light hitting a menisk in a Mak scope. It bends
outwards (away from optical axis) and spreads into components. Violet and
blue components will bend the most. Red and IR will bend the least. Now
let's see what happens when this fan hits the rear surface of the menisk.
Violet/blue entrance point is further away from the axis than red/IR one.
All rays will bend INWARDS (towards the axis). But now let's remember the rule -
violet/blue rays will bend MORE. As a result the slope of blue rays is steeper
than that of the red ones. They will CROSS somewhere. Can we make this in
such a way to make this crossing coincide with focus point ? Of course we can.
Result ? Our "single layer of glass" does NOT introduce any chromatic
aberration, even if it has some power. We have invented an achromatic menisk.
If only someone else didn't do the same more than 60 years ago. His name
is Dmitry Maksutov.

Can we apply the same logic to Schmidt plate ? No. Why ? Because Schmidt plate
is a compound curve that is positive lens in the middle and negative at the
edges at the same time. Hence spherochomatism cannot be avoided. It can be
minimized by having a neutral zone placed at 87%, but that's it. It will
never be zero (strictly speaking it will never be zero in a Maksutov, but
say properly designed Maksutov Newtonian can have chromatic residuals,
including spherochromatism, that even today's best triplet APOs can only
dream about).

Frankly, in my experience the chromatic aberration of an SCT is very, very
small, much less than that of a refractor, and I have never been able to
observe it visually (even when doing sensitive star-tests). I have also not
heard of visible chromatic aberration in a Mak/Cass.

Spherochroomatism looks vastly different from primary or secondary chromatic
aberrations. You just have to learn to detect it, and then you're cursed
forever :-)

Bratislav