Semi-Apo

Semi-Apo is a relative term and means little.

"A marketing term for a scope that is a bit better than a regular acromatic
scope.
The various glasses that can be used for a refractor varies from very poor
combinations to very good combinations that produce excellent color control.
The unfortunate thing is that the better the glass is for the job, the worse
the price becomes due to the cost of making the strange glasses. Quantity
has nothing to do with it as the strange glasses are all glasses that are
difficult to get mixed right and melted properly."

On Thu, 17 Jun 2004 01:57:27 +0000, OncoBilly wrote:

Semi-Apo is a relative term and means little.

Well said. They are also too puny to see a damn thing. Buy a reflector.

On Thu, 17 Jun 2004 01:57:27 +0000, OncoBilly wrote:

Semi-Apo is a relative term and means little.

Well said. They are also too puny to see a damn thing. Buy a reflector.

Al Wilson wrote:
On Thu, 17 Jun 2004 01:57:27 +0000, OncoBilly wrote:


Semi-Apo is a relative term and means little.


Well said. They are also too puny to see a damn thing. Buy a reflector.


:-)

Al Wilson wrote:
On Thu, 17 Jun 2004 01:57:27 +0000, OncoBilly wrote:


Semi-Apo is a relative term and means little.


Well said. They are also too puny to see a damn thing. Buy a reflector.


:-)

Semi-Apo is a relative term and means little.

"A marketing term for a scope that is a bit better than a regular acromatic
scope.

Of the roughly 2000 glasses available for making lenses, 99% of them will
result in just about exactly achromatic performance. Of these 99%, some are
less expensive, and some are super expensive. Cost has no bearing on whether a
glass can correct secondary spectrum or not. In fact, some of the most
expensive glasses will result in slightly higher secondary spectrum error.

In order to correct secondary spectrum, the designer must choose two glasses
that lie considerably off the normal Abbe line, which means at least one of
them will have abnormal dispersion and totally different chemistry than a
normal glass.

These abnormal glasses tend to be fragile because they typically use fluorites
instead of oxides in their chemical makeup. There are also only a very few of
them that are suitable for making lenses and available from only a couple of
manufacturers. All these things are well known to the optical manufacturers who
make astronomical lenses. There are no hidden designs or secrets where new
lenses made from new glasses remain to be discovered. It is also actually
cheaper to make a well corrected ED lens with 4 times better color correction
than a normal achromat, than it is to make a "semi-apo" with only twice better
correction. Meade design is an example of that.

Roland Christen

Semi-Apo is a relative term and means little.

"A marketing term for a scope that is a bit better than a regular acromatic
scope.

Of the roughly 2000 glasses available for making lenses, 99% of them will
result in just about exactly achromatic performance. Of these 99%, some are
less expensive, and some are super expensive. Cost has no bearing on whether a
glass can correct secondary spectrum or not. In fact, some of the most
expensive glasses will result in slightly higher secondary spectrum error.

In order to correct secondary spectrum, the designer must choose two glasses
that lie considerably off the normal Abbe line, which means at least one of
them will have abnormal dispersion and totally different chemistry than a
normal glass.

These abnormal glasses tend to be fragile because they typically use fluorites
instead of oxides in their chemical makeup. There are also only a very few of
them that are suitable for making lenses and available from only a couple of
manufacturers. All these things are well known to the optical manufacturers who
make astronomical lenses. There are no hidden designs or secrets where new
lenses made from new glasses remain to be discovered. It is also actually
cheaper to make a well corrected ED lens with 4 times better color correction
than a normal achromat, than it is to make a "semi-apo" with only twice better
correction. Meade design is an example of that.

Roland Christen

"A marketing term for a scope that is a bit better than a regular
acromatic
scope.

Of the roughly 2000 glasses available for making lenses, 99% of them will
result in just about exactly achromatic performance. Of these 99%, some
are
less expensive, and some are super expensive. Cost has no bearing on
whether a
glass can correct secondary spectrum or not. In fact, some of the most
expensive glasses will result in slightly higher secondary spectrum error.

In order to correct secondary spectrum, the designer must choose two
glasses
that lie considerably off the normal Abbe line, which means at least one
of
them will have abnormal dispersion and totally different chemistry than a
normal glass.

These abnormal glasses tend to be fragile because they typically use
fluorites
instead of oxides in their chemical makeup. There are also only a very few
of
them that are suitable for making lenses and available from only a couple
of
manufacturers. All these things are well known to the optical
manufacturers who
make astronomical lenses. There are no hidden designs or secrets where new
lenses made from new glasses remain to be discovered. It is also actually
cheaper to make a well corrected ED lens with 4 times better color
correction
than a normal achromat, than it is to make a "semi-apo" with only twice
better
correction. Meade design is an example of that.

Roland Christen

Mr. C.

Then just what are people like yourself looking for when you have already
invented the wheel?
Why don't you just sit back and make refractors that have "good" correction
with ED glasses for the masses
rather than "super" corrected and make even more money. ie. make 2000
dollar scopes rather than 6000.

"A marketing term for a scope that is a bit better than a regular
acromatic
scope.

Of the roughly 2000 glasses available for making lenses, 99% of them will
result in just about exactly achromatic performance. Of these 99%, some
are
less expensive, and some are super expensive. Cost has no bearing on
whether a
glass can correct secondary spectrum or not. In fact, some of the most
expensive glasses will result in slightly higher secondary spectrum error.

In order to correct secondary spectrum, the designer must choose two
glasses
that lie considerably off the normal Abbe line, which means at least one
of
them will have abnormal dispersion and totally different chemistry than a
normal glass.

These abnormal glasses tend to be fragile because they typically use
fluorites
instead of oxides in their chemical makeup. There are also only a very few
of
them that are suitable for making lenses and available from only a couple
of
manufacturers. All these things are well known to the optical
manufacturers who
make astronomical lenses. There are no hidden designs or secrets where new
lenses made from new glasses remain to be discovered. It is also actually
cheaper to make a well corrected ED lens with 4 times better color
correction
than a normal achromat, than it is to make a "semi-apo" with only twice
better
correction. Meade design is an example of that.

Roland Christen

Mr. C.

Then just what are people like yourself looking for when you have already
invented the wheel?
Why don't you just sit back and make refractors that have "good" correction
with ED glasses for the masses
rather than "super" corrected and make even more money. ie. make 2000
dollar scopes rather than 6000.

Mr. C.

Then just what are people like yourself looking for when you have already
invented the wheel? Why don't you just sit back and make refractors that have
"good" correction with ED glasses for the masses rather than "super" corrected
and make even more money. ie. make 2000
dollar scopes rather than 6000.

For what its worth:

I think the answer to that is that Roland is not in this for the money. If he
were, he would charge what the market would bear rather than what he believes
is fair.

He leaves making those lesser scopes to others who are unable to do better,
building the finest scopes and accessories he can is his passion/dream.

And I think too that part of the motivation is so that he can have the
equipment so that he can take the pictures and see the stuff that he wants to
see.

Jon