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| Chris.B... |
 Posted: Tue Oct 27, 2009 9:58 pm |
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On Oct 27, 8:34 pm, Phil Hobbs
<pcdhSpamMeSensel... at (no spam) electrooptical.net> wrote:
[quote]
For the truly discriminating audiophool, I mean audiophile. ;)
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot nethttp://electrooptical.net
[/quote]
So both of mine are bigger than yours?  ) |
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| Quadibloc... |
| Posted: Wed Oct 28, 2009 2:13 am |
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On Oct 27, 1:24 pm, "Chris.B" <chri... at (no spam) nypost.dk> wrote:
[quote]Long refractors can become short refractors by folding with optical
flats.
[/quote]
A simpler method has occurred to me. The patent I cited... one might
take it as inspiration in all its glory. If they make telescopes whose
objectives are Petzval portrait lenses... why not go all the way to
Double Gauss, and really correct the aberrations?
John Savard |
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| uncarollo... |
| Posted: Wed Oct 28, 2009 5:54 am |
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On Oct 27, 6:55 pm, Quadibloc <jsav... at (no spam) ecn.ab.ca> wrote:
[quote]But this reminds me of what is probably a far better technique to do
this...
U. S. Patent 2,406,762, by D. S. Grey of the Polaroid Corporation -
from 1946, applied for in 1943, so it's expired - reveals the secret.
Have a lens consisting of two cemented doublets.
The first consists of an acrylic element and a polycarbonate element
of equal and opposite powers.
The second consists of a flint and crown element. Assuming the first
cemented doublet consisted of a positive acrylic element and a
negative polycarbonate element, the first doublet will embody an
overcorrection for chromatic aberration, and so the second doublet,
containing the positive power of the system, would be undercorrected
for chromatic aberration.
So the power of the lens is not affected much by temperature, only the
color correction is.
John Savard
[/quote]
This has never been a secret. Lens designers have long known that
plastics can be used to create an apochromat. What you all don't
understand is that plastics are not practical as telescope objective
lenses. It's not just that the expansion co-efficient is so high
(which means that the focus point for different colors and for the
image itself would change drastically even with only 1 degree
temperature change), but that the internal quality of the material is
so low as to not allow diffraction limited performance in any
reasonable aperture size.Forget plastics.
Rolando |
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| Phil Hobbs... |
| Posted: Wed Oct 28, 2009 7:42 am |
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Quadibloc wrote:
[quote]On Oct 27, 10:05 am, Phil Hobbs
Not to mention that it has about 100 times higher thermal expansion than
glass, so it would be very difficult to keep a large cemented lens from
delaminating.
I would avoid attempting to cement the plastic element to a glass
element for that reason.
But this reminds me of what is probably a far better technique to do
this...
U. S. Patent 2,406,762, by D. S. Grey of the Polaroid Corporation -
from 1946, applied for in 1943, so it's expired - reveals the secret.
Have a lens consisting of two cemented doublets.
The first consists of an acrylic element and a polycarbonate element
of equal and opposite powers.
The second consists of a flint and crown element. Assuming the first
cemented doublet consisted of a positive acrylic element and a
negative polycarbonate element, the first doublet will embody an
overcorrection for chromatic aberration, and so the second doublet,
containing the positive power of the system, would be undercorrected
for chromatic aberration.
So the power of the lens is not affected much by temperature, only the
color correction is.
John Savard
[/quote]
But the curves aren't really the same shape, are they? I thought you
wanted an apochromat?
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net |
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| Quadibloc... |
| Posted: Wed Oct 28, 2009 8:11 am |
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On Oct 28, 9:54 am, uncarollo <chris1... at (no spam) aol.com> wrote:
[quote]the internal quality of the material is
so low as to not allow diffraction limited performance in any
reasonable aperture size.
[/quote]
You're probably right, or this would already have been done. But I am
thinking in terms of aperture sizes from, say, 50 to 80mm, and not
beyond. Not in terms of making the highest quality telescope, but
simply to make a better department store telescope at a slightly
increased cost.
John Savard |
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| uncarollo... |
| Posted: Wed Oct 28, 2009 8:13 am |
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On Oct 28, 1:11 pm, Quadibloc <jsav... at (no spam) ecn.ab.ca> wrote:
[quote]On Oct 28, 9:54 am, uncarollo <chris1... at (no spam) aol.com> wrote:
the internal quality of the material is
so low as to not allow diffraction limited performance in any
reasonable aperture size.
You're probably right, or this would already have been done. But I am
thinking in terms of aperture sizes from, say, 50 to 80mm, and not
beyond. Not in terms of making the highest quality telescope, but
simply to make a better department store telescope at a slightly
increased cost.
John Savard
[/quote]
Think 5mm aperture, and you will be in the ballpark for a plastic apo
lens.
Rolando |
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| Quadibloc... |
| Posted: Thu Oct 29, 2009 6:12 am |
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On Oct 29, 8:14 am, Don Stauffer <stauf... at (no spam) usfamily.net> wrote:
[quote]There was a period- late 70s, early 80s, when there was a lot of
product development in plastics for elements in professional optics.
[/quote]
And, of course, nowadays, virtually all eyeglasses are made from
plastic.
Of course, though, if what I want is a lens whose power does not
change much with temperature, that means that I can't use just one
plastic element, since a zero-power element from one type of plastic
would not contribute color correction. So it might be that the best
material, perhaps acrylic, can now meet the quality requirements for
inexpensive optics - but polycarbonate, the next choice, for a plastic
'flint', might still have issues that prevent its use.
John Savard |
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| Don Stauffer... |
| Posted: Thu Oct 29, 2009 8:14 am |
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Guest
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Quadibloc wrote:
[quote]On Oct 28, 9:54 am, uncarollo <chris1... at (no spam) aol.com> wrote:
the internal quality of the material is
so low as to not allow diffraction limited performance in any
reasonable aperture size.
You're probably right, or this would already have been done. But I am
thinking in terms of aperture sizes from, say, 50 to 80mm, and not
beyond. Not in terms of making the highest quality telescope, but
simply to make a better department store telescope at a slightly
increased cost.
John Savard
[/quote]
There was a period- late 70s, early 80s, when there was a lot of
product development in plastics for elements in professional optics. I
was into LWIR systems and did not follow it very much, but believe most
of it went into cameras, as original poster noted. There was a push to
develop processes that did have good quality. As well as these elements
worked in cameras, I am surprised the consumer telescopes, the type John
mentions, did not pick up on it.
As I remember, the lenses were cast in metal molds that were diamond turned. |
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| Quadibloc... |
| Posted: Thu Oct 29, 2009 1:30 pm |
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On Oct 29, 10:12 am, Quadibloc <jsav... at (no spam) ecn.ab.ca> wrote:
[quote]but polycarbonate, the next choice, for a plastic
'flint', might still have issues that prevent its use.
[/quote]
I see that currently polycarbonate, rather than acrylic, is what is
used in eyeglasses, so as to make them more compact. So possibly it,
too, is available in optical quality.
I've made a rough calculation of how such a system might be put
together at the bottom of my web page at
http://www.quadibloc.com/science/opt0504.htm
John Savard |
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| AES... |
| Posted: Thu Oct 29, 2009 8:00 pm |
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Guest
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In article
<ea7b13d6-8049-40d1-a699-077bf5d6dada at (no spam) y28g2000prd.googlegroups.com>,
Quadibloc <jsavard at (no spam) ecn.ab.ca> wrote:
[quote]I see that currently polycarbonate, rather than acrylic, is what is
used in eyeglasses, so as to make them more compact. So possibly it,
too, is available in optical quality.
[/quote]
For the interesting story on how eyeglasses became primarily
polycarbonate, and how sperm from Nobel Laureates may or
may not create Nobel Prize level babies, Google on Robert K.
Graham, or on David Plotz, "The Genius Factor". |
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