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| Quadibloc... |
 Posted: Tue Oct 27, 2009 5:18 am |
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Guest
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The refracting telescope was much improved when Fraunhofer perfected
the achromatic lens.
Newton had thought that an achromatic lens would not be possible,
since denser glasses with a higher refractive index also have more
dispersion. But it was later discovered that this relation is not
linear, and, thus, since flint glasses tend to have an increased
dispersion which is more than proportional to their increased
refractive index, the combination of a positive crown lens with a
weaker negative flint lens could allow their dispersions to balance
out while leaving useful lens power.
To combine three different types of glass to bring three wavelengths
of light to a common focus, however, does generally place optical
designers in the frustrating kind of situation envisaged by Newton.
The way in which the higher dispersion of a flint glass does not
exactly cancel out with the lower dispersion of a crown glass is
pretty much constant for all ordinary glasses.
The usual solution to this is to use the mineral fluorite, calcium
fluoride, as was done in the first achromatic lenses, microscope
objectives made by Zeiss, or glasses which contain a significant
amount of fluorite. These glasses are expensive, and vulnerable to
attack by water.
There are alternatives known; why aren't they used?
One alternative would be to use one lens element of plastic. Acrylic
is a plastic of sufficiently high quality that it can be used for
optical purposes. Multi-element plastic lenses using acrylic and
polycarbonate have been used in some cameras. But when it comes to
astronomical use, the fact that plastic lenses are affected about 100
times as much by changes in temperature as glass lenses would make a
refractor using a plastic lens element flawed when it comes to service
on cold winter nights.
A light baryta flint was used as the negative element in the Cooke
photo-visual triplet; thus, a lens with three ordinary glasses still
had enough of a difference between the three elements for an
apochromatic design. Unlike a lens using a fluorite element, though,
the telescope tube still had to be long, as surfaces would be too
curved, and other abberrations too significant, beyond about f/16.
This means that a 3" telescope would have to have a 48" long tube,
like traditional 6-inch and 8-inch Newtonians.
That, in itself, wouldn't be too much of a problem; it would simply
mean that the telescope would still "look like a telescope", but it
would mean that it wouldn't offer the compactness of modern
apochromats which do use fluorite-based glasses. A telescope that
doesn't offer any obvious benefit besides sharper images, and which
superficially resembles an inexpensive refractor, might find it harder
to command a premium price - although the premium would be lower than
that of a fluorite apo of equivalent aperture, three elements still
cost more to make than two.
But that wasn't the only problem with the Cooke photo-visual. The
light baryta flint element tended to discolor on exposure to the air.
People expect products made of modern materials to be durable and long-
lasting. However, today lenses usually recieve antireflection
coatings; could a coating, and other measures, protect the negative
element of a Cooke photo-visual from the air, making this type of
objective practical once again?
John Savard |
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| Quadibloc... |
| Posted: Tue Oct 27, 2009 8:21 am |
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Guest
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On Oct 27, 11:06 am, Salmon Egg <Salmon... at (no spam) sbcglobal.net> wrote:
[quote]My guess is that the best bet will be in the development of new glasses
in which the dispersion is "bent' to give color correction across a
broad band of wavelengths with just a doublet.
[/quote]
(smacks head)
Of course; this is why apochromatic doublets are possible. In an
apochromatic triplet, either there are two positive lenses and one
negative lens, or two negative lenses and one positive lens.
One could simply mix the glasses from the two lenses of the same sign
in proportion to their powers!
Speaking of mixing things...
Rare earth metals don't have to be expensive. The flints used in
cigarette lighters are made of rare earth metals. The trick is -
they're made of "misch metal"; that is, while the rare earth metals
are extracted from the ore, they are not separated from each other.
Since the rare earth metals (and some related metals) are nearly
identical chemically, they have to be separated by difficult and
expensive methods.
If one could use the misch metal from a particular mine to make glass
that was certain to be transparent... it might still have some of the
desirable optical properties of normal rare earth glasses but at a
lower cost (although less predictable properties can be costly to work
with).
John Savard |
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| Chris.B... |
| Posted: Tue Oct 27, 2009 9:24 am |
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Guest
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Long refractors can become short refractors by folding with optical
flats. The latest mirror coatings can provide very long term
protection and ultra-low light loss for the flats.The flats are
smaller than the clear aperture of the objective and may be mass
produced given an adequate market to support their manufacture. A long
refractor offers comfortable eye relief with commonplace eyepieces
without the need for multi-elements and Barlows. The view can often
be enjoyed from a comfortable chair given a high enough pier and a
humble star diagonal. If one were to read the comments of the owners
of D&G's refractors one would be given to believe that no other
instrument is as desirable for fine detail and high resolution.
Selective coating filters can eradicate the worst of the purple haze
which seems not to spoil the image unduly and is largely seeing
related in my own experience. There are those who believe the quality
classical refractor leaves the expensive apochromat in a lowly second
place for lunar, planetary and double star work at high
magnifications. A number of optical system rely on a smaller lens to
provide chromatic correction with a large, simple objective lens which
may be made of inexpensive, water clear, long life glass. Wind is not
much of a problem in an observatory. Even those with a roll-off roof
of adequate wall height to avoid the temptation of daylight bird
watching... |
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| Quadibloc... |
| Posted: Tue Oct 27, 2009 9:49 am |
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Guest
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This interesting page:
http://rohr.aiax.de/chapter%204b.htm
notes that the Zeiss Type B objective is an improved version of the
Cooke photo-visual using modern glasses that are somewhat more stable
- although still not free from problems - than those of the original.
John Savard |
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| dkelvey at (no spam) hotmail.com... |
| Posted: Tue Oct 27, 2009 10:03 am |
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Guest
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On Oct 27, 12:37 pm, "Uber Buble'" <U... at (no spam) nospam.com> wrote:
[quote]What's the point of all of this? Go ask Roland C.
Oh..no mention of ED glass.
[/quote]
Hi
No mention of synthetic oils either.
Dwight |
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| Phil Hobbs... |
| Posted: Tue Oct 27, 2009 10:05 am |
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Guest
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Quadibloc wrote:
[quote]The refracting telescope was much improved when Fraunhofer perfected
the achromatic lens.
Newton had thought that an achromatic lens would not be possible,
since denser glasses with a higher refractive index also have more
dispersion. But it was later discovered that this relation is not
linear, and, thus, since flint glasses tend to have an increased
dispersion which is more than proportional to their increased
refractive index, the combination of a positive crown lens with a
weaker negative flint lens could allow their dispersions to balance
out while leaving useful lens power.
[/quote]
[quote]
To combine three different types of glass to bring three wavelengths
of light to a common focus, however, does generally place optical
designers in the frustrating kind of situation envisaged by Newton.
The way in which the higher dispersion of a flint glass does not
exactly cancel out with the lower dispersion of a crown glass is
pretty much constant for all ordinary glasses.
The usual solution to this is to use the mineral fluorite, calcium
fluoride, as was done in the first achromatic lenses, microscope
objectives made by Zeiss, or glasses which contain a significant
amount of fluorite. These glasses are expensive, and vulnerable to
attack by water.
[/quote]
Achromatizing via two-point matching is really a hand-calculation
technique. Because different glasses have different dispersion curves,
I would be very surprised if anyone has used analytical three-point
matching to make apochromats in a long, long time. Getting good
chromatic performance is about minimizing the error residuals, which is
much better done with a numerical optimizer, unless I'm very much mistaken.
[quote]
There are alternatives known; why aren't they used?
One alternative would be to use one lens element of plastic. Acrylic
is a plastic of sufficiently high quality that it can be used for
optical purposes. Multi-element plastic lenses using acrylic and
polycarbonate have been used in some cameras. But when it comes to
astronomical use, the fact that plastic lenses are affected about 100
times as much by changes in temperature as glass lenses would make a
refractor using a plastic lens element flawed when it comes to service
on cold winter nights.
[/quote]
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.
[quote]
A light baryta flint was used as the negative element in the Cooke
photo-visual triplet; thus, a lens with three ordinary glasses still
had enough of a difference between the three elements for an
apochromatic design. Unlike a lens using a fluorite element, though,
the telescope tube still had to be long, as surfaces would be too
curved, and other abberrations too significant, beyond about f/16.
This means that a 3" telescope would have to have a 48" long tube,
like traditional 6-inch and 8-inch Newtonians.
That, in itself, wouldn't be too much of a problem; it would simply
mean that the telescope would still "look like a telescope", but it
would mean that it wouldn't offer the compactness of modern
apochromats which do use fluorite-based glasses. A telescope that
doesn't offer any obvious benefit besides sharper images, and which
superficially resembles an inexpensive refractor, might find it harder
to command a premium price - although the premium would be lower than
that of a fluorite apo of equivalent aperture, three elements still
cost more to make than two.
[/quote]
Long refractors are heavy. They're big, for one thing, and require a
very strong tube to keep them from deflecting under gravity. They're
also very awkward. The eyepiece is far from the centre of mass, so for
visual observation you have to use a tall tripod and scamper around like
a monkey. Their length also makes them vulnerable to wind, and very
hard to put on a motorized mount for spotting and photography. All that
effort would be far better spent making an achromatic Schmidt or Maksutov.
[quote]
But that wasn't the only problem with the Cooke photo-visual. The
light baryta flint element tended to discolor on exposure to the air.
People expect products made of modern materials to be durable and long-
lasting. However, today lenses usually recieve antireflection
coatings; could a coating, and other measures, protect the negative
element of a Cooke photo-visual from the air, making this type of
objective practical once again?
John Savard
[/quote]
<IANALD warning>
Flint-in-front designs are vulnerable to weathering, as you say, and
coatings are of limited help because most coatings are not fully dense.
A very thin zero-power element or a densified coating might be a
possibility, I don't know.
The answers AFAIK are the usual: cost and performance. Rare-earth
crowns and other modern glass types have dispersion curves that lie well
off the lead-silica 'normal glass' curve, so exotic materials like
fluorite are less necessary; wide-field reflectors are cheap and
plentiful, and they work better than long refractors.
</IANALD warning>
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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| uncarollo... |
| Posted: Tue Oct 27, 2009 10:52 am |
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Guest
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On Oct 27, 12:06 pm, Salmon Egg <Salmon... at (no spam) sbcglobal.net> wrote:
[quote]Although I am not a lens designer, the comments by Phil Hobbs seem to be
very appropriate. It is a matter of economics. As you add more pieces of
glass, design gets more complicated--heavier and more expensive.
Certainly, modern computation techniques allow for nonlinear
optimization. I presume most lens codes allow for that. The problem is
that there are going to be many local optimizations. It will be
difficult to find a global optimized maximization of a figure of merit.
With modern codes, almost anyone can carry out an optimization. It will
take a good idea, if one is possible, to cut through all the Gordian
knots of lens design to get to a simple apochromat capable of color
correction while maintaining flexibility for monochromatic aberration
correction.. I do not think lens codes are up to that kind of
breakthrough. The proliferation of computing to substitute for thinking
makes such a breakthrough less likely than without computing power.
My guess is that the best bet will be in the development of new glasses
in which the dispersion is "bent' to give color correction across a
broad band of wavelengths with just a doublet.
Bill
[/quote]
1) New glasses are NOT being developed for the amateur telescope
market because the market is way too small. The market for glass is
driven by the camera industry, and they have no use for what we
telescope people need. In fact, many useful glasses from the past are
no longer being made for environmental reasons (radiation, containing
lead and other poisons, etc).
2) Making an apochromat is a very simple thing. All you need are two
glasses with significantly different dispersion and equal partial
dispersion. Every glass company has a chart showing this graph of
dispersion versus partial dispersion, which makes it a 5 second job to
pick two glasses that will produce an apochromat (the job is made even
simpler because there are less than 1 dozen glasses that you can
choose from as the charts will quickly reveal). A modern lens design
program can then be used to calculate the curves needed to achieve
color, spherical and coma correction. Any grade school kid can do it
with maybe 20 minutes of orientation.
3) there is no such thing as a secret design or design method to
produce apo lenses. An apo is not produced by a "design" it is
produced by two (or more) glasses having widely different dispersions
and equal partial dispersion. Glass index is of no consequence, only
dispersion matters.
4) of the dozen or so useful glasses, there are really only 3 or 4
that can actually be used because the rest are not of high enough
quality to form a diffraction limited image (remember that light must
pass thru the glasses, and therefore the internal variation of
refractive index must be very very low so as not to introduce
astigmatisnm and coma). The optical designer is limited in what he can
use by what the glass companies are making in high homogeneity
material. Most optical glass has poor internal homogeneity, so you can
design all you want, and it will be for naught.
Rolando |
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| Ken S. Tucker... |
| Posted: Tue Oct 27, 2009 11:04 am |
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Guest
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On Oct 27, 1:31 pm, "Bob May" <bob... at (no spam) nethere.com> wrote:
[quote]First off, congrats on finding out that the glass curvs ar nonlineear and
thus you can't really do a short FL lens of perfect image quality. Please
do note that there are glassses that are a long way from the normal glass
line and these glasses will make reefractors of fairly high quality. It is
possible with more elements to get betttter performance but you will never
get to the performance of a reflecting system in any way. Lenses do have a
place but the specs of their usage doesn't; include any need for diffraction
limiteed (all light inside of the Airy Disc) performance in anything but
small lenses.
Bob May
[/quote]
I once built a refractor telescope from a 4" magnifying glass
that fed light into a 2" concave 8"-10" behind it, then into an eye
piece that pretty much eliminated chromatic aberation, and was
ok on spherical aberation, but I only tested it on fairly low power
to be used as a bright field scope.
Ken |
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| Salmon Egg... |
| Posted: Tue Oct 27, 2009 11:06 am |
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Guest
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<parts snipped>
In article <rKqdnWWHQozPh3rXnZ2dnUVZ_oOdnZ2d at (no spam) supernews.com>,
Phil Hobbs <pcdhSpamMeSenseless at (no spam) electrooptical.net> wrote:
[quote]Quadibloc wrote:
The refracting telescope was much improved when Fraunhofer perfected
the achromatic lens.
To combine three different types of glass to bring three wavelengths
of light to a common focus, however, does generally place optical
designers in the frustrating kind of situation envisaged by Newton.
The way in which the higher dispersion of a flint glass does not
exactly cancel out with the lower dispersion of a crown glass is
pretty much constant for all ordinary glasses.
Achromatizing via two-point matching is really a hand-calculation
technique. Because different glasses have different dispersion curves,
I would be very surprised if anyone has used analytical three-point
matching to make apochromats in a long, long time. Getting good
chromatic performance is about minimizing the error residuals, which is
much better done with a numerical optimizer, unless I'm very much mistaken.
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.
Long refractors are heavy. They're big, for one thing, and require a
very strong tube to keep them from deflecting under gravity. They're
also very awkward. The eyepiece is far from the centre of mass, so for
visual observation you have to use a tall tripod and scamper around like
a monkey. Their length also makes them vulnerable to wind, and very
hard to put on a motorized mount for spotting and photography. All that
effort would be far better spent making an achromatic Schmidt or Maksutov.
John Savard
Flint-in-front designs are vulnerable to weathering, as you say, and
coatings are of limited help because most coatings are not fully dense.
A very thin zero-power element or a densified coating might be a
possibility, I don't know.
The answers AFAIK are the usual: cost and performance. Rare-earth
crowns and other modern glass types have dispersion curves that lie well
off the lead-silica 'normal glass' curve, so exotic materials like
fluorite are less necessary; wide-field reflectors are cheap and
plentiful, and they work better than long refractors.
By the way. what is a
/IANALD warning
[/quote]
Although I am not a lens designer, the comments by Phil Hobbs seem to be
very appropriate. It is a matter of economics. As you add more pieces of
glass, design gets more complicated--heavier and more expensive.
Certainly, modern computation techniques allow for nonlinear
optimization. I presume most lens codes allow for that. The problem is
that there are going to be many local optimizations. It will be
difficult to find a global optimized maximization of a figure of merit.
With modern codes, almost anyone can carry out an optimization. It will
take a good idea, if one is possible, to cut through all the Gordian
knots of lens design to get to a simple apochromat capable of color
correction while maintaining flexibility for monochromatic aberration
correction.. I do not think lens codes are up to that kind of
breakthrough. The proliferation of computing to substitute for thinking
makes such a breakthrough less likely than without computing power.
My guess is that the best bet will be in the development of new glasses
in which the dispersion is "bent' to give color correction across a
broad band of wavelengths with just a doublet.
Bill
--
As the years go by, dying just before having to fill out a tax return has merit. |
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| Phil Hobbs... |
| Posted: Tue Oct 27, 2009 12:15 pm |
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Guest
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Salmon Egg wrote:
[quote]parts snipped
In article <rKqdnWWHQozPh3rXnZ2dnUVZ_oOdnZ2d at (no spam) supernews.com>,
Phil Hobbs <pcdhSpamMeSenseless at (no spam) electrooptical.net> wrote:
By the way. what is a
/IANALD warning
Although I am not a lens designer, the comments by Phil Hobbs seem to be
very appropriate. It is a matter of economics. As you add more pieces of
glass, design gets more complicated--heavier and more expensive.
Certainly, modern computation techniques allow for nonlinear
optimization. I presume most lens codes allow for that. The problem is
that there are going to be many local optimizations. It will be
difficult to find a global optimized maximization of a figure of merit.
You said it yourself: I Am Not A Lens Designer.  [/quote]
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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| Phil Hobbs... |
| Posted: Tue Oct 27, 2009 1:34 pm |
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Guest
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Chris.B wrote:
[quote]Long refractors can become short refractors by folding with optical
flats. The latest mirror coatings can provide very long term
protection and ultra-low light loss for the flats.The flats are
smaller than the clear aperture of the objective and may be mass
produced given an adequate market to support their manufacture. A long
refractor offers comfortable eye relief with commonplace eyepieces
without the need for multi-elements and Barlows. The view can often
be enjoyed from a comfortable chair given a high enough pier and a
humble star diagonal. If one were to read the comments of the owners
of D&G's refractors one would be given to believe that no other
instrument is as desirable for fine detail and high resolution.
Selective coating filters can eradicate the worst of the purple haze
which seems not to spoil the image unduly and is largely seeing
related in my own experience. There are those who believe the quality
classical refractor leaves the expensive apochromat in a lowly second
place for lunar, planetary and double star work at high
magnifications. A number of optical system rely on a smaller lens to
provide chromatic correction with a large, simple objective lens which
may be made of inexpensive, water clear, long life glass. Wind is not
much of a problem in an observatory. Even those with a roll-off roof
of adequate wall height to avoid the temptation of daylight bird
watching...
[/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 net
http://electrooptical.net |
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| Uber Buble... |
| Posted: Tue Oct 27, 2009 1:37 pm |
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Guest
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What's the point of all of this? Go ask Roland C.
Oh..no mention of ED glass.
http://www.birdforum.net/attachment.php?s=45b77493b44e87d65474232a3aabe42f&attachmentid=64759&d=1164415946
"Quadibloc" <jsavard at (no spam) ecn.ab.ca> wrote in message
news:f6656730-7c78-48c7-a99e-3b81db488687 at (no spam) v15g2000prn.googlegroups.com...
On Oct 27, 11:06 am, Salmon Egg <Salmon... at (no spam) sbcglobal.net> wrote:
[quote]My guess is that the best bet will be in the development of new glasses
in which the dispersion is "bent' to give color correction across a
broad band of wavelengths with just a doublet.
[/quote]
(smacks head)
Of course; this is why apochromatic doublets are possible. In an
apochromatic triplet, either there are two positive lenses and one
negative lens, or two negative lenses and one positive lens.
One could simply mix the glasses from the two lenses of the same sign
in proportion to their powers!
Speaking of mixing things...
Rare earth metals don't have to be expensive. The flints used in
cigarette lighters are made of rare earth metals. The trick is -
they're made of "misch metal"; that is, while the rare earth metals
are extracted from the ore, they are not separated from each other.
Since the rare earth metals (and some related metals) are nearly
identical chemically, they have to be separated by difficult and
expensive methods.
If one could use the misch metal from a particular mine to make glass
that was certain to be transparent... it might still have some of the
desirable optical properties of normal rare earth glasses but at a
lower cost (although less predictable properties can be costly to work
with).
John Savard |
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| Quadibloc... |
| Posted: Tue Oct 27, 2009 1:55 pm |
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Guest
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On Oct 27, 10:05 am, Phil Hobbs
[quote]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.
[/quote]
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 |
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| Bob May... |
| Posted: Tue Oct 27, 2009 2:31 pm |
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Guest
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First off, congrats on finding out that the glass curvs ar nonlineear and
thus you can't really do a short FL lens of perfect image quality. Please
do note that there are glassses that are a long way from the normal glass
line and these glasses will make reefractors of fairly high quality. It is
possible with more elements to get betttter performance but you will never
get to the performance of a reflecting system in any way. Lenses do have a
place but the specs of their usage doesn't; include any need for diffraction
limiteed (all light inside of the Airy Disc) performance in anything but
small lenses.
--
Bob May
rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net |
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| Rich... |
| Posted: Tue Oct 27, 2009 6:41 pm |
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Guest
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"Chris.B" <chris.b at (no spam) nypost.dk> wrote in news:c00c9c05-cd72-494e-9f9d-
59b432b51a32 at (no spam) b3g2000pre.googlegroups.com:
[quote]Long refractors can become short refractors by folding with optical
flats. The latest mirror coatings can provide very long term
protection and ultra-low light loss for the flats.The flats are
smaller than the clear aperture of the objective and may be mass
produced given an adequate market to support their manufacture.
[/quote]
Most people can't align an SCT secondary, let alone 2-3 optical flats and
an objective... |
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