How to convert 160mm obj into infinity-corrected objective?
Jeremy Levy
into an infinity-corrected objective. The objective is not part of a
conventional microscope, so I have some flexibility on where to put it, etc.
The question is: what kind of lens should I use, and where should I put it?
My main concern is to minimize abberations. You might well be wondering,
why not just buy another objective? The answer is that this is the only
objective that I have found that will not crack when cycled to 4 Kelvin.
Any wisdom on this would be greatly appreciated. (Please respond by e-mail:
jl...@pitt.edu)
Jeremy Levy
Leonard Migliore
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In article <84mbu9$s1m$1...@usenet01.srv.cis.pitt.edu>, Jeremy Levy
<jl...@pitt.edu> wrote:
No wisdom from me, but my wild guess is that any *simple* lens you use
would introduce more aberrations than just using the 160-mm corrected
objective as is.
Bob May
160mm focal length? What do you mean by infinity-corrected?
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Erik Reuter
>I think that I am missing a bit here. Is that a 160mm diameter or
>160mm focal length? What do you mean by infinity-corrected?
160mm is a common "tube length" for microscopes (traditional). It is
roughly the distance between the objective lens and the eyepiece. The
focal length of such an objective (designed for 160mm tube length) is
about 160mm / magnification. When the objective is focused, the object
being viewed will be slightly further away than the focal length.
A more recent type of microscope objective is "infinity corrected". The
objective lens is designed to have the image focused at infinity
(the object being viewed is precisely at the focal distance), so the
real, usable image is produced by a "tube lens" somewhere between the
objective lens and the target or eyepiece. I think this is done so that
various optics (beamsplitters, filters, etc.) can be inserted into the
tube in the collimated portion, thus introducing fewer aberrations.
I'm not a microscopist, so feel free to correct me if I have misstated
anything.
--
"Erik Reuter" <ere...@erikreuter.net> http://www.erikreuter.com/
Bob May
source light that's some diameter. This is done by taking the
converging beam from the microscope objective and diverging it into a
parallel beam. You can do this with either a pair of positive lenses
or with a single (implied is that a acromatic lens set is a single
lens) negative lens.
The regular microscope is intended to focus the light from the
objective to a point 160mm (as you said) where an image is formed and
the EP looks at that image and shows it to your eye which wants a
parallel light beam to focus to. To play around, make a small
telescope of some small power and focus it at infinity and small
enough to fit inside of the microscope. Then put a negative lens
inside the tube so that you can move it up and down (you can attach it
to the end of the telescope if you wish) and move the whole assembly
up and down until you can get a focus without having to move the stage
to refocus the image. You will probably want to make a minor
adjustment to that distance afterwords and after the parallel part of
the beam (however many times you split it up), you will want to
introduce a positive lens with the same FL as the negative lens to
allow for the focusing of the image. This should be just what you
want.
Hope this helps you and I know that others will jump in and help if
anything more is really needed.
Julio Serna Galán
Jeremy Levy wrote:
>
> I am looking to add a simple lens to convert a 160-mm corrected objective
> into an infinity-corrected objective. The objective is not part of a
> conventional microscope, so I have some flexibility on where to put it, etc.
> The question is: what kind of lens should I use, and where should I put it?
> My main concern is to minimize abberations. You might well be wondering,
> why not just buy another objective? The answer is that this is the only
> objective that I have found that will not crack when cycled to 4 Kelvin.
>
> Any wisdom on this would be greatly appreciated. (Please respond by e-mail:
> jl...@pitt.edu)
>
> Jeremy Levy
I am really far from being an expert in that field, but I would be happy
if someone sends a simple answer to that question. My guess is that you
need a negative lens close to the objective if you want to keep the tube
short. In a 160 mm microscope the intermediate image is formed in the
front focal point of the eyepiece. That's where you want to have the
object focus of your negative lens, so I would choose a focal length
somewhat longer than -160 mm (note the negative sign), since the image
focus of the objective is inside the objective. I think that the
standard for a 160 mm objective is 10 mm inside the mounting thread, so
you need to be over -150 mm. And since aberration correction is
important I would use a negative achromat. Just one problem: negative
achromats are two cemented lenses, so I guess they would not be happy
being cycled at 4 K. But maybe the lens does not need to be in contact
with the objective. Otherwise I would do what Leonard Migliore suggests:
try ignoring the problem, you could worsen it. Or maybe try with a
positive achromat further away from the 160 mm point.
Any real expert out there?
Julio Serna
Jerry
This is a standard ploy by microscope manufacturers in order to
uncouple the movement of the objective from other components and to
provide a more parallel beam for certain optical components. The extra
lens is usually an achromat and has negative focal length. For example,
if the primary image formed by the objective is 150mm from the objective
flange and the extra lens is mounted 10mm from the flange then the
nominal focal length of the extra lens is -140mm. It used to be common
for a microscope manufacturer to reduce the constraints on objective
design by leaving some abberations uncorrected and then correcting them
in the ocular. If you have such an objective then it is highly probable
that the uncorrected abberations will be compounded by any extra lens
you add. However the trend for several years now has been to put all the
needed corrections in the objective. In this case adding an achromat
will have minimal effect on image quality. I have seen singlet lenses
used in this application successfully when UV transmission was required.
Please note that the 160mm specification is the mechanical tube length
from the objective flange to the ocular flange. The primary image is
always below the ocular flange. There was no standard on exactly where
to put the primary image but it always seemed to be in the range of
150-152mm from the objective flange.
HTH,
Jerry
Mark W. Lund, PhD
think that Leonard's reply was the best, a little bit of defocus will make
the 160 mm an infinity, and the aberrations should be small. Alternatively
take a 160 mm FL negative lens and put it on top of the objective. If
it is thin enough it should not affect the aberrations much, since it is
very weak. Which method makes the worst aberrations is a toss up
without getting specific.
best regards
mark
Julio Serna Galán wrote:
> Jeremy Levy wrote:
> >
> > I am looking to add a simple lens to convert a 160-mm corrected objective
> > into an infinity-corrected objective. The objective is not part of a
> > conventional microscope, so I have some flexibility on where to put it, etc.
> > The question is: what kind of lens should I use, and where should I put it?
> > My main concern is to minimize abberations. You might well be wondering,
> > why not just buy another objective? The answer is that this is the only
> > objective that I have found that will not crack when cycled to 4 Kelvin.
> >
> > Any wisdom on this would be greatly appreciated. (Please respond by e-mail:
> > jl...@pitt.edu)
> >
> > Jeremy Levy
>
> I am really far from being an expert in that field, but I would be happy
> if someone sends a simple answer to that question. My guess is that you
> need a negative lens close to the objective if you want to keep the tube
> short. In a 160 mm microscope the intermediate image is formed in the
> front focal point of the eyepiece. That's where you want to have the
> object focus of your negative lens, so I would choose a focal length
> somewhat longer than -160 mm (note the negative sign), since the image
> focus of the objective is inside the objective. I think that the
> standard for a 160 mm objective is 10 mm inside the mounting thread, so
> you need to be over -150 mm. And since aberration correction is
> important I would use a negative achromat. Just one problem: negative
> achromats are two cemented lenses, so I guess they would not be happy
> being cycled at 4 K. But maybe the lens does not need to be in contact
> with the objective. Otherwise I would do what Leonard Migliore suggests:
> try ignoring the problem, you could worsen it. Or maybe try with a
> positive achromat further away from the 160 mm point.
>
> Any real expert out there?
>
> Julio Serna
--
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Stefan Mueller-Pfeiffer
>> Any wisdom on this would be greatly appreciated. (Please respond by e-mail:
>> jl...@pitt.edu)
>>
>> Jeremy Levy
>
>No wisdom from me, but my wild guess is that any *simple* lens you use
>would introduce more aberrations than just using the 160-mm corrected
>objective as is.
Who ist the manufacturer of your lens? Some of the microscope
manufacturers (at least Zeiss does AFAIK) sell adaption optics to use
their objective lenses with their (!) infinity corrected systems.
I.e. still not need a fitting tube lens, to make an intermediate image
at a finite distance.
Please note, that even infinity corrected objective lenses are *not*
easily interchangeable between microscope stands of different
manufactureres, at least if you pay attention to image quality.
Regards,
Stefan
--
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