Re: tube lens and spherical aberration

Posted by James Pawley on
URL: http://confocal-microscopy-list.275.s1.nabble.com/Southeastern-Microscopy-Society-meeting-tp4640594p4670301.html

Thank you, Jim and others who replied to my question! - Mike

You are welcome.

I forgot one more point that you might find interesting.

As the proper correction of SA depends on the image plane being a specific distance behind the tube lens, it follows that one can change the SA correction by changing this distance. Indeed, in older microscopes, one could adjust the tube length specifically to give the best SA correction under whatever imaging conditions were in use.

Of course, changing the tube length also changes the magnification and this is presumably the main reason why modern instruments lack this convenient adjustment.

Cheers,

Jim Pawley

              **********************************************
Prof. James B. Pawley,                                          Ph.  608-263-3147 
Room 223, Zoology Research Building,                                  FAX  [phone]
1117 Johnson Ave., Madison, WI, 53706                                [hidden email]
3D Microscopy of Living Cells Course, June 12-24, 2010, UBC, Vancouver Canada

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               "If it ain't diffraction, it must be statistics." Anon.

-----Original Message-----
From: Confocal Microscopy List [mailto:[hidden email]] On Behalf Of James Pawley
Sent: Tuesday, March 02, 2010 11:55 PM
To: [hidden email]
Subject: Re: tube lens and spherical aberration

Hi Mike,

I agree with Guy. In fact going back to the "mental optical diagram"
in your original post, the "blurry spot" that you suggest occurs in
the intermediate image plane (and, larger, on your retina) is just
the caustic that he describes. This is not only because the rays
"fail to come to a common focus," but because they do this in a
specific way: the rays from around the periphery of the optic come to
a focus that is either closer to it or farther away from from it than
the focus of the paraxial rays near the optical axis.

However, regarding: "(I suspect, it's not because even the distance
to the tube lens should make a difference)", in the case the tube
lens is essentially part of the objective. Together, (and only
together)  they will form an aberration-free image a certain distance
behind the tube lens.

Wrong tube lens, NG.

Wrong distance: NG.

In short, one can ONLY design an aberration-free optical system once
one has first defined the positions of the object and final image
planes (In incoherent imaging  such as fluorescence, the optical
system is everything between the object and final image plane). This
is why we have objectives designed for different magnifications. One
could use a 100x objective is a lens with a focal length of about 2
mm. Despite what is written on the barrel, it could be used to form
an image that was, say, only 15x larger than the object. But if you
positioned the objective to make this occur, the plane at which this
this 15x image appears will be near the back of the objective itself
and it would be highly aberrated (and aberrated not just because the
light had yet to pass through the tube lens. It would even be
aberrated if the objective was a fixed-conjugate, rather than an
infinity, objective).

When I learned microscopy, I was taught that, although SA could be
completely corrected in the LM, as all EM (cylindrical) lenses
intrinsically had positive SA it could not be corrected in EM. Now it
is the EM guys who have corrected SA (by employing multipole lenses)
and the LM'ers now have a serious SA problems because they have
started to look at thick specimens that have the "wrong" RI.

Must be time to retire.

Jim Pawley

               **********************************************
Prof. James B. Pawley,                                         Ph.
608-263-3147 Room 223, Zoology Research Building,
FAX  [phone]
1117 Johnson Ave., Madison, WI, 53706  [hidden email]
3D Microscopy of Living Cells Course, June 12-24, 2010, UBC, Vancouver Canada

Info: http://www.3dcourse.ubc.ca/            Applications due by March 15, 2010
       "If it ain't diffraction, it must be statistics." Anon.

>I don't think that is true.  A lot of published versions start out from
>an objective forming an image of a point object.
>
>Many years ago I got a bit incensed at a diagram in a textbook,
>purporting to show SA introduced by a coverslip.  The rays leaving the
>coverslip were not parallel to those entering it!  I wanted a diagram to
>show my students, and this obviously wasn't it.
>
>So I sat down with pencil, paper and ruler.  First of all I drew a
>perfect lens forming an image of a point.  Then I drew a coverslip in
>the way, looked up the sines of the angles as measured, and constructed
>the ray paths.  Lo and behold, there was the classic caustic curve.
>(And, need I say, the rays exited the coverslip at the same angle they
>entered.)  So I traced it on OHP film and used it in my lectures
>thereafter (long before Powerpoint!).  And the students were much more
>willing to take it on board since it was constructed without any complex
>formulae - nothing more than Snell's Law.
>                                                     Guy
>
>-----Original Message-----
>From: Confocal Microscopy List [mailto:[hidden email]]
>On Behalf Of MODEL, MICHAEL
>Sent: Saturday, 27 February 2010 7:55 AM
>To: [hidden email]
>Subject: tube lens and spherical aberration
>
>(I apologize if this is a second post, I got some strange errors from
>the first one)
>
>Dear List
>
>It's not a confocal question, it's strictly a wide-field question...
>When people talk about spherical aberration they typically start with a
>parallel beam falling on an objective, and then, for whatever reason,
>the objective fails to bring all the rays into a common focus. In
>wide-field, one has a luminous spot in the object space, and the effect
>of spherical aberration would be a failure of the objective to collect
>all the rays into a single parallel beam. Some rays will form a
>converging (or diverging) cone instead. Then, this cone will be received
>(I suspect that in the case of a diverging cone, some light may even get
>lost on its way) by a tube lens and form a blurry spot on the image
>plane. My question is, Is this situation really equivalent to the
>standard one considered in all books? (I suspect, it's not because even
>the distance to the tube lens should make a difference). But I would be
>very interested in the opinion of those who understand optics better
>than I do. Thanks!
>
>Mike Model
>
>Michael Model, Ph.D.
>Confocal Microscopy,
>Dpt Biological Sciences,
>1275 University Esplanade,
>Kent State University, Kent, OH 44242
>tel. [phone]
>
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>06:57:00


--
               **********************************************
Prof. James B. Pawley,                                         Ph.
608-263-3147 Room 223, Zoology Research Building,
FAX  [phone]
1117 Johnson Ave., Madison, WI, 53706  [hidden email]
3D Microscopy of Living Cells Course, June 12-24, 2010, UBC, Vancouver Canada
Info: http://www.3dcourse.ubc.ca/      Applications due by March 15, 2010
       "If it ain't diffraction, it must be statistics." Anon.
--
James and Christine Pawley, 21 N. Prospect Ave. Madison, WI, 53726
Phone: [phone]

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