Telecentric Lens Systems

[colinsk]

I am working on documenting sources for telecentric lens systems.

This was perhaps the first thing I found years ago:

http://www.edmundoptics.com/technical-s ... ne-vision/

Gene, RIP, wrote this article:

http://home.comcast.net/~g2baraff/Solar ... g/Tele.pdf

The additional reading (except for what I wrote :dry: ) is interesting:

Additional Reading
General etalon theory and solar usage:
http://en.wikipedia.org/wiki/Fabry-Perot_interferometer
http://www.cloudynights.com/item.php?item_id=1791
http://www.cloudynights.com/item.php?item_id=2015
Telecentric Lenses and Systems in Optical Metrology: The following describe various
telecentric systems but I found them somewhat peripheral to etalon use.
http://www.edmundoptics.com/technical-s ... entricity/
http://www.lhup.edu/~dsimanek/scenario/ ... elecen.htm
http://www.opto-engineering.com/brochur ... Lenses.pdf
Telecentric Systems and Etalon Use:
http://www.cloudynights.com/ubbthreads/ ... /page/4/vi
ew/collapsed/sb/5/o/all/fpart/1
http://www.cloudynights.com/ubbthreads/ ... /page/0/vi
ew/collapsed/sb/5/o/all/fpart/1
http://www.alpineastro.com/Reference_Do ... System.pdf
http://www.brayebrookobservatory.org/Br ... owLens.pdf
http://www.sungazer.net/tele.html

[colinsk]

Ok, so of the three kinds of Telecentric systems:

Object Space Telecentric
Image Space Telecentric
Double Telecentric

We appear to need Image Space Telecentrics.

From WikiPedia:

Image-space telecentric lenses

An image-space (or image-side) telecentric lens produces images of the same size regardless of the distance between the lens and the film or image sensor. This allows the lens to be focused to different distances without changing the size of the image. Image-space telecentric lenses have an exit pupil infinitely far in front of the lens; that is, if you look in the back of the lens, the apparent aperture is very far away.
At the film or image sensor, all of the chief rays from these lenses hit "straight on", or at zero angle of incidence. This property minimizes any angle-of-incidence dependence of the sensor, or of any beam-splitter prism assembly behind the lens, such as a color separation prism in a three-CCD camera. Many lenses that have been specially optimized for digital SLR cameras are nearly telecentric on the image side, to avoid the vignetting and color crosstalk that occur in color filter array-based digital image sensors with oblique incident rays. The Four Thirds System uses this approach.
Since the ray cones approaching the detector surface have the same angle of incidence and angular subtense everywhere in the image plane, the image is evenly illuminated. This feature is commonly used in photography, and is very useful for radiometric and colour measurement applications where one would need the irradiance to be the same regardless of the field position.
Lenses that are double-telecentric have magnification that is more precisely constant than those that are only object-side telecentric, because the principal ray intecept position on the detector doesn't change. This property allows precise measurement of objects regardless of position.

[colinsk]

This link is quite helpful:

http://www.schneiderkreuznach.com/pdf/d ... lenses.pdf

[colinsk]

This paper may be relevant:

Advanced technology solar telescope multiple Fabry-Pe´rot interferometer telecentric optical design
Robinson, Brian; Balasubramaniam, K. S.; Gary, G. Allen
Optical Engineering , Volume 45 (2)
SPIE – Feb 1, 2006

The next time I am at the UC Davis library I'll try to get a copy.

[Bob Yoesle]

Hi Colin,

Here's an interesting telecentric design using off-the-shelf optics from Edmund Optical I found on Peter Hobel's German site (with all his filter transmission graphs):

http://translate.google.com/translate?h ... x-tzs.html

Peter appears to have used Telescope Optics: Evaluation and Design. A Comprehensive Manual for Amateur Astronomers By H. Rutten and M vanVenrooij to write a program which was checked "OpTaliX."

[colinsk]

Bob, That is my main book on lens design that I have found useful. I also like the software but it is DOS so it is a little more difficult to use. The math in the software is really useful. If you can get a copy it will be very helpful. I should have a copy of the floppy that came with the book on an old hard drive.

[colinsk]

i took the liberty of finding the lenses used in the link you provided.

i bought a couple before they "disappear"

www.surplusshed.com item L8175 and PL1154




Don't forget that a telecentric system is only telecentric at the same focal length as the design focal length. I can't wait to see the results!

[Bob Yoesle]

Rusty - That was my thought as well! But you better hurry, I understand from somewhere Lunt may soon no longer offer the LS35, or at least the DS filter... Should be an interesting project!

Collin - I have the book and software somewhere as well - probably burried in the garage!

[marktownley]

[quote]Rusty - That was my thought as well! But you better hurry, I understand from somewhere Lunt may soon no longer offer the LS35, or at least the DS filter... Should be an interesting project!



i found the picture of the new ls35..
starizona





Reminds me of my Stellarvue SV50 Nighthawk finder scope...

[Merlin66]

What's that small brass piece just above her fingers in the Dew shield area?
Could it be a tuning knob?? Hmmmm - a small external etalon????

[colinsk]

I bought a few papers and was able to confirm that we want Image space telecentric systems and not object space telecentric systems:

Classification 2: Image-Space Telecentricity
It occurs when the system stop is placed at the rear focal plane of the lens, resulting in an exit pupil location at infinity. A shift in the image plane does not affect image magnification.



Figure 2: 0.5X Image-Space Telecentric Lens (Note the Parallel Chief Rays in Image Space)

Optical Design for a
Fabry-Pérot Image Interferometer for Solar Observations
Gil Moretto(1), G. Allen Gary(2), K.S. Balasubramaniam(1), Thomas R. Rimmele(1)
(1) National Solar Observatory, Sacramento Peak, Sunspot, 88349, New Mexico, USA
(2) Marshall Space Flight Center, NASA – Huntsville, Alabama 35812, USA

Optical Design and Specification of Telecentric Optical Systems
Michael A. Pate
Hewlett-Packard Co.
1000 N.E. Circle Blvd., MS 522A
Corvallis, OR 97330

The first paper talks about a f/300 telecentric system!

[colinsk]

And conversely, a telecentric optical
system in image space is a system where the aperture stop is located at the front focal
distance of the group of lenses in back of the aperture stop.
Another definition that is used by optical designers is that the chief ray is perpendicular
to the object (or image) plane over the full field of view in an object (or image) side
telecentric system — I call this the chief ray definition. Because the chief ray definition is
based upon the geometric optics principles, I also call this definition geometric
telecentricity which concurs with most of the existing definitions. It is this chief ray
definition that optical designers often use to quantify the geometric telecentricity of their
optical design during optimization.

[colinsk]

The one parameter is the location of the front focal point of the group of lenses behind
the aperture stop must be located at the aperture stop. Since most optical designs are
multi-element solutions the designer must often perform a gaussian reduction on the
group of optical elements behind the aperture stop to reduce it into a single lens with a
position. power, and front focal distance. For the lens to be telecentric the constraint that
the front focal point is located exactly at the aperture stop must be maintained. If this
design constraint is maintained then the aperture stop, when viewed from image space.
will appear to be located at infinity. Recall that the image of the aperture stop from
image space is the definition of the exit pupil. In an image side telecentric system the
exit pupil is located at infinity.

[colinsk]

For those that can afford ZMAX it appears to be pretty straight forward.

http://www.radiantzemax.com/kb-en/Knowl ... 50057.aspx

[colinsk]

Figure 7 from this link is useful:

http://solar.njit.edu/~cdenker/physics6 ... rument.pdf

[colinsk]

Kentischer, T.J., Schmidt, W., Sigwarth, M., & Uexk¨ull, M.v. 1998,
TESOS, a double Fabry-Perot instrument for solar spectroscopy.
A&A 340, 569-578

This paper is helpful as well.

In the telecentric configuration, lenses L1 and L2 project the
solar image (FS) into the interferometer. Lenses L3 and L4 image the
solar image onto the detector (FP2).

This is another way of saying the exit pupil is collimated.

Gene's section 6 is spot on for understanding a new design. Now the hard part is deriving a couple of lens equations to make a custom telecentric.

What Gene does not cover at all is the induced aberrations of the added lenses and how to minimize them.

[colinsk]

I found this today:

http://www.brayebrookobservatory.org/Br ... owLens.pdf

[colinsk]

I spent a few hours as UC Davis Library today. I had access to many papers that I had wanted to read. These were of interest:

The effect of telecentric use of narrow-band filters on diffraction limited imaging - J. M. Beckers

The paper is the famous Beckers' Effect paper.

Optical Design for a Fabry-Perot Image Interferometer for Solar Observations - Gil Moretto

Advanced technology solar telescope multiple Fabry-Perot interferometer telecentric optical design - Brian Robinson

[colinsk]

I also found this book:

http://spie.org/x648.html?product_id=540692

It was quite useful with practical examples of telecentric design.

[colinsk]

Here is another interesting paper:

http://atst.nso.edu/files/docs/mfpi_telecentric.pdf

[colinsk]

From the above link regarding the Beckers' Effect:

"In the telecentric configuration there is no variation in transmittance across the
image itself, but the spectral resolution is degraded, since a cone of rays of finite angular
extent converges on each image point within the etalons."

[colinsk]

This thread was continued here:

http://solarchat.natca.net/index.php/en ... ns-systems