# FOV for galilean vs keplerian telescope

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1. Jun 26, 2013

### fsonnichsen

It is my understanding that a Galilean telescope has a smaller field of view (FOV) than a Keplerian. Doing some "pencil" ray tracing on a sheet of paper I don't seem to get this result. Can anyone suggest a source that works the math for the FOV of both types?

thanks
Fritz

2. Jun 26, 2013

### Staff: Mentor

I'm not sure, but while searching I did come across this interesting website. Give it a look and see if it helps you.
http://www.scitechantiques.com/Galileo-Telescope-Anomalies-optics/

If you look towards the bottom you will see some ray tracing with an arrow pointing to the galilean design. It appears that the smaller field of view is due to the light coming out of the eyepiece the opposite way that it does using a normal eyepiece. AKA the incoming light that enters the eyepiece towards the bottom is refracted even more towards the bottom. With a positive lens as an eyepiece the light entering the bottom of the eyepiece is refracted UP. This results in a very small FOV with a negative lens as an eyepiece, but with the ability to move your eye around to see approximately the same FOV that one sees with a positive eyepiece. (According to the website)

Anyways, that's what I picked up from the site. I didn't do the math or ray tracing myself, so I can't guarantee it's correct.

3. Jun 26, 2013

### fsonnichsen

Thanks. This page offers some interesting information. It confirms my suspicion that the FOV in this case is being measured including the eye pupil as part of the aperture / stop. FOV is somewhat vague to me, being rather aligned with cameras I think. At any rate from the standpoint of light collecting ability, I think the Kepler and Galileo systems are equal for a given objective lens. (I am using these constructs as a collimator to collect light into a PMT). The Galileo is, in practice, a bit easier to deal with on the bench since it is afocal.

Thanks for the post!
Fritz

4. Jun 26, 2013

### Andy Resnick

Both Galilean and Keplerian designs are afocal. The essential difference is that the Galilean places the exit pupil on the inside of the telescope, while the Keplerian places it outside of the telescope. Thus, the Galilean design suffers from vignetting, reducing the field of view.

Its not clear if this impacts your application- if the angular FOV of each PMT is small (a few degrees), there may not be a difference.

5. Jun 28, 2013

### fsonnichsen

Andy-just curious-- how is a Keplerain system afocal? Perhaps I am misusing the term.

In the Galilean system the concave lens basically forms a collimator in the case of infinitely distant objects, thus the resulting parallel beam does not have a focal point (and is easy to route around the optical bench since I am splitting into spectrometers etc).

To my understanding the Keplerian system has its convex eyepiece lens placed past the focal point of the objective, and re-converges the light to a focal point yes? Thus it is "focal".

I concur that there is not resulting difference in the light collecting ability of either system if the PMT has a linear response across its face. (In my final design I intend to forgo the 2nd lens completely)

Thanks!
Fritz

6. Jun 28, 2013

### Andy Resnick

7. Jun 28, 2013

### fsonnichsen

OK-makes sense
thanks
fs