# Galilean beam expander and spherical aberration

1. Dec 2, 2008

Sorry for posting this post in two different place, I could not decide where is the best place to post it.

I am trying to build a Galilean beam expander/contraction using a plano-convex and plano-convace lenses. My question is how should I arrange them to reduce the spherical aberration?
Should the two plano surfaces face each other (http://www.mellesgriot.com/glossary/imagesDir/Gallilean.gif" [Broken]), the two curvatures surfaces facing one another, or some different way.

I need to know that both for a beam expander and beam contraction.

Thank you

Last edited by a moderator: May 3, 2017
2. Dec 2, 2008

### mgb_phys

General rule is 'split the powers' = minimise the amount of curvature at each face.
That is the curved face should face the colimated beam and the flat face - the diverging/converging beam.

3. Dec 2, 2008

What is confusing me is that according to this document from Melles Griot
http://www.mellesgriot.com/pdf/CatalogX/X_01_27-28.pdf" [Broken]
they should be aligned in a different way ( see figure 1.32 )

So what way reduces the spherical aberration better?

Thanks

Last edited by a moderator: May 3, 2017
4. Dec 2, 2008

### mgb_phys

I think their catalogue is just showing a diagram, this one has more detail
http://www.edmundoptics.com/TechSupport/DisplayArticle.cfm?articleid=270 [Broken]

Last edited by a moderator: May 3, 2017
5. Dec 3, 2008

I do not think it is only a diagram, they are driving an example.

Last edited by a moderator: May 3, 2017
6. Dec 13, 2008

### Redbelly98

Staff Emeritus
Surpisingly, they are showing it wrong for the negative lens. Note in Fig. 1.31, that arrangement has worse aberration than the other way around (1.069 vs. 0.272)

mgb_phy's link to Edmund has the correct figure at the top of the web page.

The way I remember it is: never have the flat face facing a collimated beam.

Last edited by a moderator: May 3, 2017