Reflection of a plane mirror at a focal point

[christopher.s]

I am trying to interface a detector system to a FT-IR spectrometer. The spectrometer uses off-axis parabolic mirrors to focus light in the center of a sample compartment, where it diverges and again strikes an off axis parabolic mirror, focusing it on a detector. I would like to place a plane mirror at a 45° angle, at the focal point of the first mirror. My question is, what kind of beam divergence can I expect with this setup? The detector element itself will be housed in a dewar behind some windows and filters, approximately 6-8" from the mirror in the sample compartment. There is a compound parabolic concentrator, located about 3inches in front of the detector, with a .500" entrance aperture. Can I expect reasonable signal from this setup? This will be used in the IR, from about 20-700cm-1. Will atmospheric interference be significant? I have attached a quick sketch of the outline of the system.

Thanks!

https://www.physicsforums.com/attachment.php?attachmentid=57447&stc=1&d=1365048951

 

[Valerie Park]

The beam divergence you can expect with this setup depends on the size and shape of the mirrors and the distance from the mirror to the detector element. Generally speaking, the beam divergence should be small enough that atmospheric interference is not a significant factor. However, if the optical path between the mirror and the detector is too long or the mirrors are not properly designed, the beam divergence could be too large to produce a strong signal. To minimize the beam divergence, you should use a high quality, aspherical mirror with a short focal length. Additionally, you should make sure the optical path between the mirror and the detector is as short as possible.