Foucault Tests & Aperture Stops

[herbhighstone]

Hello to all the folks --

Many people still use the venerable Foucault test for paraboloid mirrors because of its familiarity and long history. However, there are certain problems with the Foucault test that many people fail to recognize.

In many optical systems, restricting the aperture reduces the performance of the system. In the Foucault test, the viewing aperture can be restricted by first the pupil of your eyes (which is only a few millimeters in diameter) and secondly, the tiny aperture of "pinhole" type video cameras which are often used to make digital pictures of Foucault displays.

In both cases, the finer details of the Foucault image are removed by the small aperture. Thus the Foucault test becomes ineffective for diagnosing the fine details of the optical roughness on your glass. Too many people say that their mirrors "show no roughness under the Foucault test" when in fact their test is set up with such a small aperture that the roughness is filtered out of the picture.

There are two methods that you can use to restore the fine detail in your Foucault images. If you're using a camera, this camera should have an imaging lens of reasonable aperture located immediately behind the knife edge. Either your camera should have a built in lens at least 10 - 15 mm in diameter. Or you can set up a small viewfinder-size telescope "afocally" which means that the little 7 X 50 scope is focused on the mirror, with its exit pupil feeding your video camera.

If you're using the Foucault test visually, with your own eye as the "camera," then you certainly should try a small viewing telescope. The objective lens of this small refractor, which can once again be a 7 X 50 viewfinder size scope, should be as close to the knife edge as you can get it. The scope should be focused on the mirror's surface. You can tape a small piece of newsprint or some other printed material onto your mirror to verify the correct focus.

A viewing telescope of this kind will make your Foucault work much easier, because you won't have to force the pupil of your eye right up against the knife edge to see the image. And also, the wider lens of this small scope will gather much more of the higher "Fourier frequencies" in the image, thus giving you a better picture of optical surface roughness.

Opticians may argue against a Foucault viewing telescope merely because they don't like to see roughness on their glass. But as a telescope user, optical roughness should be of considerable interest to you. And also you can view the Foucault image much more comfortably than with your unaided eye, especially if you use a 90 degree star diagonal on your viewing scope.

Foucault viewing telescopes have been mentioned in telescope making literature ever since the original ATM series, but few people seem to use them. However, I definitely think that this viewing aid deserves a try. If you use a 90 degree star diagonal on a 7 X 50 viewfinder, the star diagonal will give just about enough extra focusing distance to allow the scope to be focused on your mirror. Try it, you'll like it!

Best wishes from Herbert Highstone

[herbhighstone]

Hello again --

In Allan Mackintosh's book entitled "Advanced Telescope Making Techniques," Ralph Dakin discusses the use of an auxiliary telescope for the Foucault test on pages 32-33.

Mr. Dakin states that it "is almost impossible" to use Foucault testers on short radius mirrors because it is difficult to bring the operator's eye close enough to the knife-blade to accept the entire light cone. However, by using a small telescope behind the knife-blade, this problem disappears and the Foucault testing of mirrors becomes "a real pleasure" (his words) It is helpful to use an erecting telescope so that mirror holes and bumps are not inverted as compared with the simple classic Foucault setup. Mr. Dakin also mentions the improved view of fine surface details on mirrors that are viewed with an auxiliary telescope.

Mr. Dakin built his own erecting telescope. However, small erecting telescopes intended for terrestrial use are now quite cheap on various Internet sites and may cost less than $15. The objective can be moved outward to focus the scope on your mirror instead of distant objects. This procedure used to be called "front focusing." A piece of plastic plumbing pipe can be used to hold the repositioned objective lens in position. Such a project shouldn't take more than an evening to assemble.

If you have a copy of the old Amateur Telescope Making Advanced book with its burgundy-colored binding, you can turn to pages 259-261 for another discussion of the advantages of an auxiliary telescope for optical testing. The index will list this information under "Telescope For Knife-Edge Shadows."

Best wishes from Herbert Highstone

[BT Technologies]

Excellent information Herbert and some real insight on the Focault test. Thank you for taking the time to post this for all to read.

Originally Posted by herbhighstone

Hello to all the folks --

Many people still use the venerable Foucault test for paraboloid mirrors because of its familiarity and long history. However, there are certain problems with the Foucault test that many people fail to recognize.

In many optical systems, restricting the aperture reduces the performance of the system. In the Foucault test, the viewing aperture can be restricted by first the pupil of your eyes (which is only a few millimeters in diameter) and secondly, the tiny aperture of "pinhole" type video cameras which are often used to make digital pictures of Foucault displays.

In both cases, the finer details of the Foucault image are removed by the small aperture. Thus the Foucault test becomes ineffective for diagnosing the fine details of the optical roughness on your glass. Too many people say that their mirrors "show no roughness under the Foucault test" when in fact their test is set up with such a small aperture that the roughness is filtered out of the picture.

There are two methods that you can use to restore the fine detail in your Foucault images. If you're using a camera, this camera should have an imaging lens of reasonable aperture located immediately behind the knife edge. Either your camera should have a built in lens at least 10 - 15 mm in diameter. Or you can set up a small viewfinder-size telescope "afocally" which means that the little 7 X 50 scope is focused on the mirror, with its exit pupil feeding your video camera.

If you're using the Foucault test visually, with your own eye as the "camera," then you certainly should try a small viewing telescope. The objective lens of this small refractor, which can once again be a 7 X 50 viewfinder size scope, should be as close to the knife edge as you can get it. The scope should be focused on the mirror's surface. You can tape a small piece of newsprint or some other printed material onto your mirror to verify the correct focus.

A viewing telescope of this kind will make your Foucault work much easier, because you won't have to force the pupil of your eye right up against the knife edge to see the image. And also, the wider lens of this small scope will gather much more of the higher "Fourier frequencies" in the image, thus giving you a better picture of optical surface roughness.

Opticians may argue against a Foucault viewing telescope merely because they don't like to see roughness on their glass. But as a telescope user, optical roughness should be of considerable interest to you. And also you can view the Foucault image much more comfortably than with your unaided eye, especially if you use a 90 degree star diagonal on your viewing scope.

Foucault viewing telescopes have been mentioned in telescope making literature ever since the original ATM series, but few people seem to use them. However, I definitely think that this viewing aid deserves a try. If you use a 90 degree star diagonal on a 7 X 50 viewfinder, the star diagonal will give just about enough extra focusing distance to allow the scope to be focused on your mirror. Try it, you'll like it!

Best wishes from Herbert Highstone