RONCHI

Advanced and Special Set-ups

Particularly suited to Ronchi Testing.

Copyright – P. J. Smith

But permission is given to distribute this material in unaltered form as long as it is not sold for profit.

Ronchi tests are, strictly speaking, applied to an incoming wavefront. This wavefront may be generated by a large array of possible optical set-ups.

If all but one element in the set-up are of guaranteed quality, and the optical set-up introduces zero or known aberrations, the arrangement becomes a test set-up for that one unknown surface or element.

Each of the following may be used in this way.

This is the classic test for a single Concave surface at Centre of Curvature of a spherical surface. The grating and source are close to each other and may in fact be merged by using a portion of the grating as the source. There is detail on the characteristics of this option elsewhere. Of course the distances may be unequal, in which case the surface tests out as an ellipse. The surface being tested may be subsequently used for either a reflective or refractive role in some other optical arrangement.

A lens designed to work at definite object and image distances may be tested. The grating must be placed at a position distant from the source. This test must be limited to close to monochromatic light in most cases.

Few realize that the Ronchi Test may be ‘reversed’ by placing a grating at the source and directly viewing the image of the grating by looking directly into the emerging light beam. Sometimes this is convenient because there is no need for the head to block off the beam of light. This variation is often called the ‘Lower’ test. Usually it is not quite as sensitive although it can be made fully sensitive again by using a sufficiently wide telescope objective to catch and focus the emerging rays. In this case the grating is used at the focus of the auxiliary objective lens. Of course, the objective must be of excellent quality and be wide enough to catch at least half of the incoming beam.

This is the exact analogue of Case 3, but tests a mirror rather than a lens.

Don’t forget that stars provide a ‘perfect’ pinhole source at infinite distance. Thus, a telescope may be tested at prime focus with a grating. Be aware that the atmosphere distorts star images, which degrades this test. The operator must decide if atmospheric seeing is sufficiently good for reliable testing. Convenient sometimes but of limited usefulness.

Other Examples

These illustrate a few interesting features such as :-

Autocollimation
Use of an Oil surface as a reference flat
In the right and lower diagrams, use of a folded telescope for remote viewing of the Ronchi test pattern. Note that the telescope in this case need not be of high quality as it is used after the Ronchi grating.

Here a slit forms a source for a Schmidt and the image is translated via a viewing telescope to an external position where a Ronchi grating may be used. In this case, the Objective lend must be of unimpeachable quality.

This is equivalent to the previous set-up except a ‘virtual pinhole’ is formed via a laser beam shining onto a small polished steel ball. The laser beam can simply shine through the centre of the glass in the primary since it need not be free of aberrations until after it is reflected from the steel ball.

This is an example of the first use of the ‘Lower’ test of a Schmidt where the Ronchi image is viewed by looking into the exiting beam. A straight edge is placed against the exit pupil and the straightness of lines estimated against this. The accuracy required of a Schmidt is a little less than that of most other optics.

Small and miniature lens surfaces may be tested by this modification of a microscope below.

Warning

The laser diode is used with collimating lens removed and the system arranged to filter or attenuate the brightness if necessary. Users should acquaint themselves with the dangers of using lasers and, if appropriate, replace the laser diode point source with some other source.

It is extremely difficult to test miniature surfaces by normal means. This provides a ready solution. It will test both convex and concave surfaces. The other useful aspect of the device is that it becomes an excellent spherometer for miniature surfaces with the addition of a dial gauge to measure the focussing travel of the microscope barrel.

Other special set-ups are considered under Null Tests.l

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