RONCHI INTERPRETATION

ANSWERS

Copyright – P. J. Smith

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

To cement the above material in ones mind, it is suggested users attempt to fully evaluate the following before looking at the answers.

The consequences of misinterpretation and subsequent misfiguring are frustrating, painful, and time consuming. It is worth putting some time into assuring your interpretation is accurate.

It is highly suggested you use this in conjunction with my downloadable program “RonchiZ”. With this you can choose a defective mirror and view Ronchigrams (also Foucault and Wire test images) simulated at any position of the grating and with different PV errors.

(a) One method is to move the grating forward. A decrease in the number of bands indicates it is outside

(b) It has a low center so the addition of glass in the center would make it spherical. Since this is possible, we must reduce the edges instead.

(d) Moving from outside to inside COC inverts the curvature of Ronchi bands.

(e) COC.Does this represent a small or large departure from spherical ?

(f) Different zones of the mirror surface have different COC’s which means they have different curvatures. See more under Advanced Interpretation.

(a) It depends somewhat on which reference sphere is chosen but the center is somewhat depressed.

(b) Consider the above profile. If the reference sphere is BOB it may be considered turned down. If The reference sphere is AOA it is slightly turned up. BUT the effect is really much too far in from the edge to be called either Turned up or down. A far more appropriate description of the surface is that it has a broad raised zone at B with respect to the reference sphere AOA.

(c) Greatest slope is approximately when R = 0.8 and 0.2. It is easy to see this from the mirror profile above. On the Ronchigram the quick change of direction of the bands is visible as we travel around the R = 0.9 position but this is not so evident at R = 0.2. This is because it is hidden within a blind spot. A finer grating or a scan would show this.

(d) Slope changes fastest at about R = 0.5. There is also a quick change at the center discontinuity but it is hidden behind the center band. This is best seen from the profile above.

(a) This is a turned down edge.

(b) A very good sphere.

(c)

(a) The edge is raised.

(b) The centre is raised (neglecting the turned edge).

knowing grating period and F:NO of the test beam.

(d) Because the centre is raised, the reflected returning wavefront is ahead or ‘raised’ as well. Thus it is advanced.

(a) Low centre.

(b) The test wavefront is retarded by the low centre.

(d) Since the test wavefront is retarded in the centre, the only way a refractive surface can do this is by being a little thicker. Thus the centre is raised. We may also deduce this by considering surface curvature but the concept of wave path difference is easier.

(a) The defect represents a low area on a reflective surface. It is really the same as Exercise 5 but the low area is localised. This could be caused by cooling the area involved.

(b) The defect represents a high area if the surface is refractive. Similar to Exercise 6. This could be caused by heating the local spot.

(c) If in doubt, raise the surface by local hearing and watch the Ronchi bands to confirm whether the surface defect is high of low. This can be done by holding a soldering iron close to the surface or even placing the thumb in contact with the surface for 10 seconds.

(d) Since the wavefront has been more retarded in this region, the local refractive index must be higher.

(a) The central zone has its COC is closest to the grating.

(b)

(d) Note that since the drawing below is with respect to a reference sphere, the general shape of the surface is not spherical. Rather it is an aspheric deeper than a sphere.

(e)

(a) It depends a little on exactly which reference sphere is chosen but this is a reasonable interpretation.

(b) The following are typical of what will be seen when grating is outside COC.

The Ronchi test is excellent for detecting turned down edge on all spheres. Of course there is a limit for other reasons on testing very deep spheres.

It also works well on parabolas of higher F:RATIO but for deeper mirrors confusion with the strongly curved Ronchi bands makes it unusable. As for all tests, the closer to a null test the more sensitive is the test.

Since a large inner portion of the first Ronchigram represents a perfect sphere, this has been chosen as the reference sphere. Others could be used.

The second is the same as the first but the entire surface has been warped downwards in the centre.

The maximum slope (with respect to the reference sphere used in (a) occurs at R = 60 and R = 80. This can be deduced from the Ronchigrams because these are the zones of maximum bend of the Ronchi bands. Find the zones of maximum band bending then drop down to the scale to confirm this.

GOTO RONCHI INDEX