Mirror null test

The charts in the original article are only for use with paraboloidal mirrors. Values of the pinhole to lens spacing b may then be determined. For the best null, a ray trace may be performed to determine the optimum lens to test surface distance d. If you are setting up the Dall null test for a general conic surface, remember to input the proper conic constant into the ray trace program. In practice, the spacings for a given test set up should always be ray traced to obtain the optimum values.

In doing this, I have discovered that the best null may be obtained by setting up the lens with its convex side toward the mirror. This is opposite of the set up Dall recommends in the ATM article. This can make a significant difference in the quality of null obtained with faster mirrors.

Before deciding that this is the best approach, however, you should look at the tolerances involved with each method and how they differ.

In general, the setup Dall recommended gives more latitude in tolerances. You should also note that Dall's setup will require a smaller lens. The Dall Null Test has been said to be very critical in its set up and use. The tolerances in component spacings have at times been reported to be very tight. I suspect that this belief stems from the requirements of using a good lens more than anything else.

My ray tracing of the test has shown that this need not be the case for many mirrors the ATM may produce. Only for large very fast mirrors do the tolerances become tight. Even then, with care, the amateur should be able to use the test to good advantage. The optics in question were both paraboloidal mirrors for Newtonian telescopes. In each case, I first used Dall's recommendation of having the flat side of the lens toward the mirror.

I then repeated the tests with the lens turned around so that the convex side faced the mirror. I chose a plano-convex lens of 6" focal length and 0. I assumed the lens to be made of BK7 crown glass. I set up the ray-trace with light from the source passing through the lens the reflecting back from the mirror. I allowed the program to find the focal point of that setup. The lens to mirror distance was then adjusted so that the source and knife edge focal point were at approximately the same distance from the mirror.

The program was then allowed to change to source to lens separation until it could minimize the total spherical aberration in the system. This marks the best null. To read out the value of the null, the program was asked to refocus for minimum optical path difference and then display its value. To find the tolerances for the setups, one parameter was varied at a time until the Strehl ratio dropped to 0.

The spacings were not optimized, only changed and then evaluated for degradation effects on the wavefront. The results are shown below. This 6" focal length lens must be spaced about 3. The pinhole and lens assembly, in turn, will be spaced a little less than " from the mirror under test.

Again, the tolerances indicated in this setup are for changing only one parameter at a time. Under these conditions, the lens needs to be at least 0.

When the lens is turned around so that the convex side faces the mirror, the following results are obtained. A better null is achieved given a perfect set up, but some leeway is lost in the tolerances allowed in the set up. The user must weigh these two factors in deciding whether to use this form of the test. It should also be noted that the lens must be larger when the convex side faces the mirror. Click here to see what's new.

A circular null computer-generated hologram CGH was used to test a highly paraboloidal mirror diameter, 90 mm; f number, 0. To verify the null CGH test a classic autocollimation test with a flat mirror was performed.

Comparing the results, we show that the results of the null CGH test show good agreement with results of the autocollimation test. David E. Stoltzmann and Peter Ceravolo Appl. Stephen Rolt and Andrew Kirby Appl.

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Contact your librarian or system administrator or Login to access Optica Member Subscription. Cited by links are available to subscribers only. Figure files are available to subscribers only. Article tables are available to subscribers only. Equations are available to subscribers only. CGH Structure Parameters a. Allow All Cookies. On its part, the lens induces nearly identical aberration of opposite sign, which results in a formation of near-perfect focus that will make possible to null accurately made surface.

This initial location not necessarily results in the aberration offset, and usually requires minor adjustment the figure is usually more accurate when measured from the center of the lens, due to lens thickness being a factor not accounted for in the Gaussian lens formula that the relation is obtained from.

Since lens' spherical aberration changes with R 3 , relatively small variations lead to the desired aberration level, with the change in effective lens diameter due to needed axial adjustments having relatively small effect on the aberration. Main limitation of the Dall null test results from the need to bypass correcting lens with the converging cone. It requires tilting the mirror it can also be thought of as placing light source off the mirror optical axis , which inevitably induces astigmatism.

For mirror with the stop at surface, it is independent of object distance, thus as given by Eq. The point of cone separation is, therefore, about mm from mirror's center of curvature, but the actual lens shouldn't be farther than mm from it.

Needed lens-to-source separation L O is dependent on the lens focal length f L and lens-to-c.