Oldham Optical: Testing Methods

Mirror manufacturers need to have a dedicated "Test tunnel". This will be a space set aside in the factory which is long enough to test mirrors and can be maintained at a very even temperature, while allowing easy access to the individual components of the system under test.

There must be no draughts or significant air convection as this implies there is a temperature gradient in the tunnel which will affect the results.

In our new factory we have several test tunnel positions, - the one in the photograph opposite is along one of the walls. It is a continuous timber bench, waist high and about 24' long. 

Testing of Concave Surfaces

There are several satisfactory ways of testing concave surfaces. We generally use the methods detailed below:-

For spherical mirrors, one method is to set up a point light source at the centre of curvature of the Mirror, (Twice the Focal Length.), and observe the image formed.

A cheap laser pointer with the lens system removed is perfectly adequate as a light source.

In practice the light source must be set up just off axis so the image is formed close to, but not coincident with the source, else it could not be viewed. The image is viewed either with an eyepiece or on a knife edge and is interpreted to show defects in the mirror. 

For non-spherical mirrors, like Parabolic, as used in a Newtonian Telescope, the light source can be "abberated" to correct for the surface not being spherical.

For mirrors up to about 300mm diameter, this can be done very efficiently with a single Plano-convex lens placed close to the light source. The exact distance from the light source is a function of the diameter, focal length and conic shape of the mirror under test.

A knife edge is placed close to the focus of the converging rays, and by small increments edged into the beam until the image seen begins to darken. In a perfect mirror, the smallest movement of the knife edge at this point will make the image go completely dark. In practice the dark area spreads over the mirror gradually as the knife edge is incremented. This test is known as the "Dall" Null Test after the Englishman who invented it, - Horace Dall.

These tests are quite practical for an enthusiastic amateur making his/her own mirror and telescope.

The test is usually good enough as a final test for small mirrors up to 300mm Diameter and 1/4 Wave accuracy. It does become less accurate for larger mirrors and we use other tests to confirm accuracy for these.

Note Pictures taken of "real mirrors" will always show some shading unless the mirror is incredibly perfect. The picture here is the null of a 500mm mirror to our normal minimum specification of PV 1/4λ Wavefront error. 

If the knife edge is now moved forwards and backwards along the optical axis using a micrometer, individual readings of the micrometer can be taken as the shading spreads over the mirror. These can be used to calculate the RMS surface smoothness, - which for one of our normal specification mirrors will be less than 1/15λ.

For larger mirrors, we often use the Dall Null test as a preliminary test during the figuring as it is so easy to set up, but we follow on with either a Double Pass, which is described below, - or we may resort to the James test, which is similar in principle to the Dall Null, but uses a large spherical mirror to abberate the light source.

Ronchi Test

If the knife edge is replaced with a diffraction grating of say 4 lines/mm placed just short of the focus, the resulting image seen through the grating will be a series of straight lines if the mirror is perfect, or with bends and kinks if there are problems.

The example here shows a 500mm mirror to our standard specification of PV 1/4λ Wavefront.

Note that because we use a light source that is abberated to correct for the curve used, (Usually Parabolic), the Ronchi test always gives a series of lines. There are many references in texts and on the web to Ronchi tests that show zones or symmetrical patterns, - these are obtained from non-abberated light sources. We believe that using an abberated light source for testing is more efficient.

The Ronchi test's big strength is that it is very simple to set up. If the result seen is straight lines, - it confirms the mirror has the right curve and figure. Curved lines indicate the mirror has the wrong curve. Zones or turned down edges can be easily seen.

This test is not generally used to work out a numerical value for the error remaining on the mirror.

Double Pass Methods

Testing accuracy can be doubled by using a double path method. This as its name implies, - passes light off the mirror surface twice thereby magnifying any faults. The point light source is positioned closer to the mirror at the Focal Point. A large optical flat with a central hole reflects the light back down to the mirror for a second pass. The Optical flat should ideally be as big as the mirror under test. We generally apply this test in the later stages of figuring for mirrors of 300mm or over. There is some more information on how the Double Pass Null test operates on another page of this website about Interferometers.

The two Mirrors used in Cassegrain systems may be initially tested as separate entities, - but it is standard practice to carry out a Double Pass test of both components together.

Note Cassegrains do not usually have parabolic mirrors, but the Double Pass method can be used  irrespective of the individual conic sections used on the Primary and secondary mirrors.

The point light source position on axis is often more critical, due to the longer effective focal lengths involved, and a beam splitter may be used to view the image formed. An optical flat at least as big as the primary mirror is essential to test Cassegrain systems.

Testing of a large Newtonian or Cassegrain system by double pass is quite easy for the amateur if he/she has access to an optical flat of the same size as the primary mirror. Perhaps the local Astronomy society has one?  However Optical flats are actually a lot harder and more expensive to make than a parabolic mirror of the same diameter. As a result, - there are very few optical flats of over 20" diameter in the UK today. (But we have one of course!)

Testing of Convex Surfaces

The main use of convex surfaces is as Secondary mirrors in Cassegrain systems. Testing of Convex surfaces can be a problem. For Dall-Kirkham secondary mirrors which are spherical, a concave proof plate is manufactured and used as a proof plate. The testing process is similar to that outlined for Optical flats below, except that the concave proof plate is used as the base.  

The Dall-Kirkham secondary mirrors are generally made out of simple plate glass but a good quality optical glass, (usually BK7), is used for all other types of Cassegrain secondary mirrors.

For these mirrors we test by viewing the curved surface from the back, through the glass making up the mirror. This makes the convex surface appear concave to the testing process.

With a few "tricks of the trade", this allows the normal methods for testing concave mirrors to apply. Following testing as an individual unit the mirror is then paired up with a primary in a double pass test.

For all main Mirror testing, we generally use Red light of 625nM Wavelength as sources are now so readily available.

If we are providing a Certificate of Conformity, we usually convert the figures obtained at 625nM to equivalent values at Yellow light of 546nM.

Testing of Optical Flats

Elliptical Flats are required for Newtonian and similar telescopes and require an entirely different testing method. Instead of using a point light  source, a light source physically bigger than that of the flat under test which produces diffuse parallel light is used.

The flat under test is placed on a known good  optical flat. The pair are illuminated from vertically above and the result viewed from a near vertical angle. 

Due to the small air wedge that always occurs when two glass plates are placed together, a series of fringes will be seen. Basically a series of straight regular fringes indicate the flat is good. Curved or irregular fringes indicate defects.

 Examples of what could be seen are illustrated on the right of this page and an actual photograph of a flat being tested is enclosed on the left. More detail on what quality of surface is needed on an elliptical flat to match a primary mirror is on another page of this website.

(Note the fringes on this flat appear curved which would suggest the flat is "Poor". The picture was taken at a large angle to the vertical and it is this that makes the fringes curved. Please be assured It is not a poor flat!)

Oldham Optical are happy to test mirrors and optical flats produced by third parties, - professionals and amateurs alike. We can produce reports or can re-work, re-aluminise, etc as required. Please feel free to make contact to discuss your requirements. 

Mirrors of 20" Diameter or over, - (or if requested for smaller as well), are provided with a "Certificate of Conformity" that serves to confirm the specification of the mirror, - Please use the link to go to the Second Testing Page

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