There is an incredible amount of interest in testing mirrors using Interferometers and we are always being asked if testing with Interferometers is better than a conventional Null test?
\ y}!yrQ The answer is a resounding "No"
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/G8 Interferometers cannot compete with the "Time Honoured" method of a knife edge and the human eye in a Double Pass Null test for accuracy. If they could, we would be using an Interferometer instead of conventional methods.
:+? w> But it is amazing the faith placed in an Interferometer result! We are occasionally getting challenged about the specification of our mirrors on the basis of a poor Interferometer test. There seems to be a semi religious belief that the Interferometer result is correct and our method is wrong, - when in reality it is the opposite way round!
{Yc #XP Even firms in the optics industry who should know better are being taken in by Interferometer results. We are making some information available on the level of accuracy that can be expected from our tests compared with Interferometer tests.
QMQ\y8E "Conventional Methods" - The Double Pass Null Test
aJ3 .D What follows is a description of the Double Pass Null Test as carried out at Oldham
Optical. This is the basic test we use on all large parabolic mirrors. The description is very simplified and is aimed at peak to valley (PV), measurement, but all the Double Pass strengths are brought out to illustrate why it is such a good test of a mirror. This test is also known as "Auto-collimation" and most professional mirror makers agree with us that it is the definitive test of a parabolic mirror.
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- xE%`X 5oTj^W8M( The diagram adjacent shows the basic arrangement of a parabolic mirror set up under test facing an optical flat that has a central hole. A point light source is set up near the focal point of the mirror and shines through the central hole onto the surface of the parabolic mirror.
d #vo)> The light reflects back parallel to the axis of the system to the optical flat which reflects it back along the same path to the parabolic mirror again.
AQU^7 O It reflects off the parabolic mirror a second time and returns to a focus near the original light source. In practice the light source has to be set up just slightly off axis so the focus of the reflected light can be accessed.
<oKoz0! A knife edge is set up at the exact point of focus. The knife has micrometer adjustments to allow it to be adjusted slowly and accurately into the returning light cone.
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o The detector used in the Double Pass Null test is of course the "Mk1 Eyeball". In our case the person wielding the eyeball has developed the skill from carrying out the test a great number of times. While an amateur setting up this test for the first time would certainly benefit from being led through the test by a more experienced person, - once he has been led through the test once, - he would probably be able to repeat it on his own.
Gzw9E.Hk The point being made here about the Double Pass Null Test is that if you have access to an optical flat, - through an Astronomy group for instance? - all the other equipment is easily made or readily available and the test is easy to do.
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y{k65dk - 6cVJu%<V The next diagram is an enlargement of the light rays passing the knife edge. If the mirror is the perfect parabolic shape then all the rays of light will come together at the focal point. If the knife edge is moved on its micrometers it will be possible to find a single position at which all the light rays are cut off by the smallest vertical movement. The observer would see an instantaneous Null, (total blocking of all light), as the knife edge is moved into the beam. (vertical movement as shown on the diagram.)
G 5!J9@Yi 'Z}3XVZEN In practice, it's not possible to make an absolutely perfect mirror, although some of us can get fairly close! When the parabolic surface is not absolutely perfect the light rays coming back will not pass through one fixed point. They will range around the nominal focal point.
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!l|fzS8g ZFFKv In this next diagram the range is shown by the solid and dotted lines. Say in this next example that light rays from the centre of the mirror are represented by the solid lines and rays from the edge of the mirror are represented by the dotted lines. Everywhere else focuses somewhere in between.
*$ kpS ph If the knife edge is adjusted to the same point as in the first diagram, then only part of the mirror (the centre), will be Nulled. There will be a dark centre on the mirror where it is Nulled and the image on the rest of the mirror will still be light.
s,|"s|P Once at this position, horizontal movement of the knife edge will make the dark centre expand out to a ring and continue to expand out across the surface of the mirror. The ring will reach the edge of the mirror when the knife edge is in the dotted position shown corresponding to the rays from the edge of the mirror. The horizontal movement of the knife edge needed to move from the Null at the centre to the Null at the edge is a direct measure of the surface error on the mirror.
}v4T&/ vt- So once the test is set up and adjusted, only one movement of the micrometer is needed to take the test results.
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S-2@:E ;^f ;< There is a relationship between the Focal Ratio of the mirror and the horizontal movement of the knife edge to work out the error on the mirror.
r>, s-T!7 An easy way to show it is a graph like the one adjacent.
CwdeW.A" j From one simple measurement and the use of a graph, the Double Pass Null Test directly measures the error on the surface, (or on the Wavefront of course!)
8_=MP[(H The knife edge movement is not great. A typical value may be around 0.1mm. This might at first seem small and difficult to measure, but that's exactly why the knife edge is equipped with a micrometer movement that can measure horizontal distance to an accuracy of better than 0.01mm. Mechanically the set-up can theoretically measure PV Wavefront on our 20" mirror to an accuracy better than 1/100λ. However its not quite as good as that because the exact position at which the Null reaches the edge of the mirror is partly subjective. Some figure better than 1/30λ is readily achievable.
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