Oldham Optical: Testing No 2 Page

Certificate of Conformity

We produce a Certificate of Conformity as proof that a mirror meets certain specifications.

Our standard Certificate comprises a number of A4 pages containing the specifications and pictures. They are usually supplied laminated back to back in a plastic sheath. Follow the link on the right to see a sample Certificate.

Our standard Certificate has the physical sizes of the mirror, - Diameter, Focal Length etc, - and details of the test procedures used including a diagram of the arrangement. The certificate will contain figures to confirm the surface accuracy, - this will usually include the maximum Peak - Valley figure and the RMS Surface Value.  It may contain pictures of the tests or graphs and tables of results derived from feeding error measurements into a professional optical programme.

It may be details of a Double Pass test and/or Ronchi grating tests sufficient to prove the accuracy of the mirror surface. Can be more exotic tests such as a "James Test" for instance.

The exact requirements detailed on the certificate can be as directed by the customer when the order is placed.

A certificate costs £40 for mirrors under 20" Diameter. From 20" and upwards, - a standard Certificate is provided free.

We do not provide a certificate automatically for smaller mirror sizes and we suggest one is not required if you are buying from our standard product range which is already guaranteed to a minimum specification of PV 1/4λ Wavefront accuracy. (1/8λ wave surface accuracy.)

However, - you might want one if the mirror is being built to a special or enhanced specification, - or for eventual sale to a third party who may require more proof of the specification than sight of our standard invoice would give. You may just want one to boost your own confidence in our product?

So you probably wonder why we charge extra for a certificate for smaller mirrors? - especially since we have to test the mirrors anyway during manufacture!

It's because it takes as long to produce the certificate as it does for Oldham Optical to completely manufacture a 6" or 8" mirror! - The test tunnel must be reserved, - Pictures taken of the tests and written interpretations produced to go in the body of the certificate. Taking pictures is quite a problem,

Oldham Optical's Workshop is situated in a fairly quiet built up area of Scarborough, but we are in a busy Holiday resort on the East Coast of Yorkshire. The workshop is within easy walking distance of the town centre.

Our Test tunnel which is described on another page is also an extremely sensitive detector of vibration in the surrounding area. It can detect people walking on solid concrete floors nearby, - it can detect passing cars on nearby roads, - It can detect building work in nearby streets and within about 100M range, it can detect and "track" automatic washing machines through the "wash", "Rinse" and "spin" parts of their cycle!

When the test tunnel is used for setting up and adjusting mirrors, we view the results by eye, and over a few seconds of viewing, the eye is extremely good at rejecting the effects of vibration, - unfortunately, - It is not the same with taking pictures with a camera!

Typical shutter speeds used are 1/60th of a Second, and vibration usually shows up in pictures taken during normal work hours. It can be more dominant than defects in the mirror. Photographs taken at these times would simply not do justice to the mirrors.

It is often necessary to leave photography for mirror accuracy to the "unsocial hours", and even then, - as you can see from the second picture of Scarborough, which is of the South Bay taken from the castle headland, - there is still a lot of nightlife.

(Our Workshop is about half way up the hill at the left of the picture at about at the level of the line of lights.)

This explains why we charge for a certificate for smaller mirrors, - the cost of producing the certificate is a significant fraction of the mirror price. For larger mirrors the cost is absorbed in the manufacturing costs.

(And its been a good excuse to "plug" our town!)

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