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.
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 specifications of the mirror.
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!)
Examples of what could be seen are illustrated below and an actual photograph of a flat being tested is displayed 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.