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.
Please note there are alternate ways of testing a convex mirror, - but they are all more complex. They are not as efficient and Oldham Optical does not believe they are as accurate.
This method of testing through the back of the mirror does require it to be made from a glass with good optical transmission properties. This rules out the use of Pyrex, Suprax and Zerodur etc, which can have differences in transmission characteristics across the diameter of the mirror sufficient to render the testing method unreliable.
The use of a glass with a higher coefficient of thermal expansion is not a problem as the secondary mirror is always small compared to the primary mirror and in practice it will still stabilise before the low expansion primary does.
Overall the use of optical glass in the secondary allows a more efficient testing method to produce a better result at a cheaper price.
The main use of convex surfaces is as secondary mirrors in Cassegrain systems. Testing of convex surfaces is more difficult than concave surfaces.
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. For this test to work - the mirror must be made from a glass with good optical transmission characteristics. BK7 is used for nearly all Cassegrain secondary mirrors.