Foucault test

On 21 September 2019 I attended a Foucault- and Ronchi-test workshop at the observatory in Almere, the Netherlands. The workshop was well presented, a pleasure to attend and very educational.

Being not a mirror-maker myself I did little with it afterwards until the next course, concerning the Bath-interferometer, was announced by the same observatory. Once that resulted in a working bath-interferometer, I needed a way to accurately measure the Radius of Curvature (RoC), for which the Foucault-test was advised.

The first version that I made of this simple mirror-testing instrument is shown in figure 1, but was not correctly build. Even though it did deliver Foucault images, the construction with the knife only covering the light source (see figure 3), did not allow to accurately measure the RoC. In this construction the RoC is the average of the distance to the knife and the part in the lens that vignettes the returning beam. As the latter position is not well known, an accurate measurement is impossible.

So a second Foucault-test device was created, now with a knife running all the way from the light source to the camera-lens (see figure 4). As camera a ZWO ASI290MC was used with the ZWO 2.8-12mm zoom-lens. The whole contraption was mounted using two M4 screws on the T-bar of the Bath-interferometer, making it easier to switch between the two.

Downside of the ZWO lens is that it has a very small aperture by which the lens tends to take over the function of the knife-edge when not properly positioned behind the knife. In order to get the aperture correctly behind the knife-edge I widened the hole for the 1/4" screw, that holds the camera in place, to a 6.5mm x 8mm slot. This allows to slide the camera sideways until the knife comes into view. The camera needs to be positioned in a way that the visible shadow of the knife just 'touches' the mirror.

Using this set-up it is now possible to measure the RoC by adjusting the XYZ-mount until paraxial focus is reached.

The current set-up could do with a longer focal length lens as the mirror-under-test remains fairly small, so I ordered 25mm and 35mm fixed focal length lenses in the hope to improve on the image scale.

In the weeks following the mirrors of two Newton telescopes at InFINNity Deck, a Bresser 130 f/5 and a SkyWatcher 300PDS 12" f4.9, were tested using both the Bath-interferometer and the Foucault test. Both methods showed that both mirrors suffer from astigmatism (see figure 5 and figure 6).