Ronchi-test

In 2020 I purchased a Gert Neumann photographic Ronchi in order to be able to do single-pass Ronchi tests on my GTT60, but also to check the mirror distance of RC-telescopes during collimation.

In order to be able to use it I used a bracket that I once designed to do afocal imaging with the GTT60. This bracket (see figure 1)consists of a 2" to 1¼" adapter to which I mounted a round aluminium plate. To this plate three brass rods are attached using M3 screws and a second brass plate connects to that using M3 screws, while springs allow to have a variety of lenses and eyepieces of different lengths. A standard T2 nose cone is screwed into the photographic Ronchi after which it can be inserted into the adapter and the camera, with a CS-lens mounted, can be mounted behind it (see figure 2).

For these set-ups I have three CS-lenses available: the ZWO 2.8-12mm zoom-lens, an unbranded 25mm f/1.4 CCTV C 1/2 Lens, and a Fujian 35mm F1.7 CCTV TV Movie lens.

This whole set-up can then be inserted in any 2" focuser. Using a real or artificial star (see the Collimator page) a single-pass Ronchi test can then be performed. For this the camera is focused at infinity and the telescope is aimed at a star. Once it is properly aimed focus can be varied to get more or fewer lines in the ronchigram.

The downside of this method is that it is a single-pass test, which is not as sensitive as the double pass, and that it needs clear skies or an artificial star to be used. When used on a real star, seeing affects the image as can be seen in below video.

In figure 3 the lens of the GTT60 can be seen at full 60mm aperture (top) and stopped-down to 30mm (bottom). The lens clearly has a central zone with a different curvature than the outer region. Stopping down this lens significantly improves its performance.

Below video shows the same lens while being aimed at a real star under good seeing conditions and while being racked through focus.

In order to overcome the detrimental effect of seeing, I built myself a Collimator, which I equipped with a 9μm optic fibre. This set-up give very stable views of the ronchigram, but still is single-pass. It allows to a certain extent to test the inter-mirror distance of Ritchey-Chretien (RC) telescopes. The test is, however, not very sensitive, indicating that a slight change in the inter-mirror distance does not significantly affect image quality.

The set-up can be seen in figure 4 where an 8" RC is tested in front of my 12" collimator.