Skybadger Observatory
Astronomer and tinkerer, This site is about projects in Astronomy..
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DPAC is the use of a telescope and an optical flat to test the telescope optics under auto-collimation. The stellar source is placed at the focal plane, where the eyepiece would normally go and used to illuminate the optical path, which is reflecte don itself back to the eye, also at the focal plane. USe of the optical flat mirror means the test is twice as sensitive as a star test. If the eye is also presented with a ronchi screen, then we can see the optical quality of the system. However an optical flat of the size needed for larger scopes is expensive. A 12" flat being of the order of £300+. At some point I will make one, but in the meantime, there is a work-around. And that is to use an oil-bath as a fluid mirror. The oil is disturbed less and settles quicker than water and only a few mm is required to form a reflective surface. My setup for the 12" can be seen below. The telescope is inverted over the oil bath, sitting on a board with a 12" hole in it. The oil bath is in the box below the scope. The flat board is fitted with three levelling screws bringing the scope and fluid mirror into optical alignment. As the focuser is adjusted, the number of ronchi lines seen in the image of the mirror varies from many to zero at precise focus. The direction of the curvature of the line away from straight indicates the optical characteristics of the system and can be compared with modelled ronchigrams made using programs like RonchiXP.