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I have polished the mirror and now it’s time for testing the surface. The foucaulttest consist of a lightsource also called the artificial star placed in radius of cuvature (two times focuslength). By refracting the light inside and outside focus it is possibly to analyzing the surface. The light source is in radius of curvature 3 cm. left from center axis, digitalcamera in focus right to center axis and the knife-edge moves from right to left.

With my little digitalcamera the mirror makes these images in foucaulttest:


Knife-edge cut light outside focus Knife-edge cut light inside focus.









When testing the focal plane the lightsource is placed in radius of curvature, a white screen is placed in focus and by moving the light, I testing the focuslength is equal on different points on the mirror. The test is repeated 5, 10, 15 and 20 cm. from optic axis and the mirror is also turned arround five times, approximately 72 degrees each time. The test showed that focuslength is uniform and therefore the image is formed on a curved focal surface.

The miror was also tested for astigmatism. If there is astigmatism the image out of focus will be eliptical and not round. And the eliptical images (inside and outside focus) will be right angles to each other. With the white screen inside and outside the focal plane I could not registrate any astigmatism.

With an eyepiece in focus the mirror is tested for represent the artificial star sharp and distinct. The test was excellent – the artificial star was represented very fine.

Ripple surface was controlled by visuel test and the mirror was accepted.

The last test is test of polishing. Light from the sun concentrated by a magnifying glass controle the surface. If the surface make different kind of reflections then the mirror is not polished enough. I decided to polish 30 minutes more and then it got a smooth and well-polished surface. At the image below you can see how I check up the surface, here there is a little scratch:


I have contacted Vacuum coatings, maybe they shall aluminize my mirror. In the meantime I will construct the tubus and mounting (Dobson).

Category: Astroblog
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