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Goodmorning everyone, I state that mine is an academic question, because I haven't tried to polish mirrors for a long time. But my curiosity remained. Somewhere, the last century, I had read that one could simply try to place an as-yet-unmetalized mirror in place in the telescope, after which the focal plane was explored with a foil, and placing a star in the center would have seen it extinguish the illuminated surface in a linear fashion (“null test”). It seems that Dobson used it like this. I would like to know if anyone has tried it, and if so, your impressions (I had tried to polish a 13cm glass. f/10 and looking into it… hmm it seemed to work, but once aluminized I decided that much more was needed…).
Cordial greetings.… curiosity remained. Somewhere, the last century, I had read that one could simply try to place an as-yet-unmetalized mirror in place in the telescope, after which the focal plane was explored with a foil, and placing a star in the center would have seen it extinguish the illuminated surface in a linear fashion (“null test”). It seems that Dobson used it like this. I would like to know if anyone has tried it, and if so, your impressions (I had tried to polish a 13cm glass. f/10 and looking into it… well it seemed to work, but once aluminized I decided that much more was needed…).
Cordial greetings.Good evening Max.
Let me give you your name, since you were one of “our” Scratchers
And. What you have described is the homologous test of Foucault's, in the variant that allows you to execute it as you described, on a source placed at infinity.
The Foucault test, on the other hand, is performed “Indoor”, illuminating the mirror with a point source located not at infinity, but at the distance of its radius of curvature, which is notoriously equal to twice the focal length.
In both cases, “cutting” the mirror reflection… If the blade is at the focal point of the source at infinity, corresponding to the center of curvature of the double focal distance of the artificial source, the complete darkening of the mirror surface is obtained, without being able to appreciate any lateral origin of the shadow.
(From this it follows that: If instead we were at a distance “intrafocale” less than the focal one, you would see the shadow of the blade come to obscure the image in accordance with the movement of the blade itself;
While if you were in position “Extrafocale” i.e. further beyond the focus (where the reflected rays cross) one would see the shadow of the blade enter to obscure the image in the opposite direction to that of introduction of the blade).John Dobson, per “light” the curvature of a mirror, frame at “star test” under high magnification a point source to see its diffraction rings. Then check by slightly pushing the eyepiece into intrafocal, that there are no rings brighter than the others in the center of the mirror; then he pulls the eyepiece slightly out of focus, and check that there are no rings that are brighter than the others towards the outer edge.
And finally he concludes that the brightest rings in the center seen in an intrafocal position diagnose a mirror with a dish that is too deep in the center; while vice versa brighter rings at the edge, seen in extra focal length, they diagnose a dish that is too flat in that area.On this very topic, see the following short excerpt from John Dobson's film
That fragment is extracted from the complete construction movie that you can find here: https://www.grattavetro.it/auto-costruzione-di-specchio-e-telescopio-o400mm-f6-con-video-tutorial-di-john-dobson-parlato-in-italiano/
obviously, in order to see the diffraction rings, the primary mirror must be acclimatised to room temperature. Otherwise
the hot mirror creates a strong turbulence of the air inside the optical path, which transforms the image of the diffraction notch into a chaotic bubbling.in conclusion, It is much better, from i pigeon method, choose the bending radius test, using a Couder mask, to “delete” slowly, area by area of the mirror determined by the circular crown on which the pairs of concentric windows of the mask open. That way you can fix it area by area, progressively bringing the mirror to perfection.
Thank you, later I also found the rough description of the test. Pure academic curiosity, because after spending hours and hours trying a 30cm f/4, I had given up by falling back on a 13cm. I'll leave that to those more experienced than me, I now devote myself to mechanics for which I am much better suited (https://onedrive.live.com/?authkey=%21ABZwwfIQbEfGYvI&id=50DB6FBE84E59CC1%21767&cid=50DB6FBE84E59CC1). A greeting.
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