Testing at Focus
A more positive test than the one just described is made at the mirror's focus, using parallel light. The set-up for this test is shown in Fig. 40. The mirror and a silvered or aluminized optical flat are aligned with their axes coincident.
(A perforated flat is used in this arrangement, although a small prism or diagonal could be centrally placed before an unperforated flat at a 45° angle, and the pinhole and knife-edge moved around" to the side.)
Rays from the illuminated pinhole, which is placed exactly in the focal plane, are reflected, parallel, from the paraboloidal mirror to the flat. Thence they return to the mirror, still parallel just as if they had proceeded from a star. The mirror reflects these rays to a point image in the focal plane, and when the knife-edge cuts into that image, the mirror is seen to darken
evenly and instantly, just as does a spherical mirror when tested at its center of curvature.
The accuracy of this test is further amplified by the double reflection from the mirror. The flat used must be accurately plane to within 1/10 wave length of light, so it is not a test that the average amateur is likely to employ.
But it would be interesting to make the test with your finished telescope on a star, such as Polaris, which is stationary enough for the purpose. Get the star centered in the field, remove the eyepiece, and see if the mirror "blacks out" as a knife-edge is cut slowly across the focal plane.
next- test for astigmatism
|
