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Originally Posted by legswilly
Hello. This is my first post and I have two questions.
Is there a mirror maker in the Port Macquarie/Wauchope area? I hope to get come information on testing equipment and tests.
Why are the secondary mirrors on reflectors made an oval shape? The reflection from the primary mirror creates an oval shape on the seconday, it being at 45deg angle. Could a round mirror, or even a rectangular or square one of sufficient size be used and would it be cheaper to make? Has anyone tried to use a piece of a normal mirror like a bathroom one?
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Welcome. Not sure of where any makers are and perhaps others will answer. In answer to your second question:
Any shape mirror will do but the elliptical if correctly sized is most efficient at producing a fully illumination circular field and without introducing odd diffraction effects in the process. The reason is that an elliptical secondary will show a circular reflection through the focuser when intercepted at 45 deg. which if sized correctly will show you the circular primary when you look through the focuser (no eyepiece).
When buying a secondary you need to know the minor axis of the elliptical since this will in effect be the diameter of the reflection at 45 deg. and equal to the minor axis. The major axis of the elliptical is 1.4142 x the minor axis; 1^2+1^2=1.4142^2 but it is not necessary to know this for any calculation involving the secondary.