Telescope Grinding
Friday, December 10th, 2010 at
12:06 pm
Very old mirror/lens grinding machine. This machine can grind and polish using different stroke patterns, ie w-stroke, short strokes, long strokes, etc. Currently this machine can only operate a mirror/lens up to ~250mm size glass. I keep the lens/mirror in a bath with a pump to agitate the water and cerium oxide or whatever I happen to be grinding/polishing with. I usually have a small DC motor with a propeller to agitate the solution but I have had some issues lately with the motors. Post any questions and I will try to respond asap. Thanks and enjoy!!
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US $34.99
The pads are from Willman-Bell I believe. They are optical polishing pads. They have an adhesive on the back and in the shape of flower petals. Be careful, they are aggressive!
What material are the pads that you show?
Thanks,
Frank Spillman
Yes you have that right. Although you have to factor in for the diameter of your tube etc. and how much back-travel or in-travel you want from your focuser.
How do you know where to put the secondary mirror? (I know the basics) So let’s keep the secondary out of mind for simplicity. Say you have a mirror that has a 4.5 F ratio. Does that mean the light from the primary mirror comes to a point at 4.5 times the diameter of the mirror?Example: You have an F 3 mirror, the mirrors diameter is 3 inchs. Does the light come to a point at 9 inchs? Thanks!!!!
The F ratio is determined by the sagitta, also known as the depth. The F ratio equation is (r²/sagitta)x 4=f/l. I have a tool called a spherometer. I made it using a dial indicator and a solid piece of square wood about 20″ in length. The indicator measures to .001″. You have to zero on the edge of the mirror and I find the lowest spot in the center and that gives me my sagitta. ALL lenses and mirrors are first made spherical, then polished to parabola, ellipsoidal, etc.Google ronchi for tests.
So, how do you determine the F ratio? Do you just measure how deep the parabolic shape goes compared to the diameter of the glass? And what other kind of test do you perform to determine the quality of the mirror?