All Newtonian telescopes require a secondary mirror — a flat mirror held at roughly a 45-degree angle to reflect the light from the primary out to the side. Generally this secondary mirror is an ellipsoid, in order to waste as little light as possible.
One major problem is figuring out how to hold this secondary mirror in place securely without interfering with the passage of light from your distant target. The secondary mirror can be held on a stalk, or on crossed arms like a spider’s web.
The images below show how Ramona D made a spider using a piece of extruded aluminum tube with a square cross section, several bolts, a spring, a piece of plastic dowel, some pieces of steel strapping tape, a few thumbscrews, and various small nuts and bolts. She did a very neat job, including threading and tapping several small holes in the aluminum tube.
The idea is not original to me: I got the idea from somebody else on line, but unfortunately, I don’t recall the name of the person to whom I should give credit.
Here are some photos that probably do a better job of explaining how to make it than I could explain in many, many paragraphs.
Jim Simpson said:
Nice design and well executed. I assume that the diagonal mirror is just held in place using some RTV?
Yes, pure silicone caulk.
Bob Bunge said:
Many years ago I made a similar spider for my 20-inch using sheet aluminum to make both the vanes and square section. The secondary is 3.1-inches, so I had enough room to bend the aluminum around with an over lap on one side and hold it together with some rivets. I thought about using square pipe like she did, but didn’t have access to a large enough piece at the time. I use a wood block with a hold drilled through the center for the secondary holder, similar to her plastic dowel.
I glue the mirror on with 100 percent silicon glue. You can use a razor blade to cut through the glue when you need to remove it, but in 25 years, I’ve never had a failure of the glue. Just make sure the surfaces are really clean when you glue together.
I don’t use a spring in the center. The center bolt pulls the mirror block against the adjustment screws. Here is a not so good photo:
She did a super job and is to be congratulated for a wonderful piece work!
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