A Center Donut for your Telescope Mirror

DISCUSSION: Regardless of the spider, if you mount the secondary to the holder with silicone, this is a good way to accomplish that task. It's pretty simple. I use 100% clear silicone -- any brand will do.

When gathering the "pieces" you need to mount the spider, here's a list that works for me:

  1. A tube of 100% silicone. I like to use the brand that has a "cap" so I can re-seal the silicone and keep it for the next application.

  2. Gather 3 "spacers". The little squares are actually cut from shoe-box cardboard. You can use just about anything about the thickness of shoe-box cardboard.

  3. In this picture, I also show a sheet of plastic "bumperons". These are available from a variety of sources -- TAP Plastics. 



Run a bead of silicone around the perimeter of the secondary holder and then place the spacers at approximately 120-degrees in the silicone. Note: You "don't" want to place the secondary against holder without a spacer of some kind. The slight gap allows the silicone to cure.

Once the silicone and spacers have been put onto the secondary holder, carefully center the holder over the back of the mirror. Of course, you'll want to watch the orientation of the mirror, being sure the edges are correctly oriented.

I use a piece of masking tape to hold the mirror in place. Allow the silicone to cure for 24 hours.
Place the secondary on a piece of smooth paper. Don't use a paper towel, for example, as the spray paint will "wick" onto the front surface of the mirror. You can always get a piece of butcher paper from your local market. Don't "over spray", take your time and all will be well!!

Hold the paint at a good distance, 20" or so, and apply 2 or 3 thin coats until the back of the secondary is covered.*

*OK, so the disclaimer!! Painting the back of your secondary has some risks. You may get a bit of paint on the front surface of the mirror. If so, just clean it with reagent-grade acetone. Still, being careful is the best way to avoid of this. This procedure is a "recommendation" -- it isn't totally necessary and the risk is "yours" (sorry about that).

Once you've completed painting the secondary, allow it to dry. From that point on, you're ready to go..


For more information about your requirements:
--Dennis Steele
--Tel: 650-315-6578 (anytime)
--Email: densteele@dobstuff.com



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