Mirror Holder

A mirror holder or cell will hold the primary mirror safely inside of optical tube. You should make a good quality, sturdy mirror cell to prevent any damage to the mirror.

I used same 3/4 inch marine ply which I used to make the DOB mount. I cut two disks, one 150 mm (mirror dia) and another 190 mm (tube inside diameter)

Steps

  1. Cut the 190mm (back plate) and 150mm (mirror plate) disks from the plywood.
  2. Cut the sides of back plate in circle or straight as you wish. This is to hold it easily and to help air circulation around the mirror.

  1. Draw following size circles on the back plate and mirror plate
    • 4.5 inch dia circle on the mirror plate and the back plate
    • Mark 3 points 1200 apart on this circles to drill 8 mm holes
    • You would see 3 circles on my back plate which you can disregard. The outer circle is 6 inch dia representing mirror dia and inner circle around 3 inch dia to set counter retaining bolts (or locking screws) which I did not use on this design. Locking screws are used to lock the mirror cell after alignment of the mirror.
  2. Drill three, 8 mm holes on both the plates 1200 apart.
  3. Use three, 6 mm T-Nuts and fix them behind the mirror plate. You need to drill 18 mm wide holes slightly deep to accommodate and countersink the T-nuts

  • These are on back plate for counter retaining bolts which I did not use for 6″ design
  • Countersink T-nuts on the back side of mirror plate
  • 3 inch, 6 mm screws countersink on the mirror plate and running through the T-nuts set at the back side

  1. Use three, 3 inch, 6mm screws and tighten it through the T-nuts on the mirror plate.
  2. Put 1 inch, good quality springs on each of these screws.
  3. Place 3 metal washers over the springs and then place the back plate aligning the holes.
  4. Place 3 more metal washers on the protruding screws on the back plate.
  5. Tighten 3 fly nuts on each screws. These will be used to adjust the mirror for the optical alignment. You can get the fly nuts from hardware shops or from gas stove repair shops. Just make sure to get the right size nuts which fits on the screws.
  1. Get three, 2 inch metal angle brackets.
  2. Arrange them on the mirror plate as shown at 1200 apart
  1. Glue 3 strips of felt cloth on the front surface of mirror holder. I used Fevibond to glue it. You can use round or square piece as you wish but place them around 2/3 distance from mirror’s center. Felt strip will provide soft back support to the mirror.
  2. Glue 3 pieces on the inside surface of angle bracket. It will make sure that metal is not directly in contact with glass mirror. It has also ensured a snug fit.
  1. I decided not to bend the clips to grab the mirror or to use silicon glue to glue it to the holder. I want easy removal of mirror when needed and providing safe support at the same time. To prevent mirror moving or falling off, I also used masking tape around it to provide extra support. All this will make sure there is no unnecessary pressure on the mirror or at its edges.
  2. This mirror you see is un-coated. I plan to do star testing before sending the mirror for aluminizing.
  3. Your mirror holder is done!

Note: My mirror cell is based on the design given on Stellafane page https://stellafane.org/tm/dob/ota/cell2.html. Please have a look at it. You may want to modify the design based on your need and resources available.

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