AVC College Astronomy Lab Telescope Projects
Telescope Mirror Cell Assembly
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Dob AZ base assembly
Dob box support fabrication
Mirror Cell Assembly
Tube assembly for the telescope
Create the Spider and Focuser
Final assembly of all parts

The mirror cell assembly is probably the most important assembly in the telescope.  The spider and secondary mirror holder should also be made with some precision.

 

Telescope mirrors need support.  The mirror is a disk of glass. This glass has several problems in the gravity and the atmosphere of the Earth.

 

First is the temperature ranges over which the glass mirror will be seeing in its life or just the day you want to take it out and look at stellar objects.  The glass of the mirror will expand and contract with the temperature changes.  The concave surface of the mirror that reflects the stellar light into and image can be badly distorted by any temperature changes.

 

If you look at the 6” mirror you will see that this glass is greenish. This is called plate glass. The green color comes from the lead used in making the glass. This is one of the worst glasses that a telescope mirror can be fabricated from. This glass will expand and contract widely over temperature changes. So, when you want to take your Dob out to view night sky object you must be sure that the mirror has acclimated to the outside temperature.  The rule of thumb is it takes about 2 hours per 1” of thickness of the mirror to stabilize to a constant outside temperature.  So, don’t take the scope out from an 80 degree house and expect it to make round images. It will take the mirror 2 to 4 hours to cool down to the outside temperature.


Figure 1: A 6" telescope mirror and its secondary diagonal mirror

The second problem with all telescope mirrors is the gravity sag due to the Earths gravity. Most all telescope mirrors are supported form the flat back. There are many ways to support the glass to minimize the gravity warping effects. The rule of thumb is that a mirror with a thickness to diameter radio of 1:8 can use simple support of the back of the mirror.  Mirrors that are thin and have a ratio of 1:10 will need extra support to help them provide the best images.

 

For the 6”f5 plate glass mirror we see it is about ¾” thick and 6” in diameter. Therefore the ratio is   .75” to 6” or  a ratio of  1:8. So, this is good and the mirror will only need the simplest support.

 

The simplest support for a mirror is 3 points at 120 degrees around the center of gravity of the mirror.  There is a radius out from the center of the mirror in which the volume of glass on the inside of the circle is equal to the volume of glass on the outside of the circle.

 

Mathematically the equation would look like this where Rin is the inside radius of the circle and Redge is the outside diameter of the mirror.  The relationship of the area of a circle is Pi times radius squared.

 

So the area inside Rin equals                                =  Pi * Rin * Rin,

the area from Rin to Redge equals                        =  Pi * (Redge –Rin) * (Redge – Rin)

 

If you solve for Rin you get the equation       Rin =  0.707 Redge =  .707 * 3”

 

So you want to put three small cork dots about  ¼” diameter at 3” * .707 or   2.12” out form the center of the mirror back.

 

So, flip the glass mirror over to the uncoated side. Place at 3 places and 120 degrees apart, small paste on dots of cork out 2.12” from the center of the mirror.  These 3 cork pads will support the weight of the mirror equally when the mirror is sitting on its back in the telescope.

mirrorpads.jpg
Figure 2: Wood mirrocell on the left, and the 3 cork support pads on the back of the mirror

Now go cut the 6” mirror wood circle and the tube back insert circle.  We will now assemble these circles.  Paint them so they will be protected from water and rain.  Paint the wood flat black on the side that will face into the tube. Paint the edges black also to help minimize reflections.

cutcircles.jpg
Figure 3: Cut the plywood for the mirror cell and the tube back plate

Mark the 6” circle with lines 120 degrees a part radiating from the center of the circle.

 

Hardware that you will need now to assemble the mirror cell is as follows.

 

  1. 6” wood circle for mirror cell mount
  2. 8” wood circle for telescope tube end support
  3. 3 bolts 3” long by ¼” threaded
  4. 6 washers for the bolts
  5. 3 standard 1/4"x20 nuts
  6. 3 standard ¼”x20 wind nuts
  7. 3 springs about 1” long and about ½” inside diameter
  8. varnish to paint the wood
  9. flat black or ultra-flat black spray paint

 

Begin the assembly by marking to drill for 3 bolts.  Drill 3 holes ¼” diameter through the mirror cell mount  about 1” inside the edge of the wood circle at 120 degrees apart.

 

Using a larger drill open the ¼” holes so that the head of the bolt will be counter sunk into the wood. You do not want the metal bolt to touch the mirror glass at any time.

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