Telescope Mirror Cell Assembly

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

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

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 are usually made from a disk of glass. The glass is subject to temerature changes and gravity sag.

A telescope mirror must be supported so that gravity of the Earth does not warp the mirrors reflecting surface. Just as objects on Earth are held to Earth by gravity forces, a round glass disk used for a telescope mirror is subjected to gravity forces as it sits on a table or points upward to the heavens.

The temperature ranges over which the glass mirror will be subjected during viewing can vary many degrees. The glass of the mirror will expand and contract with these temperature changes. The concave surface of the mirror that reflects the stellar light into and image can be distorted by even small temperature changes. Some telescope mirrors are made from expensive low thermal exapansion glass, but this 6"f5 Dobsonian mirror is just standard glass.

If you look at the 6” mirror you will see that this glass has a greenish tinge. This is glass 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 air temperature. The rule of thumb is that it takes about 2 hours per 1” of mirror thickness for the glass to stabilize to a constant outside temperature.

So, don’t take the scope out from an 80 degree house temperature and expect it to make good images. It will take the mirror 2 to 4 hours to cool down to the outside temperature.

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

The second problem with all telescope mirrors is that gravity can bend or sag the glass due to the Earths gravity.

Most telescope mirrors are supported form there flat back side. There are many ways to support the glass to minimize the gravity warping effects.

One solution is to make the glass thick. 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 larger than 1:10 will need extra support to help them provide the best images.

For this Dobsonian 6”f5 plate glass mirror we see it is about ¾” thick and 6” in diameter. Therefore the ratio is 0.75” divided by 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 weight of glass on the inside of the circle will be equal to the weight 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 radius of the mirror. The relationship of the area of a circle is Pi times radius squared.

The weight of the glass at any radius is

= Density_of_glass * Thickness * Pi * (Rin * Rin)

The weight of the glass outside an Rin radius to the edge of the mirror, must equal the same inside wieght.

This is

= Density_of_glass * Thickness * Pi * ((Redge * Redge) - (Rin * Rin))

Pi and the Thickness and the Density drop out on both sides of the equation.

If you solve for Rin inside radius you get the equation

2 * (Rin * Rin) = (Redge * Redge)

or Rin = Redge / Sqrt(2)

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

So, flip the glass mirror over to the uncoated side. Mark 3 places that are 120 degrees apart. Place 3 cork pads each at 120 degrees angle and 2.12" out from the center of the mirror. The three pads will support the weight of the mirror equally when the mirror is sitting on its back in the mirror cell.

Figure 2: Wood mirror cell on the left, and the 3 cork support pads on the back of the mirror

Now 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 inside the telescope tube. Paint the edges black to help minimize reflections. Paint the edge clips and its screws to minimize reflections.

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

Mark the mirror 6” wood circle with lines 120 degrees apart radiating from the center of the wood circle.

Begin the assembly by marking 3 drill hole positions about 1" in from the edge of the mirror wood circle. Drill 3 holes ¼” diameter through the mirror wood circle at 120 degrees apart.

Using a 3/4" flat spade drill counter sunk holes into the top of the wood circle where you drilled 1/4" holes. You do not want the top of the metal bolts to touch the mirror glass at any time.

Figure 4: Drill and counter sink the 3 mirror cell bolts into the wood

Next, align the 6” mirror wood circle with the center of the 8” wood back plate. Then continue to drill the ¼” holes all the way through both pieces of wood. It is important that the mirror cell wood be very close to the center of the telescope tube. So drill carefully.

Mount the hardware as follows. Place a 3” bolt through the 6” mirror cell wood circle so the head is counter sunk into the wood. Then on the other side place a flat washer and use the standard nut to tighten the bolt against the 6” wood circle.

Figure 5: Mirror Cell bolt parts. Bolt, washer, spring, wing nut

If you drilled the holes straight enough then all 3 bolts should line up with the 3 holes in the tube base wood circle. Place a spring over each bolt and add another washer. Then, push the 3 bolts through the tube wood circle so they stick out the other side. Place a washer and the wing nut screw on the end of the bolt. Tighten the wing nuts tell you compress the springs.

Squish the two pieces of wood together so the springs are in tension and run the wing nuts up to hold the two wood circles in position.

Use the flat black paint and paint the top of the bolts on the mirror cell side.

Prepare the tube back plate so that the wood circle fits smoothly into the back of the telescope tube.

Figure 6: Tube wood back circle to hold the mirror cell inside the tube

Now mark the edge of the tube wood back plate where you will put 3 screws in the perimeter of the wood to hold it inside the telescope tube.

Now push the mirror cell into the tube so that the boots do not stick out the back end. Push the tube wood circle in far enough so that when the telescope tube sits on the tube the bolts do not stick out.

Measure up the back of the tube to the back of the wood tube wood circle. Obtain an average distance. You want to install the tube wood circle end square to the end of the tube.

Measure the distance up the tube to drill three holes from the outside diameter of the tube through the cardboard and into the edge thickness of the wood circle.

Figure 7: Mirror against mirror cell wood and tube wood circle

Some of the Dobsonians use a quick trick to attach the mirror to the wood circle. They use RTV glue or some sort of rubbery silicon glue. If you place 3 blobs of this glue on the back of the mirror and then center the 6” wood mirror cell on the mirror and press them together you have attached the mirror to the wood permanently.

This I don’t recommend. The RTV is generally a good soft pliable material. But over time it can get hard and pull on the back of the mirror and warp the mirror.

The 6"f5 mirror will rest on the three cork pads attached to the back of the mirror. The mirror will be attached to the mirror wood circle by using edge-clips. Edge-clips can be made of many things, brass, aluminum, rubber and plastic. The edge clips will block a small amount of the star light hitting the mirror, but will keep the mirror from falling or breaking loose of the wood cell. If the RTV or glue gives way when you are transporting your telescopes it is possible that the mirror will roll down the tube and hit the secondary mirror causing chips to both.

Figure 8: Put small cork pads on the back of the Mirror at .707 * R of the mirror

You will use brass edge-clips that are made by bending over about 1/2” of a brass strip. Cut 3 pieces of brass from a sheet into strips about ¾” wide and 2” long. You will cut off any excess later.

Sand or file the edges of the brass so they are not sharp.

Bend one end of the brass strip over about 1/2” from the end. Attach one cork pad to the under side of the 1/2" lip close to the tip. Lay the1/2” bend on top of the mirror and let the rest hang down the side of the mirror wood circle. Now, attach another cork pad in line with the center of the edge of the mirror on the inside of the brass tab.

The tabs will keep the metal from touching the glass mirror.

Align the cork pads so they isolate the brass tab from touching the glass.

Now mark the brass tab to drill a hole through the brass and into the edge of the wood mirror circle. Mark the brass and drill a small hole though the brass. Then find 3 wood screws about 1” long with flat heads. Pre-drill a small hole for the wood screws. Then assemble all three brass tabs and screw them to the mirror cell wood. One each brass edge clip at 120 degrees around the edge of the mirror.

Figure 9: Mirror and mirror cell wood where an edge clip will be placed

The brass edge clips will just touch the glass mirror top to keep it from tipping or sliding around. You may need to slot the hole you drilled into the edge clip where it will attach to the mirror cell wood circle. Drill the brass tab hole a bit larger than required so you can slide the tab up and down.

Now mark and trim off any excess brass that sticks out past the bottom of the mirror cell wood circle.

Now remove the tabs and paint them flat black.

Wash the 6” mirror concave reflecting surface with liquid soap and warm water for the last time. Do not touch the mirror front surface after you wash and dry it. Clean paper towels can be used to blot and lightly dry the mirror front surface. Do not use rags or paper as they can scratch the mirror coating.

Prepare the inside of the telescope tube by spraying it flat black. The inside of the tube should not show any reflections.

Figure 10: Telescope tube must be painted black inside

Figure 11: Mount the Mirror Cell so the bolts do not stick out

Check if the telescope tube is now ready. It should be finished painting on the inside and the outside.

Then, assemble the final mirror cell and install it in the telescope tube.

Install the edge clips for the final assembly of the mirror cell.

Press only hard enough to make the cork pad of the edge clip lip firmly touch the mirror. Now, tighten that side screw. Do this for all 3 screws.

You may need to bend the 1/2” tab so that the cork pad will just touch the mirror. Do not touch the mirror aluminized front surface with your greasy fingers. The oil from your hand or fingers can etch the coating and this will cause the aluminum coating to break down and turn dark.

Do not install or attach any other telescope parts onto or into the telescope tube. The mirror cell assembly should be the only piece attached to the inside of the tube right now.

You will now measure up the telescope tube inside from the front of the mirror the focal length of the mirror. The 6”f5 mirror has a focal length close to 30”. Therefore any star light coming from infinity stellar objects will focus in front of the mirror at a focal point 30” from the mirror.

If you have not fabricated the spider and its secondary mirror holder do that next.

The eyepiece focuser can only be installed on the outside of the tube after the spider and diagonal secondary mirror has been installed.

Using the 3 mirror cell wing nuts, align the mirror while looking down the front of the tube. Place two strings across the 8" tube so they from a "+" at the center of the tube.

Look along the center of the cross strings and find your eye reflected back onto your eye. Align the mirror using the 3 spring loaded bolts until your eye does reflect back on itself and at the center of the 8" tube.

You are done here. Continue with the spider assembly.

Figure 12: Place two strings across the front of the tube and align mirror to the center "+"

Get help on Dobsonian type telescopes by going to GOOGLE.com and search for things like "Dobsonian", "Dobsonian Telescope", "Dobsonian Mount".

You may run across several different types of Dobsonian mounts with features you would like to add to the current design.

Also you will find that some of the modification we did to the class telescope are not found in other telescopes. Yours is better.