Almost there :) A few more steps and you will have a complete, working Dobsonian telescope!

Mounting the focuser

This is a fairly simple affair. Align the focuser over the center of the focuser hole, mark the drilling holes and drill 2 mm holes for the screws, if you use woodscrews (works very well on PVC) Or, you can drill larger holes and use bolts and nuts to attach the focuser.

Mounting the mirrors

A word of caution: NEVER touch the aluminized surface of the mirror!

Lets start with the primary mirror. Put your mirror cell on a table, or other leveled surface. Use 9 felt pads to provide a soft surface for the mirror to rest on. Then make 3 blobs of aquarium grade silicone on the surface of the cell. Carefully unpack the mirror and place it centered over the mirror board. Cover your mirror with cotton or a soft cloth to protect it. Leave the silicone to cure for at least 24 hours.

Mounting the secondary mirror

Place the secondary holder that the surface for the secondary mirror remains in a horizontal position. You can clamp the spider assembly in a vice or similar. Apply a coin size aquarium silicone blob at the center of the mirror holder, and place the secondary mirror carefully and centered over the holder. Adjust if necessary. Again, leave the silicone to cure for at least 24 hours

Mounting the spider

This is a typical commercially made spider.
Basically, you need to drill 3 or 4 holes (3 or 4 vane spider) to be able to attach it to the tube wall.

But where?!

You need to measure the distance from the center of the secondary mirror to the spider vane. Now, measure the same distance from the center of focuser hole, towards the front end of the tube. Dill your holes here.

This way, when you mount the spider , the center of the secondary mirror will be exactly, or very nearly centered under the focuser. You can make adjustments for this with the center bolt on the spider assembly.

Mounting the homemade spider

No holes. No bolts. The spider you made is a self holding, self centering, curved vane spider. As most observers agree, it is optically superior to a classic, straight vane spider, especially when observing the planets or double stars at high power.
All you need to take care of is that the spider vanes are touching the tube wall at 120 degrees apart.
You can remove it in matter of seconds. The spider is held in position by the tension of the bent spider vanes alone.
I have such a spider in my telescope for over 2 years now and it has not moved since.
But, if you feel uncomfortable about this, apply a few drops of epoxy resin on the contact points with the telescope tube once you have adjusted the position of the spider vanes.



Mounting the primary mirror cell

Homemade or commercial, you need to attach the mirror cell to the back side of the tube. To attach your homemade cell, all you need to do is to drill three 4 mm holes, 120 degrees apart, 9 mm from the edge of the tube. Sink in the screws heads. Put the mirror cell inside the tube, and place the collimation bolts away from the holes you drilled to attach the cell to the tube. Use 4x 20 mm woodscrews to fasten the cell.

For a commercially made cell, refer to mounting instructions of the manufacturer

You now have a complete OTA.
ow, you can attach your finder scope, and put the tube inside the altitude bearing box. Also put the eyepiece in the focuser, choose a middle one (as per weight) if you have more eyepieces. This is necessary so you can balance the telescope. Balancing is done by simply sliding the telescope tube inside the altitude bearing box. When you found an optimum balance point, you can fasten the telescope tube by drilling a 3 mm hole trough the bottom of the altitude bearing box and the tube, and drive in a 4x20 mm woodscrew.


Next step is collimating the telescope. A telescope is like a musical instrument. If badly tuned, it cant play nice music. If the optical axis of the mirrors are not adjusted, the image contrast and detail will suffer tremendously.
To do this, we need a collimation tool. There are many variations of this on the market, from simple sight tubes to holographic lasers, costing anything in-between 19$ and 200$. But you don't have to spend any money on it :)

You can make a simple collimation tool in matter of minutes. Get a empty Kodak film canister, cut off the bottom, and drill a small hole (2 mm typically) in the lid center. What this tool does is to force you to place your eyeball at the center of the focuser if you wish to look inside....

When you assemble your telescope, and take your first look trough the collimation tool, it will probably look something like this, a complete mess :)

What is what


Start with centering the secondary mirror under the focuser. You can adjust the distance relative to the primary mirror with the center bolt of the spider assembly, and the tilt with the 3 collimation bolts. The secondary holder can also be rotated around the center bolt if necessary. At this step, you wish to see the entire primary mirror reflected in the secondary. You will also see a black ring around the mirror , that is the mirror cell. The ring and the mirror should appear concentric. The small reflection (secondary in primary) is adjusted in the next step


Now we have to "put" the reflection of the secondary mirror in the center. This is done by adjusting the collimation bolts on the primary mirror cell. You will note how big your new telescope is because you can't reach the bolts and observe trough the focuser at the same time. So, do it in small steps, 1/4 turn, one bolt at the time, and check what happened. Slowly bring the reflection in the center. You now have a collimated telescope ready for first light!

Collimation can be compared to shoelaces. When you do this for the first time, you can spend as much as 10 minutes, and it looks so frustrating and impossible. But, with little practice, collimation is a 30 seconds process. Check your telescope collimation before every observing session, especially if it was transported a long way and bumped around.

The finished 150 mm Dobsonian telescope

1999-2005 Berislav Bracun