The optical tube assembly (OTA)

This is the raw PVC sewer pipe , originally intended for a job not so glorious , but you can give it a much more noble purpose by turning it into a precision astronomical instrument :)

PVC tubes are tough, easy to work with and relatively cheap. They are also a thermal insulator, your telescope will be much less sensitive to dew than the one with a metal tube.

Start with cutting off the wider part of the tube. This is easily done with a jigsaw. Mark your line around the tube, drill a 10 mm hole next to the line (on the part that is wasted!) put the blade in the hole and cut the tube. Use flat black spray to paint the inside of the tube and let it dry.

Dimensions :


 

150 mm F8 primary mirror 200 mm F6 primary mirror
focuser height 100 mm 100 mm
mirror face to focuser hole 1011mm 993 mm
mirror cell height (2) 90 mm 90 mm
focuser to front of tube (3) 100 mm 120 mm
total tube length 1200 mm 1203mm

The parts dimensions listed here are more or less typical. If you make your own mirror cell by the plans provided here, and your mirror is 20 mm thick, the mirror cell height (including mirror) will be 90 mm. If your mirror is 30 mm thick, you will have to add 10 mm to the "mirror cell height" All other dimensions remain the same.
But if your focuser is different as the 2" , 100 mm tall Meade DS described here, you will have to alter the "mirror face to focuser hole" dimension.
Example : A low profile helical focuser is typically 50 mm tall, this means you need to ADD the difference of 50 mm into the "mirror face to focuser hole". As a consequence , your tube total length has grown by 50 mm. If your focuser is taller, 120 mm for example, you need to subtract the 20 mm difference from the "mirror face-> focuser hole" dimension. The part of the tube pass the focuser (labeled 3 on the drawing) is not critical.. but you will want at least 100 mm here, or even better, 200. This prevents a lot of stray light that comes from streetlamps etc to interfere with the light coming from the stars. If your cargo space is tight, you can cut the tube just behind the spider vanes.... but it is recommended to make a removable dew and light shield out of cardboard or similar that you can attach on the telescope at the observing site.

The focuser, commercially made

This is the focuser I used on most of the telescopes I built. It is a Meade 2" DS series focuser. It comes with adapters for 2" and 1.25" eyepieces. At 19$, it is a good value for the money, although it is not a high end product, but it will serve its purpose very nicely. Highest quality products like JMI can easily cost a couple hundred $. Alternatively, you can buy a 1.25" or 2" rack&pinion metal focuser at the Orion website

Homemade focuser

This is a 2" helical focuser made out of common plumbing parts. This particular item is called "toilet flush valve" and it will cost you around 6$. If the saved 10$ is worth the effort, it is up on the telescope builder to decide. An even simpler focuser form is the drawtube focuser. Basically, it is made out of 2 tubes, usually plastic, that fit into each other, and the top tube has 1.25" ID. It can even be made out of cardboard.
Another alternative is the PVC focuser

Mirror cell , commercially made

Commercially made, fancy, high tech mirror cells are very expensive.
The one in the picture, 200 mm mirror size, will set you back 220 €!

Homemade mirror cell

This is a homemade mirror cell. Basically, it is made of 2 particle board discs, 6 screws and nuts, 3 compression springs and 9 felt pads. How much does that cost? 10$? :) This is one of the parts that really saves money if homemade! And it works the same! A typical 150 or 200 mm mirror is thick enough not to require a floating mirror cell like the one above.

Spider, commercially made

A commercially made spider can cost you anything in-between 20 and 200$. But it can also be easily made.

Homemade spider

Hmm... what do we have here? A short piece of plastic tubing cut at 45 deg, a plastic hexagonal male-female adapter, 3 small woodscrews, 3 hacksaw blades and 3 screws :) Voila, a spider ! This is how it looks like mounted in the tube.

The curved spider vanes give a spike-less image

Eyepiece and finder scope

To observe with your telescope, you need at least one eyepiece.
If you are in doubt which one to buy, you can't go wrong with a 26 mm Plossl. This is a good, primary, low power eyepiece for your telescope. It will give you 50x magnification with the telescopes described here.
And to observe, you also need to aim the telescope at the object you wish to see. I would recommend a 8x50 finder scope, (esp. for the 200mm scope) but a smaller one, 6x30 will also do.
If you are on a tight budget, you can make both eyepiece and finder scope from surplus lenses

Drilling the focuser hole

The best tool to make the focuser hole is the hole saw. For the Meade 2" (and most other 2" focusers) you will need to make a 60 mm hole. Measure carefully. For 1.25" focusers, you typically need a 40 mm hole. The "mirror face-> focuser hole" distance from the table above refers to the CENTER of the focuser hole, not the edge!

Also, make sure to secure the tube FIRMLY with a clamp to your workbench.
Drill SLOWLY (low RPM) and GENTLY (the weight of the drill is too much.. hold it up, the saw should barely glide on the tube)
This way you will make a clean cut hole, with a low injury risk.

Alternatively, you can draw a 60 mm circle, and drill many small holes along the line. You can make this "toothy" hole clean with file and sandpaper

In addition to the main hole, you will need 3 or 4 small holes, for the screws to mount the focuser on the tube. Their number and position depend on your particular focuser.

Plans for the primary mirror cell

Top view Side view

Rear view

This is how our simple, but fully operational mirror cell looks like. Basically, it is made out of 2 particle board discs, the top, smaller one having the same diameter as the mirror, and the larger, that will cover the rear of the tube, has the same diameter as the inside diameter of the telescope tube. You can see 3 bolts with springs. These are used to adjust the aim of the mirror, or collimation.
This is essential for good quality images in the eyepiece.

Lets cut the particle board discs first. Optionally, you can cut out a 80 mm opening in the "tube board" and install a 12 V cooling fan, from a PC power supply for example.

The fan speeds up the cooling down of the mirror. Now is a good time to check if the "tube board" fits inside the end of the tube snugly.

If its too large, work the edge with rough sandpaper. If its too loose, you can wind a couple of turns of electricians tape around the edge until you have a snug fit.

 

For this part, you can use any flat piece of metal, at least 3 mm thick. On it, you need to drill two 4 mm holes, so you can attach it to the mirror board with 4x16 woodscrews. On the other end, drill
a 5 mm hole, and tap in a M6 thread. You need 3 such pieces.
Attach the 3 plates to the "mirror board" with screws, 120 degrees apart. Make sure that the part of the metal plate does not stick out more than the diameter of the "tube board"! Example :
Mirror board diameter 150 mm
Tube inside diameter190 mm
Max overhang15 mm (you need some clearance)

Now, put the "mirror board" on top of the "tube board" , center over center. Trough the M6 threads, mark the drilling holes for the collimation bolts. Try to be as accurate as you can. Remove the mirror board and drill 7 mm holes on the marks. Use a drill press if you have one. Paint the mirror cell parts flat black.

Assembling the mirror cell
 

Installing your mirror is the LAST thing to do!

Take a collimation bolt (M6x60) , put a washer on it, slide the washer to the bolt head and put the bolt trough the hole on the "tube board".
Now put another washer on it, and the compression spring on top of it.
Take the mirror board, and screw the collimation bolt in the thread, just a few turns.
Do the same with the remaining bolts and springs. Now tighten the collimation bolts so that the springs get about 1/3rd compressed.

Try to tighten them evenly, check if the boards are parallel You have just made yourself a mirror cell :)

Homemade spider (secondary mirror holder)
 

These are the "raw components" for our telescope spider. Missing in the picture is a compression spring. PVC tube diameter should be the same, or slightly smaller than the secondary minor axis.

Hacksaw blades can be "cut" to length by holding the blade at the point where we want to shorten it in a vice or pliers and simply brake it off.
This is easy, hacksaw blades are very brittle.


After we cut a piece of PVC tube at 45 degrees, we need to close the tube openings. You can, for example, use a empty CD box, cut approximately sized (larger) pieces, glue them with epoxy resin on the PVC tube and then sand off the excess plastic. But, before that, epoxy a M6 bolt on the center of the part that is cut perpendicular. This holds the center bolt and the spring so you can adjust the "vane holder --> mirror holder" distance. The 3 collimation bolts tilt the mirror holder, so the telescope can be collimated.

On the hexagonal adapter, drill three (picture below) 3 mm holes in the corners and tap in a M4 thread. This is very easy in plastic.
Attach the spider vanes (hacksaw blades) to the flat surfaces of the hexagon (vane holder) with the 4x16 woodscrews, trough the holes already in the blade. Secure with a drop of epoxy resin.
 

 

Inside life of the spider assembly

Now we have all the parts needed to assemble the telescope! :)

----> Assembling and finalizing the telescope

1999-2005 Berislav Bracun