Pipe/Optical Tube

An optical tube holds the primary mirror cell, secondary cell and the focuser unit. I used a 200 mm OD rigid PVC pipe for my 6″ F/8 telescope.

Please note it would not be easy to get the PVC pipe over 140 mm OD from regular hardware shops. You need to search dealers in your city who sells larger diameter pipes. If you are in Mumbai, you can go to Narayan Dhuru Street-Masjid area. There are many traders there who sells larger diameter pipe. You also need to check them if they will cut and give few feet of length from full pipe stock. You will get it anyway from the traders there. You can also check the construction sites if they have this size pipe in spare (They may give it to you for free!).

For my 6″ F/8, I purchased a 200 mm OD, 4 feet length rigid PVC pipe. It is 5 mm thick and quite rigid. Do the following once you get the pipe.

• Wash the tube well. Use a #180 waterproof sand paper to clean and roughen the outer and inner surface. Use #220 waterproof sand paper to smoothen the surface.

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• Make a hole for focuser around 4-5 inch away from one of end of the tube. We will call it front end of the tube going forward. I made it around 4.25 inches away from the tube front. You can do it after painting the inside of tube. In my pics it was done before I painted it.
• Make a circle using the compass. It should around the outer diameter of the focusing tube.
• Make another slightly bigger circle out of it as a guide to cut the hole.
• Use a drill to make small holes between these two circles.
• Cut the entire perimeter using a thin drill bit and power drill machine. You can use any other method too.
• Clean the edges using a round file and a sand paper.

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• Buy a matt finish blank paint from the paint shop. You may need to tell them to give you black board paint.
• Prepare to paint the inner surface of tube to absorb stray light and to prevent any glare.
• BEFORE YOU START, MAKE SURE TO KEEP THE WORKPLACE AS CLEAN AS POSSIBLE! use newspapers!
• Add some turpentine oil in the paint to make a thin consistency.
• Fix a used kitchen sponge to one end of a stick for painting. Don’t use painting brush! I used a spare aluminium pipe as stick however a wooden stick will work as well. Use stick around half the length of tube for easy work.
• Dip the sponge in the thinned paint and start painting the inner surface of tube. Use inside-out motion to paint the entire surface.
• Start painting from one end of the tube. Paint inner 50% from inside to out. Turn the tube and paint in the same manner of remaining 50% of the tube.
• Use medical gloves to prevent your hands getting stained.
• Keep turning the pipe and spreading the paint using the sponge stick to prevent uneven paint buildup and to get smoother finish.

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• After around half an hour or so when paint starts getting dry, use the sponge stick to pat the entire inner painted surface. It will help to create a textured surface which will absorb more stray light. See the below pics. The particles you see in the last image are sponge particles which will go away by the swipe of a hand after complete dry.

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• Let the paint dry for 24 hours.
• Once dry, clean the outer surface again with #180 or #220 waterproof sand paper.
• You can paint the outer surface if you wish
• Your Optical tube is ready for the next step!

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Go back to Assembly