Amateur Telescope Making - Dr Chong

This was extracted from @Dr. Pierre Chong's Wall. I compacted it into a document for easy viewing.

 

 

Since 1987 we have made 50 Dobsonian Telescopes. Each telescope was completely home-made. The only telescope accessories that were purchased were the eyepieces. Listed below is a detailed description of the processes involved in making this type of reflecting Newtonian telescopes.

 

 

Process :

 

(1) Obtain thick pieces of plate glass , up to one and a half-inch thick.

 

(2) Cut out circular blanks of glass using the "Milo Tin Method". The glass blanks used were usually 7-inch in diameter.

 

(3) Make a mirror grinding table.

 

(4) Purchase the following materials: rough grinding powder, medium grinding powder, fine grinding powder, polishing powder, polishing pitch, two sheets of 4 feet X 8 feet plywood (half-inch thick), various tools for metalwork and woodwork, etc.

 

(5) Do the rough grinding process. For a 7-inch mirror blank with 48-inch focal length (about f/7), this process will take about 2 hours

 

 

(6) Do the medium grinding process, which will take about 3 hours

 

(7) Do the fine grinding process, which will take about 4 hours.

 

(8) Use the pitch to make the polishing lap. By using the polishing lap, do the polishing process which will take about a week!

 

(9) From Process (5) to Process (8) we were using the one-third-centre-over-centre stroke. This will make the bottom surface of the mirror blank to be a very accurate circular concave surface. The top surface of the tool blank will be a very accurate circular convex surface.

 

(10) This is the most exciting part of the whole ATM project, that is, the parabolizing stroke! This process will convert the circular surface of the mirror to become a very accurate PARABOLA. Why is the parabola so important? Answer: Only a parabola will focus a parallel beam of light onto an exact point! No other surface can do it!

 

 

(11) Building the Foucault Tester. It is very simple to build a Foucault Tester and this equipment will be able to measure very accurately the shape of the mirror surface.

 

(12) Use the Foucault Tester to measure how good is the parabola on the mirror surface. The Foucault Tester can measure a surface accurate to a millionth of an inch. Wow! It is that accurate.

 

(12) After the many rounds of Foucault image testing the shape of the mirror surface has been found to be a very accurate parabola.

 

(13) Time to make the secondary mirror (which is an optical flat). It is quite easy to make the secondary mirror provided it is a small one.

 

(14) With both primary and secondary mirrors polished, it is time to coat them with a very thin layer of spectroscopically pure aluminium. This can be done using a Vacuum Coating Unit. After this coating process both primary and secondary mirrors were coated with a very thin layer of shining alulminium thin film.

 

(15) Time to make the wooden tube and mount of our Dobsonian. We are talking here about making an ALTITUDE-AND-AZIMUTH MOUNT, sometimes shortened to Alta-Azimuth mount. This mount is very easy to make and it is very stable and can hold a very heavy weight and yet it can still move very easily.

 

 

(16) With simple carpentry skills we can make the tube and alta-azimuth mount of the Dobsonian telescope. Things like sawing, drilling holes, hammering nails, sand-papering the outside of the telescope, painting, etc. The next process will really make our hearts beat very fast and the excitement UNBEARABLE!!

 

(17) We now put the final touches on the telescope with the primary and secondary mirrors all in place and properly collimated. The next thing to do will be the famous "FIRST LIGHT MOMENT!!!!!!". This is the time when you first use the telescope to see an astronomical object and evaluate how good is the optics of the telescope.

 

(18) Usually I look at the following three objects for FIRST LIGHT MOMENTS. They are Saturn, Jupiter and the Moon. With previous experience in using other telescopes I would be able to tell how good is the optics of the currently made Dobsonian telescope. At magnifications of up to 200X, if I can see well Saturn's rings, bands of Jupiter and pin-point moons, and the Moon's craters well defined, then the optics is classified as good enough for me! And then inside myself I can hear myself shout WoWoWoWoW!! When you can see an astronomical object very well with a telescope that you have made with your own hands, the feeling is SO SPECIAL and is irreplaceable!! After making the first 7-inch f/7 Dobsonian, the next logical thing to ask oneself is to make another one (with different aperture), and the next one, and so on.

 

(19) So far USM-ASTRO have made 50 Dobsonians, and this is a small number. We are now reviving our ATM Section in USM. Any school or college astronomy club, teacher, student or member of the public who is very keen to make his or her Dobsonian please contact Dr. Chong.

 

(20) Remember, ATMing sure is a lot of superfun! So far I have given a very brief summary of all the processes that are involved in making a Dobsonian. In future postings, I will be presenting very detailed information of how to make the Dobsonians, where to purchase the materials locally, design and use of Foucault Tester, etc. Videos on ATMing will also be posted in future!