150mm F6 Dobson -
This Is Where It All Began (2003)
When I was only eight years old or so, I found a book about amateur telescope making from our local library. Since then I have always wanted to make my own telescope. However, I was only a little boy and decided to wait for a couple of more years.
As years went by I had bought a couple of telescopes. First I had the famous Russian Tal-1 telescope and then 200 mm Skyliner made by Helios. Even though I had a good telescope to observe with it didnít extinguish my desire to make my own.
When I was in a Upper Secondary School I just couldnít hold it anymore. I sat down with a cup of coffee and piece of paper and started sketching my first telescope. The first design was a 150 mm F4 Newtonian (diameter of the main mirror 150 mm with the F-ratio 4). However, I was told that a F4 mirror is not very easy to make especially if you are grinding your first one. Therefore I decided to make a F6 mirror instead of F4. I knew I was participating an English language course in Australia next summer and of course wanted to have the telescope with me under the southern hemisphere. I designed a truss Dobsonian telescope that I can dissemble easily and take with me to other side of the world. This designing phase took around thirty hours.
I decided to build my telescope in school during my shop classes. First I made a main mirror holder. Next the secondary mirror holder was done and little by little all other parts. It was funny to see how my class mates were filing the same little key chains entire semester and I was building a telescope next to them. However, because time was very limited and all sorts of breaks were disturbing my work I wasnít fully satisfied with my work quality. Most of the telescope is made in school because it was very well equipped and full on free materials!
Grinding the Mirror
I decided to grind the main mirror by myself. Otherwise itís not real telescope making right? My father told me to buy a finished mirror for many reasons but I told him that I knew what I was doing...(not really)! I occupied my familyís bathroom for a couple of weeks and started to grind my first telescope mirror.
The abrasive I used for grinding was carborundum. Itís very hard material and cuts glass nicely. This first grinding stage is called ďrough grindingĒ. The main idea is to smooth all uneven surfaces and make such grinding strokes that they localize pressure at the centre of the mirror disk and hence result in rapidly the desired concave surface...in theory.
First I bevelled all four edges of the glass blanks in order to prevent chipping. I applied some abrasive and water onto the glass blank and started to grind as they said in my book. In fact, my Finnish book about amateur telescope making was my main source of information during my project. After six hours of grinding the surfaces were all faced and all bumps had disappeared.
The first day I ground ten hours straight without eating or drinking. I was almost dead and the funny part was that I didnít even see any concavity at all! What a lovely hoppy I thought. Apparently my straight strokes were too short because I tried to be too careful. I also didnít apply enough pressure. The second day I ground another ten hours but this time I made longer strokes. Yes, it works! By the end of the second day the focal length of my mirror was 1.5 m. By the end of the third day the focal length was 93 cm. Auch...my goal was one metre but I decided not to start lengthen it. Now I had the right concavity and I was done with rough grinding.
ďRough grinding is a caveman job, so do it like a caveman. Eat well, sleep well, and work like hellĒ Ė John Dobson
The next stage in mirror making is fine grinding. It took much less than I thought and I finished the job in one day. I covered my work post with wet toilet paper in order to prevent undesired particles flying around and found their way between my mirror and grinding tool. I used the same technique for polishing stage as well of course.
Polishing and Figuring
I didnít have much trouble grinding the mirror but itís polishing where I faced some difficulties. Pure pitch and cerium oxide is used to polish the smooth fine ground surface. Melting pitch wasnít too difficult but I had problems when I pushed my mirror against the hot pitch tool. First time I was too excited and forgot to make grooves for airflow. After the second attempt I ended up with a big air bubble between the mirror and the pitch. I couldnít separate the mirror anymore and I had to freeze everything. Frozen pitch is very brittle and easy to break. However, my next attempts where better and finally I managed to pour a smooth layer of pitch on my grinding tool. Later I learned that the pitch thickness should be much greater, around 10 mm. And it's also fine to apply pitch many times. Playing around with pitch is really something that takes some time to learn.
In order to ensure good surface contact and abrasive distribution between the mirror and pitch tool the surface of the pitch tool must be grooved. First I tried a hot knife but it didnít work very well. I decided to take the tool into a sauna. Now it was easy to work with soft pitch and I simply pushed the grooves into pitch using a ruler. When I was done with the grooves I put the mirror top of the tool and made a 30 min hot press. First I used soap to prevent clinging but then I realised itís better to use cerium oxide. I also learned itís not fun to mess around with pitch in your home. I found little drops of pitch in my house even half an year later!
I faced some problems during polishing phase. When the mirror was mostly polished the edge of the mirror was still dim. Telescope makers call this Turned Down Edge or DTE. Itís the most common problem among beginners. I tried to polish the tool on top for awhile but it didnít help. Later I heard I should grind the mirror both mirror on top and the tool on top. I masked the mirror down to 13 cm in order to achieve better image quality. Hence my mirror is actually 13 cm F7. Mirror making is really something you have to learn by doing...not by reading books.
After polishing the mirror was spherical. The next step was to figure the mirror into a parabolic shape that is required for a Newtonian telescope. It took around three hours to figure the mirror. I made a couple of careful rounds and then examined the shape of the mirror using a Ronchi grating. The mirror is parabolic when Ronchi lines are curved. It took me 65h to make this mirror. Other parts of the telescope took around 45h.
When I received my mirror from aluminizing I had to try it straight away. I placed the mirror on a pile of books and put the upper secondary cage front of it. I held an eyepiece and tried to find focus. Suddenly I saw a sharp image of our carís back light. Yes! It works!!
Mechanical Parts of the Telescope
Primary Mirror Holder
I copied the design for my primary mirror holder from my Finnish amateur telescope making book. The basic idea is to have your mirror between three supports that are welded into a base sheet made of aluminium. Surfaces that are in contact with the mirror are all covered with soft pads. Three springs allow collimation. If I have to make a new primary mirror holder I would simply glue the mirror on a plywood base. The holder I made is way too complicated and doesnít look nice either.
Secondary Mirror Holder
For the secondary mirror holder I used steel and sheet metal. Aluminium would be nicer but my school didnít have it. I trilled a 9 mm through hole into a 20 mm steel rod. Then I trilled another 10 mm hole mid-way. This allowed to mount a screw inside the rod. In the end I sawed the rod into an angle of 45į. The secondary mirror that i bought is then glued to the angled surface.
I used the same rod for the part that holds the spider vanes and collimation screws. Four cuttings are made in order to help mounting the vanes. The vanes are glued into their cuttings. Outer ends of the vanes are bended in to 90į. There is a M8 hole in the middle of the rod for the screw that holds the assembly together. I also welded three M4 nuts for the collimation screws. It was really difficult to get those aligned properly. Again...not too elegant and nicest design but it works.
Ylšosa, peililaatikko ja ristikko
I cut the rings for the upper secondary cage out of 9 mm thick plywood. Between the rings I screwed four 15 mm thick wooden rods. The focuser plate is also 9 mm thick and 70 mm wide. For the truss four L-profiles with welded nuts are mounted to the lower part of the cage.
The mirror box is made of 9 mm plywood as well. The outer dimension are 220x220x250 mm and there are 100 mm diameter declination circles attached both side of the box. The same method for the truss attachment is used in mirror box as in upper secondary cage. The L-profiles are just longer. The truss itself is made of eight 600 mm long aluminium tubes with outer diameter of 10 mm. Both ends of the tubes are crushed. A black shroud is covering the truss in order to prevent any stray light. When I assembled the telescope for the first time the scope turned out to be front-heavy. I had to put extra one kilogram of weight back of the mirror box.
The Dobsonian Mount
The Dobsonian mount is made of 15 mm plywood. Itís 260 mm wide and 300 mm high. The diameter of the bottom sheet is 300 mm. The mirror box and mount are designed so that during transport the mirror box can be placed inside the mount. Also the upper secondary cage fits inside the mirror box. I also have a nice custom-made bag for transport.
It wasnít too easy to find a suitable focuser for my telescope. It had to be extremely low in order to fit the upper secondary cage inside the mirror box. I didnít find any focusers that low and I had to figure out something by myself. I just went to our warehouse and start looking for some random piping parts. Suddenly I found just perfect threaded parts that can hold a standard 1.25Ē eyepieces. The focusing length is 20 mm. The focuser turned out to be very light as well. Problem soved!
Didnít look too promising all the time but yes, it looks like a telescope in the end. And it works well! I encourage everyone to grind their own mirrors and build their own telescopes. Itís really not that hard...all you need is patience. I have to say that the sky looks much more beautiful through your own telescope. ..after all that sweat and tears!
Just one more warning! You will get hooked on this! Iím already thinking about making a bigger mirror... :)
Please find the drawings for this telescope below: