Dobsonian Telescope¶
I’m not an expert in astronomy, I’ve never built (or even used) an astronomical telescope before, but it is something I have always wanted to do. It hadn’t occurred to me that you could build a telescope, but years ago I ran into some instructions to build a Dobsonian telescope. At the time I found it quite difficult to find the optical parts and even struggled to source the specified 10-inch cardboard tube, and moved on to other projects. Recently I stumbled across the Stellafane guide, an excellent introduction to Amateur Telescope Making and with renewed enthusiasm I set about sourcing the parts I would need. I was excited to discover that telescope parts could be sourced quite easily online and that the cost wasn’t too unreasonable. Ironically, the large cardboard tubes still seem quite hard to find.
Design¶
The Stellafane guide is a very good introduction and provides all the details you need to get started. I won’t try to duplicate it here, I will just document some of the design decisions I made for my particular scope.
I opted for an 8-inch reflector, and this choice was mainly down to economics. I found a mirror set including a 200 mm (8 inches) primary mirror and a 40mm secondary mirror. I found that the price of the mirror seemed to increase quite significantly beyond this size. I think that a 6 or 8-inch reflector should be a reasonable size for a first telescope. The mirror has a focal length of 1600mm which gives a focal ratio of about f8. I think that the relatively high focal ratio should give quite high magnifications and should be good for observing planets.
As far as possible I have tried to follow the Stellafane guide, for a first telescope I think it makes sense to use a tried and tested approach. The most significant deviation I have mode is in the construction of the tube. I have struggled to find suitable cardboard tubes [1] so I wanted to explore some other options. It is quite easy to build a square, or hexagonal tube using wood or plywood. Aluminium or PVC tubes might also be options. A different approach entirely would be a truss-tube-based design.
In the end, I thought it might be fun to build the tube using 2020 T-slots. These aluminium extrusions work like a scaled-up version of Mechano and are often used to build CNC machines and 3D printers. Although I haven’t seen many other telescopes using these profiles, I thought it might be worth a go. Apart from being easy to get hold of, the main advantage of using this type of material is the flexibility it affords, allowing the design and dimensions of the scope to be easily adjusted.
The size and focal length of the primary mirror are the primary factors that determine the dimensions of the scope. Stellafane provides an online design tool that can be used to work out the main parameters.
The rest of the design is a balancing act, involving tradeoffs and compromises.
The first tradeoff is the size of the secondary diagonal mirror. Making the mirror too large will block too much light from reaching the primary mirror, too small and not all the light reflected from the primary mirror will reach the eyepiece.
Since I already have a 40mm diagonal mirror, the decision has been made for me and it turns out that this is a good size in any case. The only other dimension I can change is the distance from the secondary mirror to the eyepiece, once this has been decided, the distance from the diagonal to the primary is calculated by the tool. The goal is to avoid vignetting (blocking the light path) by the tube or the focuser while maximising the illuminated diameter.
The distance from the primary to the eyepiece is a combination of the tube diameter, the thickness of the tube and the height of the focuser. Constructing the tube using 2020 t-slots allows us to make the tube any diameter we want, but the thickness of the tube is 20mm which is much thicker than a cardboard tube. The focuser I bought has a fairly large minimum height and lots of travel. A tube width of 250mm seems to be a good compromise giving a 12mm 100% illuminated diameter and no vignetting.
The Newtweb utility generates a handy ray trace which helps visualise the design and diagnose the cause of any vignetting. Once the key parameters have been designed, the utility calculates a handy table with all the measurements.
Optical Tube Assembly¶
Now we have the dimensions it is a simple matter to design the optical tube assembly using the 2020 profile.
The tube is a basic cuboid, at first, I just used corner connectors but found that the tube wasn’t stiff enough and needed some trianglation.
I added corner blocks to each of the corners and aluminium gussets in the centre and this solved the problem very nicely. I only added the gussets to the top and bottom of the tube since the altitude bearings on the side will add support to this dimension.
Some of the hardware had a silver/chrome finish, I have tried to reduce unwanted reflections by either painting these matt black or by covering them with self-adhesive black felt. The open-tube design may also benefit from a shroud and additional flocking, and this is something I may address in the future.
Mirror Cell¶
For the mirror cell, I stuck quite closely to the Stellafane design, but I did have to modify the lower part to fit into a square rather than a circular tube. I incorporated plenty of ventilation into the design to assist in the cooling of the mirror. I cut the parts from 18mm plywood and again painted them matt black. I used 2020 corner pieces to fix the mirror cell into the tube. One nice thing about this is that the distance between the mirror cell and the secondary mirror can easily be adjusted.
I stuck with Stellafane’s recommendation to use silicon adhesive to mount the mirror, but I did take the risk of substituting ‘coffee stirrers’ for ‘popsicle sticks’!
Spider and Secondary Mirror Holder¶
The spider and secondary mirror holder need to be adjustable in a couple of different dimensions to allow the scope to be properly aligned (collimated). This is another place where we can take advantage of the inherent adjustability of the t-slots. The secondary mirror holder is a short length of t-slot cut off at 45 degrees, and the mirror is glued to this. The secondary mirror holder is suspended from the edges of the tube using a length of 2mm x 10mm aluminium strip. I have made this strip relatively thin to reduce the diffraction caused by blocking the light path. The strip is attached to the tube using 2 corner blocks. This allows adjustment from side to side, into and out of the tube as well as rotation in two axes.
Again the secondary mirror is attached using silicon adhesive, using a ‘coffee stirrer’ as a spacer while the adhesive cures.
Focuser¶
This is another area where economics plays a factor, I opted for a relatively inexpensive 1.25-inch rack and pinion focuser. This seemed like a good compromise between cost and quality. The focuser is an all-metal design that seems to be well-made. The focuser is attached to the tube using a 6mm sheet of black phenolic resin, and a matching sheet sits behind the focuser.
Alignment and Focus Test¶
Before committing to the final positioning of the primary mirror, I wanted to check that I could bring distant objects into focus. Having performed a rough alignment I mounted a 25mm eyepiece and attempted to focus on some distant (and not-so-distant) objects. I live about 4 miles from the Malvern Hills <https://en.wikipedia.org/wiki/Malvern_Hills> and can see these from where I live. I hadn’t yet built the mount, so used dining room chairs to point the telescope towards the horizon. I found that the calculated position of the primary mirror worked almost exactly as predicted. With the focuser extended by about 25mm from its minimum height, I was able to bring the Malvern hills into focus and was able to see individual branches of trees that couldn’t be seen with the naked eye, which was quite promising.
The focuser I bought has quite a lot of travel and with the focuser close to fully extended I was able to focus on objects a hundred meters or so away.
What next?¶
Well, now the optical tube assembly is built and we can focus on distant objects. We have taken a collection of parts and we have a working telescope - although not yet a very practical one. In the next part of this project, I will be building a traditional Dobsonian mount. Then I can accurately colimate the telescope and get some first glimpses of the night sky…
