Making a 4.25 Inch Dobsonian Reflector Telescope





          In 2010 I took a telescope making class at the Randall Museum.   In that class I attempted to make a 12.5 inch telescope.  This attempt failed.   The most notable part of this failure was in breaking John Dobson's cardinal rule, namely if you are going to break your mirror, do it early.  I regrouped and decided to start small.  


          Now having finished this scope, I believe it was the correct decision.   Along the way I've heard a number of arguments against this approach.   The most notable one goes like this;  you are probably only going to make one scope, so make it as big as you can imagine.     From my perspective, this approach has a number of flaws.    To begin with, making a telescope requires a number of skills.   These all may be developed as you proceed, but as with most things in life, you learn by making mistakes.   In the case of a telescope, any of these mistakes can cause you to create an inferior device, or in the worst case, ruin the project entirely.    In addition, a larger scope can take a long time to finish.   I can't think of anything more disappointing than spending 6 months to a year grinding and polishing and figuring a mirror, and then ending up with poor optics.


          On the other hand, grinding and polishing a 4.25 inch mirror goes very quickly.   It went so quickly for me, that when I discovered during the polishing phase that I had gone through my grits too quickly, it was no big deal to go back and do the entire fine grinding again.   Over the course of three weeks I probably spent at most 16 hours doing the mirror, and that includes both passes.


          But the mirror is only half the project.   There are a number of other things you will need to construct to make a telescope.    Some or all of these my be purchased.    But even knowing which items to buy and which to build is helped by experience.    Working on a smaller scope gave me the time to at least attempt all the components.   In the end I opted for a purchased secondary and eyepiece, but not before building my own.   


A Caution


          I suggest that you do not attempt to build a telescope if your goal is to save money.   While it is possible, I think it is rather unlikely.   You would be better off scanning eBay and Craig's list for a bargain.    The reason to build your own telescope is because you want to learn how.   This is a singularly rewarding experience, but it is quite unrelated to the quest of looking through it at the heavens.    To use a scope you need your eyes, a good location, often some warm clothing and an interest in what you are looking at.   To build a scope you need patience, elbow grease, some carpentry skills and a love for figuring things out.    There will be a lot of figuring things out.    Much of my work was done at Tech Shop in San Francisco, where I was able to use their wood shop and milling machines.