Grinding a Telescope Mirror: The Non-DIY Project


John Dobson

I did not know John Dobson, nor do I know someone who knew him, but I feel like I did from all the testimonials I have read.  At the least, my telescope build is an extremely distant branch of his legacy.

Mr. Dobson is the namesake for what is commonly referred to as the Dobsonian telescope.  He did not invent this type of telescope, but instead ingeniously brought together a number of amateur telescoping making (ATM) techniques.  This compilation is a method with general designs for how to build your own Newtonian reflector on an altazimuth (up/down left/right) swivel mount.  Sometimes you see references to only the mount as the Dobsonian part, but a true Dobsonian refers to the complete package, from the mount to the tube to even hand grinding the primary mirror.

This latter part, concerning the primary mirror, is what I stumbled on conceptually at the beginning of my telescope build journey.  When you start any type of project, and especially when you undertake what we call DIY projects today, you will have many “make or buy” decisions.  How much of the project will you, personally, create from raw materials, and how much will you rely on pre-built/pre-manufactured components?

Platonically speaking, there is no such thing as a true DIY project.  I am not going to grow my own forest to harvest trees for wood, nor start a lumber company to secure the requisite labor and machinery for my platonic lumber mill.  Nor would I obtain raw silicon to fabricate my own nano logic gates for a homemade CPU.  Still, there is a generally accepted boundary for raw materials – products that are not a specific end to themselves but are intended to be reshaped and combined with other raw materials into some form of finished component.

The primary mirror of a Newtonian reflector is indeed the main component of the telescope.  Its aperture determines everything else about the telescope’s dimensions and how “powerful” the final instrument will be.  The creation of primary mirrors is a deep step into the peculiar world of optics.  Remember the Hubble Telescope’s original blunder of having the wrong curve on its mirror?  That’s optics.  Whether we are talking big or small mirrors and lenses, the universe of optics and optical creations are not really an end-consumer endeavor.  There is a level of precision required unique to that industry.

If you follow any guide on John Dobson’s telescope build strategy, you will quickly learn that construction of the primary mirror was the core task of his method.  Below is what I assume was an old VHS era documentary on Mr. Dobson’s step-by-step approach, and most of it (a little over half) is about grinding and finishing the primary mirror.

If you watch this, or follow another guide on the Internet for grinding your primary mirror, it seems to be truly a daunting task.  It is beyond hard work and effort and closer to a stint in a hard labor camp.  Why would anyone do this to themselves?

I am in no way criticizing the method John Dobson laid out.  Too often, we judge the past by our perspectives grounded in our present.  60 years ago, the nature of amateur telescope making was very different.  There were no online guides, no easily searchable list of vendors to purchase obscure products from.  If you wanted a big telescope, I’m guessing the overall costs were too prohibitive for anyone except established institutions.  If you wanted to build a nice big telescope of your own to see the universe, you had to build your own, even scavenging for your raw materials at times.  This, I surmise, was the world of John Dobson and the source motivation for what would become the Dobsonian design.

I asked myself, “could I grind my own primary mirror?”  My weak answer was…maybe.  I have completed DIY projects before, but the grinding of a primary mirror seemed beyond my need to satisfactorily say that I could build my own telescope.  There is so much more to it than just the primary mirror – the tube, the many proper measurements, the mount wood cutting, the secondary mirror’s spider, the swivel construction, to name a few.  I decided that acquisition of the primary mirror, and all the optics in general, would be a firm “buy” decision for my telescope project.

There are other reasons to refrain from a homemade primary mirror as well.  I concluded, after all the investigations I did into the task, that there is no such thing as a true homemade primary mirror.  A DIY build means you can run to your local hardware store, buy the parts, and then construct what you need in your garage or other appropriate home venue.  Construction of a primary mirror requires, as a final step, the aluminization of the mirror’s surface.  This critical step is not a home DIY task.  You would need to find an industrial optics company willing to perform the aluminization for you.  You can spend weeks of your life grinding the mirror, then be lost because you cannot find an aluminizer.  Unless you know someone, you are going to be left having to ship your precious near-finished glass to an unknown company, somewhere and at great cost, hoping it will eventually be returned as the desired finished product.  I’m not saying it couldn’t be done, but I safely believe it is too much of a risk of both effort and money, especially when you can buy a finished primary mirror relatively easily today.

I say “relatively” easily to buy a primary mirror, because even that was a challenge, although nowhere near as hard as grinding one yourself.  For what I call consumer high-range optics, it can be very hard to find a supplier for this type of work.  Only a few online merchants offer shopping-cart style access for primary mirrors, and their supplies are limited.  Many companies post that they will make custom mirrors, but usually at a high cost, or only make very large custom mirrors, like 16″ and above.

We live in a much different world today from when John Dobson started building telescopes.  The bottom line is that, unlike most DIY projects, it is going to cost you more to build your own Dobsonian, regardless of make-vs-buy for the primary mirror, over purchasing a commercial Dobsonian or general reflector from one of the big established merchants.  So from the DIY perspective, your best route is to find one of the vendors or re-sellers of the commercial primary mirrors supplied to the Meades, Celestrons, and Orions of the market.

Who should attempt to grind their own primary mirror?  For one, masochists, and I mean this in all seriousness.  Another group that could reasonably give it a shot are those involved with any type of materials shop, from wood to metal, where building anything is just part of your routine.  And those connected to building components for the optics industry could certainly do this as well.

For the rest of us, if you really enjoy a challenge, then grinding a mirror is for you.  But for nearly all stargazers contemplating building their own telescope, I recommend purchasing all your optics, including the primary and secondary mirrors, focuser, eyepieces, and finder scopes.  Your homemade telescope will be so much more than a few specific components.  It is the journey, the knowledge you will gain, and the final satisfaction garnered from creating something far greater than the sum of its individual parts.

When I Decided to Build a Telescope


When you enjoy using your telescope to look at the night sky, you are bound, one day, to decide that you want a second telescope.  You may have “aperture fever” to gather more light, to see more.  Or you may have a reflector and want to complement it with a refractor.  Big vs. small.  Stationary vs. portable.  Sketching vs. astrophotography.  There are many reasons you will (and you will) consider having a second telescope.

Almost a year ago, I bought my first “real” telescope, a 127mm Maksutov-Cassegrain.  It is relatively small in the overall scale of telescopes, but I did not want to buy too much in case I quickly lost interest.  But I did not lose interest, and really enjoyed using it to view the Moon, Jupiter, Mars, Saturn, and the Sun (with appropriate solar filter).  I have also used it to view deep space objects like the Andromeda Galaxy, star clusters, and nebulae.  Although the latter are viewable, it is clear to me that my small 5-inch Mak-Cass is primarily intended for solar system observations.

After a few months, I decided to investigate getting a second telescope that (1) had a noticeably larger aperture and (2) would specialize in viewing deep sky objects.  I immediately understood that the most obvious trade-off would be in size and weight.  If I desire, I can take my 127mm Mak-Cas anywhere with minimal effort.  But something far bigger would likely require greater setup and planning to transport, even just from my house into my yard.

I also wanted to at least leave the door open for astrophotography.  With the Mak-Cass, I use the afocal method to take pictures.  Afocal photography is just a fancy term for holding a camera up to an eyepiece (versus the more sophisticated method of bypassing the eyepiece and using a proper T-ring or other adapter for a DSLR camera).  I have a few afocal adapters that allow me to take pictures with my smartphone; most of the pictures on this blog were taken afocally.

After reviewing the major types of telescopes, I was leaning towards a Newtonian reflector, specifically a “Dob” or Dobsonian.  I will blog about the Dob’s namesake, John Dobson, in a future post.  Reflectors in general are the most economical per size, and are intended solely for night use (since the images appear upside down, you can’t really use them for terrestrial applications).  They are also primarily intended for eyepiece viewing, versus photography, but some sort of photography method is entirely possible if you want to.

I went down the path of reviewing Dobs from the major telescope companies (there are three primary ones in the U.S.A. – Celestron, Meade, and Orion).  During my investigations into price and features, somehow I came across the idea of building a homemade telescope, from scratch.  This intrigued me, as I had built a few other projects around the house before, even though I am not a craftsman by trade nor do I have any type of workshop (except for the half of my garage that becomes a de facto workshop when I do one of these projects).

If you search online for homemade telescopes, you will see a wide range of designs and efforts.  Some look crazy.  Some are well beyond anything I would attempt.  But others seemed far more modest and manageable to execute.  After a lot more research, I decided to give it a go.  I figured that a telescope, after all, is ultimately a technology hundreds of years old, so with the right planning and designs, it should not be too difficult for anybody like me to build one.

And building a telescope, I thought, would come with a great sense of accomplishment.

So I intend to write about my telescope building experience.  I will not write a “how to” guide, as there are a number of perfectly acceptable guides on the Internet, as I will reference.  Instead of a step-by-step chronology, I plan to give insights into my efforts over the several months it took to plan and build.  When I am done, you will probably have a adequate idea anyway of how I built my 10-inch Newtonian reflector on its Dobsonian mount.

In my next telescope build post, I will discuss one of the first major decisions – whether to build or buy the primary mirror.