I Designed and 3D-Printed a Newtonian Reflector Telescope. The Views Are Amazing
By: Rafael Fiol
Tags: Astronomy, Telescope, 3D Printing, 3d Modeling, Fusion 360

When I pointed this thing towards the sky for the first time and peered through the eyepiece, two thoughts immediately struck me: 1) holy s*** it works!, and 2) wow, space is awesome.

Regarding my first thought… I wasn’t so much shocked that it worked, but I was shocked that it worked so very well. There’s a lot that goes into engineering a good telescope, and I was not sure if 3D-printing with plastic filaments would measure up. To my surprise, this thing works great.

To be fair, with its 6" (153 mm) primary mirror, this is considered a small reflector telescope, and the engineering problems increase dramatically with larger telescopes. Nevertheless, even in this size class, mirrors have to be collimated, the optical tube can’t flex when you tilt it, the focal length has to be measured and engineered accurately, and the primary mirror has to have proper plate and edge support.

I’ll show you how I solved for these engineering requirements, but first it’s time for a little background.

What’s a Newtonian Telescope?

A Newtonian telescope is a type of Reflector telescope invented by Sir Isaac Newton in or around 1668. Today, this type of telescope is popular with amateur and DIY telescope makers because it has a relatively simple design (when compared to Refractor telescopes). Newtonian telescopes are usually less expensive than comparable quality/size telescopes of other types.

Generally speaking, there are two broad categories of telescopes: Reflector and Refractor. As a reminder, the 3D printed telescope we will be discussing is a Reflector. There are tons of great resources online that go into great detail about differences (and similarities) between the two. But for this article, I’ll keep it simple and say that Refractors use lenses to gather and focus light, while Reflectors instead use mirrors.

A Reflector telescope has two mirrors: the Primary (also called the Objective) and the Secondary. The primary mirror has a concave or parabolic surface and it is located at the tail end of the telescope. Its job is to reflect incoming light (which enters through the front of the telescope) towards a smaller secondary mirror positioned closer to the front of the telescope. Because of the parabolic shape of the primary mirror’s surface, the light is reflected along a cone-shaped path, towards a very specific focal point near (but just beyond) the secondary mirror. That secondary mirror is positioned at a 45-degree angle so as to reflect that light out towards the side of the telescope tube, where an eyepiece is attached. If the positions of the mirrors are accurate, the cone-shaped light comes to a point (the focal point) within the viewing limits of the eyepiece.

Light enters the tube and is reflected from the primary mirror, to the secondary mirror, to the eyepiece.

Downloading and 3D Printing My Telescope.

I designed this telescope using Autodesk Fusion 360. It’s an absolutely amazing piece of software. If you’re a Fusion 360 user, you can download my Fusion 360 project file and customize the design as needed. Or, you can simply download the STL files and send them directly to your 3D printer.

All of the project files are available on my website www.BigBigSpace.com. You’ll see two telescopes (at the time of this writing) on that website. The article you are reading now will focus on the telescope titled “Newton to See Here.” This scope is the second iteration of my 6" Newtonian telescopes, and a significant improvement over the first scope.

My first scope, named “Reflector? I Hardly Newt Her!”, is available for free. It’s a fine scope and works great, but I’ve learned a lot since then through field tests and studying proper telescope design. Those learnings are applied in the newer scope.

Is Everything 3D-printed?

The two mirrors and the eyepiece (focuser) are not 3D-printed. Additionally, the truss tubes are also not 3D printed. The truss tubes in my project are 16mm (outer diameter) x 500mm (length) carbon fiber tubes which I purchased from Amazon. I chose them because they are light and very strong. You can substitute extruded aluminum tubes if you want to save a few bucks. Everything else (except for some screws and fasteners) is 3D-printed.

The Primary Mirror

Obviously, you can’t print a mirror (yet). You’ll have to purchase a mirror or source one from a refurbished or discarded telescope. I purchased my mirror new from AngenaAstro.com. This one is guaranteed to fit the dimensions of the telescope, and has the proper focal length (750mm) for the truss tube design in this project.

Knowing the focal length of your mirror is critical. The focal length and tube diameter will determine your mirror placement. For our mirror with a 750mm focal length, we’ve placed our secondary mirror 540mm away from the primary mirror, leaving us 210mm to account for the radius of the tube and the length of the focuser.

The 3D-printable mirror box and upper optical assembly (UOA) includes truss tube connectors angled precisely so that our truss tubes (positioned at angles for rigidity) separate our mirrors by 540mm.

Primary Mirror Cell

The primary mirror rests securely in the Primary Mirror Cell. This is an important part of the telescope that has a lot of responsibility. It must hold the mirror in place, making sure that it does not misalign as the telescope is moved on your alt-azimuth mount.

Primary Mirror Cell with 3-Point PLOP and Whiffletree Edge Support

The mirror cell has to hold the mirror very gently and only touch the mirror in certain spots. Because as it turns out, mirrors are solids but not solid enough! They actually flex and change shape as pressure is applied. If the primary mirror flexes too much or in the wrong way, your view of the stars and planets will go from Wow! to Blah.

Fortunately, people way smarter than me have created formulae and software to help us design the optimal mirror cell. The most popular software is PLOP (PLate OPtimizer). PLOP can be used to calculate floating support points for the rear of the mirror. In other words, the primary mirror will rest on the support points (which are calculated to precise locations) in order to minimize deflection.

3-point PLOP with 31.4 radius for 150mm Primary Mirror

Using PLOP, I opted for a simple 3-point support design. PLOP tells us that the support points should be 31.4mm from the center. With our 6" primary mirror, this is more than adequate. Larger mirrors typically require more complex arrangements and floating 18-point designs are very common.

In addition to the rear support, the edge of the mirror must also be supported so that as the telescope is tilted, the mirror stays in place and does not warp (too much) as the pressure shifts to the edges. PLOP won’t help us here, but there are other tools and well-established best practices that will. Read this if you want to learn more. With our scope, I went with a modified whiffletree design — four edge support points separated by 45° (22.5° and 67.5° from the vertical).

Note that a true whiffletree design should pivot at the midpoint (45°), but I opted to keep it simple (no pivot), since this is a small mirror.

The Optical Tube (a/k/a Truss Tube)

I chose to use Carbon Fiber tubes on this telescope, which I purchased from Amazon. I chose them because they are light and very strong. You can substitute extruded aluminum tubes if you want to save a few bucks. The trusses need to be 480mm. The tubes I found on Amazon are 500mm, which means you’ll need to cut those down 20mm. No big deal. Easy to do.