After building the Hadley, I wanted something more. There were a few ways to take better astrophotos: aim properly, collect more light, motorize a mount to follow earth’s rotation, or build a bigger telescope to collect more light. I chose to build a bigger telescope.
The Leavitt is the product of a year of work. This telescope is an 8" diameter f/4.5, with a focal length of 48.5". Its 3-pole design makes for easy adjustment of focal length to accomodate a camera fits into a car and weighs less than 20 pounds.
The mirror is an 8" diameter f/4.5, with a focal length of 89.5". I polished and parabolized it myself from January 2024 to August 2024. Its short focal length means the telescope’s views are less magnified and therefore brighter, perfect for deep sky objects. It has also given me great views of Mars, Jupiter, and Saturn.
The body
For the body, I used the existing Leavitt 3D printed telescope design. It uses three 0.5" aluminum rods and 3D printed parts. Each circle is split into three sections joined by screws to fit onto a standard smaller-than-8" print bed. It took two 1kg filaments of PLA, and since .
I designed a few of my own replacement parts. The original mirror cell (the part that holds and tilts the primary mirror) held the mirror in place with glue on three points, but that glue may slightly deform a mirror by a few nanometers. Since my mirror turned out to be accurate to within 1/20th of a wavelength, instead of glue I designed a mirror cell that encloses the mirror in plastic using three spokes with built-in clips stopping the mirror from falling out.
The mirror mount is an all-new design inspired by the Hill Mount I made for the smaller Hadley telescope. I don’t have access to woodworking tools, so I took a page from the mount I designed for my Hadley and designed a 3D printed truss structure. The telescope’s width is more than the size of my 3D printer’s bed, so I had to design it with a front crossbar.
Compared to the Hadley’s Hill Mount, which is 2.5’ tall, my Leavitt mount is 3’ tall. Because the Leavitt’s mirror has a longer focal length, its telescope tube is longer, so I thought the extra height was needed to stop the tube from hitting the ground when aimed upwards. However, I didn’t account for the extra weight of the mirror, which made the telescope tube bottom-heavy and moved the pivot point closer to the mirror. That meant that the extra height worked against the mount and made it more prone to tipping over. It works, but the design could be made more stable someday.
The mirror
I didn’t buy this telescope’s mirror - instead, I manufactured it myself. Amateur mirror making has a long history, and there are textbooks published a hundred years ago that document processes that still works to this day. This was my first time polishing a mirror, and I’m very proud of the end result: a parabolic surface accurate to within 1/20th of a wavelength of visible light - 25 nanometers or so. I worked on this mirror from January 2024 to August 2024, and first light was in December 2024, just in time to see the Mars conjunction.
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I finally used my new 8" telescope with a working mirror cell! Report:
Jupiter is nice and bright!
M42 is so much brighter than in my first telescope! It’s green and I can the bat wings and texture in the trapezium along with four stars!
There are so many stars everywhere!
High power reveals flaws. High power stars look like ovals. After some investigation, my secondary mirror wasn’t centered in the eyepiece view.
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The part of the telescope that holds a mirror in place is called the “mirror cell”. The simplest way to hold a mirror in place is with three dabs of silicone glue. If glue is too hard, it can pull on the mirror and the stress will distort its shape on a scale of hundreds of nanometers, which is bad.
Small mirrors can usually get away with just three dabs of glue.
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Someone brought an extremely accurate “double pass autocollimator” tester to an amateur telescope maker meetup… and it showed an uniform shade of gray across the whole mirror, meaning no defects (ignore the circular reflections from the test stand). That means my mirror is an extremely smooth parabola! I’m done mirror grinding!
🎉 First mirror: complete! 🎉 That’s so cool! I thought I was slightly overparabolized based on my Foucalt testing.
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8" mount v1 complete!
After several days of designing, 3D printing, and experiencing the joys of my favorite telescope related activity, hacksawing through metal, I managed to assemble a complete mount! Design-wise, I had to replace 0.5" screws with 0.75" screws in a few places because the redesigned EMT tubes were too close together. The telescope uses only 0.5" screws, so it would have been nice to match, but it’s fine for an one-off.
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Every telescope needs a mount, to hold it up and let it rotate. 6 months ago, I thought that I was close to the finish line, since all I had to do was make a mirror and a mount! Then the mirror took six months. Now I need to sit down and actually finish this mount. My 4.5" scope uses a truss of EMT steel tubes and the scope sits on a wide C-shaped part.
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I did a tiny bit more parabolizing, then re-measured my mirror. Last time, my spreadsheet and measurements told me my outer zone needed to focus tiny bit farther and my lower zone needed to be a tiny bit closer. To dig the inner zone out, I did some W strokes with lots of mirror movement on the tool edge, and then used long center over center strokes with a tiny offset to dig everywhere but a tiny strip of outside.
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I’ve switched to using the Foucalt test and a Couder mask for parabolizing. The Foucalt test is more precise (but more annoying) than the Ronchi test, but my local astronomy club has a foucalt testing rig with a micrometer that lets me measure very precise details of where different regions of the mirror focus light, so it’s more accurately moveable than my handheld Ronchi tester. I built a cardboard “Couder mask” to block off regions of my mirror to see where the edge and center are focusing light to separately.
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I’m at the final step of polishing a telescope mirror: turning a sphere shaped piece of glass into a parabola shaped piece of glass, called “figuring” or “parabolizing”. I messed up, twice, and since then I have been regrinding my glass back to a sphere for figuring attempt #3. Today, after several months, I finally have a spherical enough surface to start figuring attempt #3!
Here’s both a Ronchi test and a Foucalt test.
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It’s been a while but I gave the 8" mirror some grinding. I’m on my way back to sphere, undoing parabolizing attempt #2. 30 minutes of grinding seems to have created some turned down edge and a hole in the middle. Great.
I put the mirror into the telescope, leaned it against a big crate again, and I was able to use it to see the moon! It’s amazing that even though I’m not done figuring, it’s smooth enough I can see things in space.
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My local astronomy club has a foucalt tester, and today I tested that mirror after 30 seconds of parabolizing. It revealed a big turned down edge (yahoo…) and some slight zones in the center of the mirror. I knew those zones existed - heck, you can even see the slight non-sphericalness in the second image of my “close enough to spherical!” celebration - but the radius of curvature changes rapidly where one zone meets another instead of smoothly blending.
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