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’m parabolizing my telescope mirror - I’ve ground it to a sphere shape, and now I’m grinding a tiny bit more to change its shape precisely into a parabola.
I’ve done 3 grinding sessions for a total of 34 minutes of grinding. Here’s a Ronchi picture of my current progress. Mel’s online Ronchi calculator has overlaid some semi-transparent white lines which show the ideal Ronchi grid of a somewhat parabolized mirror.
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I’m in the home stretch - my goal is to turn this sphere shaped mirror into a parabola shaped mirror.
The final stage of mirror grinding is called either parabolizing, because you’re making a parabola, or figuring, because you have to measure carefully and that involves numbers which are also called figures. You use a stroke that goes up and down fast and side to side slowly in a big zigzag, called a W stroke, to remove a tiny bit of glass from both the center and the edge, as seen in the first picture (from Mel Bartels’ site).
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This site is my secret weapon: https://www.bbastrodesigns.com/ronchi.html
It’s a slightly janky javascript app made by some old telescope maker in Oregon, and it lets you get quantitative measurements from all those Ronchi pictures. Very important.
Looks like my outer zone focuses light at around 0.066 in from my desired radius of curvature, and inner zone focuses light at around 0.1 in. (There’s probably big error bars on both those numbers).
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I’M SO CLOSE TO A SPHERE
(You know you’re a sphere when the lines are completely straight. See this infographic for more)
total mirror grinding time: TWENTY GODDAMN HOURS
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I did 20 minutes without my center paper cutout and managed to dig a tiny zone in the center. Yaaaaay. Then I spent two hours ignoring it and trying to get the outside zone down.
On the plus side that outer zone at 70%+ diameter looks spherical now.
Now I have a choice: I think I’m close to a parabola. With that deeper center, if I can flatten the inner edge of that transition I’m very close.
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I’m using an advanced technique: cutting out pieces of paper and putting them under the tool while pressing to avoid certain areas of the tool touching the mirror. I know I have a hole in the middle, so by blocking the middle from wearing down I can concentrate my wearing on the outer zones without making the center even deeper.
Looks like after a few hours I’ve managed to get the outer zone almost completely spherical!
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Check out this infographic of how to read the Ronchi tests of my telescope mirror! I’ve been posting lots of pictures like this as I grind, and this tells you how to interpret what the pictures say about a mirror’s shape. The Ronchi test can be used to roughly measure a mirror’s shape and see any turned down edge (“TDE”), and with a computer program to analyze them, even give some quantitative measurements.
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Two hours of mirror grinding brought me from the first pic to the second! The center zone got so big! It’s so much straighter! I’m much closer to a spherical mirror!
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The center zone, visible in the first pic as the inner area with red on the left and black on the right, is a bit more smooth and has grown from around 50% diameter to around 80% diameter. The outer zone is still a gradual slope but it focuses light to around a centimeter or so further than the inner zone. Previously that outer zone was extremely too tall, so my goal was to reduce it, and compared to a few weeks ago it looks reduced but not gone.
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Here’s my mirror in a Ronchi test and a Foucault test. I’ve managed to reduce the turned down edge to a very small area but it’s still there, visible in the ronchi as hooks at the edges and in the Foucault as a slight black zone at the bottom left of the image. I’ve also managed to dig a huge hole in the center of the mirror trying to fix that edge…
