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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It looks like there were pockets of air under the pitch. Leaving it alone for a week meant the bubbles seem to have popped and some areas sank downwards.
This turned down edge is slowly going down but it still feels like an endless quest with no end in sight.
Total grinding time so far: 8 hours
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This pretty art piece is actually a precise measurement of my progress in telescope mirror grinding. It looked fun enough out of context I decided to post it!
You’re looking at multiple overlaid square images. Each one is a Ronchi test, which tells me the shape of my mirror on a nanometer scale in the shape of the red stripes. As I polish the mirror, it changes shape, so after 20 minutes of polishing I want to measure the new shape.
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you can mirror grind in two ways: mirror on top or tool on top. Apparently if you have a turned down edge to fix it you do center over center strokes with amplitude 1/3 of the length, according to a video by Gordon Waite. I did it for two hours yesterday and made little progress. Today I took another look at the video and noticed he was doing it tool on top.
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Figuring sucks.
My mirror should be a sphere. It isn’t a sphere. I’ve introduced a “turned down edge”, where the edge is ground lower than the rest of the mirror and you have to remove all the glass in the center to fix it. You can see it in these ronchi test pictures, each taken after a few sessions of 30 minutes of total polishing. The straight lines show that part of the mirror is spherical.
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To test the shape of a mirror, amateur telescope makers have a few tests which involve bouncing light off the mirror to see its shape. One cheap one is the Ronchi test, which sends light through a grating of fine lines, bounces off the mirror, and then you place the grating at the radius of convergence of your mirror so it blocks part of the light and reveals the mirror shape.
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After all that setup, actually polishing a mirror is surprisingly simple. First you put your mirror on your tool and apply pressure so the pitch flows and takes the shape of your mirror (which happens faster if it’s hot, so you can leave the pitch lap in hot water to heat it up and speed up pressing). Then you take your mirror, put it on your tool, and push it back and forth without applying any downwards pressure.
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I’m making a tool called a pitch lap to grind an 8" mirror. Previously, I discovered the best way to find dental stone is a dentist, and made a yellow plaster disk.
Pitch lap step 2: Pour the pitch!
Pitch is a weird material. It’s a liquid so viscous it looks like a solid. At high temperature it’ll pour like honey, at low temperature it’ll act solid but very very slowly flow.
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Previously, @Beasmeeply generously donated me an 8" mirror blank, kick-starting an attempt to finish grinding it and make an 8" telescope. There are four stages of mirror grinding, so to figure out which step I needed to start with, I put the mirror into a Foucault mirror tester, and it gave a smooth-ish image, telling me BeasMeeply had gotten through the first two stages of mirror making, rough and fine grinding, and it was ready to polish.
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Finally finished making a pitch lap for @BeasMeeply’s mirror! Yahoo! Now I can start polishing… as soon as I get some cerium oxide to polish with.
