My Second Telescope: the Leavitt on Hill's Space

Capturing Galaxies with Cameras

hillexed@email.com (hillexed) — Thu, 24 Apr 2025 13:52:15 +0000

Today, I tried finding some galaxies. I couldn’t see M101, couldn’t see M105, but did see M81 and M82 by eye. It was tough, but I finally took a phone pic. I decided to give Pingu Camera a try… and the phone plus printed adapter plus phone remote shutter plus waiting a while for vibrations to die down worked! I got a picture of both M81 and M82!!

Big Baffles

hillexed@email.com (hillexed) — Tue, 15 Apr 2025 18:55:12 +0000

My 8" telescope has been upgraded with some new printed shields to block unwanted light near the top.

Pics of said light blocking baffles. Also added a mount for my pifinder.

2025 Total Lunar Eclipse

hillexed@email.com (hillexed) — Fri, 14 Mar 2025 08:13:51 +0000

Lunar eclipse! Funny enough the telescope made things brighter and therefore even during totality it looked less contrasting through the telescope than by eye

Oh my I can see features on mars

hillexed@email.com (hillexed) — Fri, 24 Jan 2025 03:42:12 +0000

Rough mars sketch! The atmosphere is not cooperating but there’s this big Africa-shaped gray region!

Mars Behind Moon

hillexed@email.com (hillexed) — Tue, 14 Jan 2025 04:15:36 +0000

Goodbye mars! (Zoom in!)

Taken using my 3D printed telescope, with a crowd of friends invited to watch the occultation of mars by the moon! The phone I was taking photos on ran out of battery, so I hastily threw a friend’s iphone into the holder just in time to capture these.

Leavitt's First Light

hillexed@email.com (hillexed) — Wed, 01 Jan 2025 00:00:00 +0000

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. I don’t have enough room to fix it all the way before it hits the spider. Time to redesign.

Mars looks blurry. No detail. May be due to seeing. A hint of ice cap but not much else, and then my eyepiece fogged over.

Focuser is bad at holding eyepieces. They keep sliding out. Something with set screws is probably a better idea. I managed to take a few pictures but the weight of a camera meant it was no longer face-on and had distorted stars.

At least M44 looks great!

The Mirror Cell

hillexed@email.com (hillexed) — Fri, 20 Dec 2024 00:00:00 +0000

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. Because my mirror was so accurate, I was advised to make a “flotation cell”, which holds the mirror in place without glue. I designed this modification of the stock Leavitt mirror cell in Onshape. The gray edge supports are also what stop the mirror from falling out if the telescope is turned upside down. These parts are printed separately and held tight (but not too tight that they press on the mirror and distort it) with a screw that screws into the base. Making the edge supports separate removable parts is important so that I can get the mirror in and out.

This mirror cell works… okay. I don’t see much distortion after the mirror has cooled down. The height of the edge supports means it sometimes scrapes against the inside of the telescope tube if pulled too much in one direction.

Introducing Pingu Camera

hillexed@email.com (hillexed) — Thu, 12 Sep 2024 04:32:14 +0000

Introducing… Pingu Camera. I bought a used DSLR for astrophotography four months ago hoping it would take better photos than my phone. I 3D printed an adapter for it to fit into a standard eyepiece slot. Unfortunately it was a bit of a failure for two reasons: firstly, this camera isn’t mirrorless, so it had a shutter inside it, and when that shutter flips up to take photos it sends vibrations through the entire telescope. I couldn’t get any good photos of planets. Secondly, the sensor is so far back into the camera that I have to get out a screwdriver and slide the entire upper tube a few inches closer to the mirror to achieve proper focus.

8" Telescope Making: Remember to Measure your Owls

hillexed@email.com (hillexed) — Thu, 01 Aug 2024 16:47:32 +0000

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. I also removed the horizontal bar across the front of the telescope when breaking the top part into two parts; I might need to figure out how to add that back if they’re wobbly.

However, this mount has a few problems. Firstly, It’s slightly too short. Because this Leavitt telescope design was downloaded from the internet, I had to measure its width using a tape measure to load into my CAD program as a reference. I measured it around 1/4" too short - turns out it’s actually 29.8 cm wide, meaning my mount has a gap between where the scope should go. Also, I managed to forget a screw socket (which I can fix by using longer screws), and it looks like there’s a bit of wobble. Maybe I’ll do a version 2.

8" Telescope Making: Draw The Rest of the Owl

hillexed@email.com (hillexed) — Thu, 01 Aug 2024 14:37:48 +0000

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. I modified that design for this 8" telescope’s bigger bearings… only to realize before 3D printing that this 8" scope is 10" wide, too big for a support to fit onto my 3D printer. I sat down and modified the design so the telescope sits on two separate parts instead of sitting on one wide C-shaped part. I’ll probably need to add an extra reinforcing bar on top to compensate for that extra bar I removed. Hope it works and I don’t need more revisions.

My gearbox finally has gears in it

hillexed@email.com (hillexed) — Sun, 14 Jan 2024 23:09:28 +0000

This is going to turn the turntable that my telescope will sit on

Gearbox has some decent progress

hillexed@email.com (hillexed) — Sat, 13 Jan 2024 05:54:02 +0000

I’ll print out a prototype and see if it holds everything the way I want it to.

Question I still need to answer: how do I stop the axles from fall out

Gearbox design going... okay

hillexed@email.com (hillexed) — Fri, 12 Jan 2024 23:19:36 +0000

I don’t know how to design a sketch in one document (for say a stepper motor mount plate) then import it into a different document in a different location (for, say, mounting that stepper in a different place)

First gearbox test

hillexed@email.com (hillexed) — Fri, 12 Jan 2024 19:00:14 +0000

Looks like the belts and pulleys I got fit! Now to design the gearbox and 3D print it so my telescope can move very small amounts accurately

Grabbing Gears

hillexed@email.com (hillexed) — Tue, 12 Dec 2023 19:22:32 +0000

I found a design by d.revan to motorize a telescope using GT2 timing belts and it’s cool. He uses:

GT2 belts and pulleys instead of gears
608-2RS skateboard bearings to let the shafts spin (smart!)
a giant mega printed gear attached to wood for the final gear stage
4:1, 4:1, then 15:1 gear stages

…but I tried seeing how much the parts would be on AliExpress, and it was surprisingly hard to find gears with the right 8mm bore and wider than normal 10mm belt width. And the shipping added up - you’d see a $1 part plus $4 shipping. And buying two different parts from the same seller was twice the shipping! Just the tiny metal rods were $3 with $15 shipping, let alone gears!

So instead I asked around the synthesizer community, and found robotdigg.com, which is a janky 3D printer china company with a slightly hanky website but sells GT2 belts and pulleys for very cheap. And thankfully it all ships from one place, so only one shipping fee! I was able to buy the parts for one gearbox for $25!!!

So $50 later, I bought enough for two gearboxes, one per axle.

Now to CAD it up! D.revan’s design is slightly different from mine because I’m using smaller pulleys, but the idea seems similar enough that I think it’ll work. If I can motorize my telescope for $50 that’ll be incredible.

The Mirror Making Process

hillexed@email.com (hillexed) — Mon, 01 Jan 0001 00:00:00 +0000

(This page is a work in progress!)

I dove into the rabbit hole of optical fabrication and made my own mirrors for my 8" Leavitt telescope.

When we think of mirrors, we think of mirrors like those on a bathroom wall, which are flat. Telescope mirrors are curved, to focus light to a point. This curvature focuses parallel light rays everywhere on the mirror’s surface to the same place, increasing the amount of light the eye can see when looking through a telescope. This curve must be smooth to within a fraction of a wavelength of visible light - 500 nanometers. It is incredible that we can make surfaces that smooth using just muscle power.

There are four main steps to mirror making: rough grinding, fine grinding, polishing, and parabolizing.

Rough grinding carves a depression of the right curvature in a piece of glass.
Fine grinding smooths the surface using a succession of smaller and smaller grits.
Polishing uses a new tool and cerium oxide to make the glass reflective for the first time, creating a reflective sphere.
Parabolizing, also known as “figuring”, slightly adjusts the extremely precise sphere to create an extremely precise parabola.

Then there is one final step: aluminizing. After the first four steps a mirror is a fully usable precisely shaped reflective piece of glass, but covering the glass with a thin layer of shiny metal increases the amount of light reflected to around 90%. This step requires a vacuum chamber. I don’t have a a vacuum chamber. Instead, I did what most amateurs do and shipped my mirror to someone else. Aluminizing was its own shenanigan - more on that below.

The Materials

Telescope mirror glass has gotten expensive nowadays. I was very lucky: I got mine through social media. I had blogged about the process of creating my first 3D printed telescope, Hadley. When I mused online about falling down the rabbit hole of mirror making, I recieved a wonderful comment: @BeasMeeply on cohost had an abandoned mirror making project that had sat in a closet gathering dust for 10 years, and graciously offered to send it to me for the cost of shipping. After that, I had no choice but to dive in.

Making tools

Making a mirror involves rubbing two surfaces against each other to make each other smooth, one concave (the mirror) and one convex (the tool). In the olden times, glass was cheap and a second piece of glass would make a good tool. Now that 8" glass circles are almost $100, a cheaper way is to cast a plaster disk. There are two requirements for this: 1) waterproof plaster, and 2) porcelain tiles embedded in the plaster, to match the hardness of the glass and ensure the soft plaster doesn’t simply get ground away.

Dentists use waterproof plaster called “dental stone” to take tooth impressions, and it works great. Unfortunately, dental stone is usually sold in 45 lb boxes for $50, and I only need 2lb or so. I got my dental stone from… having a dentist appointment and asking my dentist nicely!

I got hexagonal porcelain tiles from Home Depot for $5. I learned the hard way to keep the webbing on.

Dental stone hardens incredibly quickly - within 10 minutes of adding water and mixing. I learned the hard way to set a timer for four minutes before pouring.

Polishing

When I got the mirror, first I used spherometer to measure the curve - same curvature all around, accurate to within 0.0002 inches. I put it in a foucalt tester, and after lots of help and fiddling I got some pictures of the result:

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The fact that it’s transparent enough that I can see through the glass and get a result in the foucalt test tells me @BeasMeeply got through grinding and polishing, and now I can skip to making a pitch lap.

Homemade 3D printed ronchi tester

I built a 3D printed Ronchi tester to test my mirror’s surface shape. Handheld and easily portable, it uses an LED, a 9 volt battery, a switch, and a Ronchi grating of evenly spaced lines sent to me by a telescope-making friend in France. It lets me bounce light off the mirror and

For more on the Ronchi test, see here.

Depending on where each part of the mirror focuses light, the pattern of lines that is reflected back to the LED will look longer or shorter. I made this infographic of how to interpret test results:

The Ronchi test is great at detecting astigmatism, turned down edges, and overall parabolization progress. It is so sensitive that you can see air turbulence or uneven heating from the wobbling of the lines! However, that sensitivity requires taking pictures of the Ronchi pattern and comparing them on a computer, using this wonderful calculator by Mel bartels. A spherical mirror shows up as perfectly straight lines up and down. However, detecting straightness by eye (or even by computer) is very tough, and if you want a 1/10th wave finish or better, slight surface issues can be visible if you are good at dialing in a Foucalt test to the center of curvature but escape a Ronchi test. I have found that sometimes seeing an issue in a Foucalt tester, I have gone back and seen the issue in the Ronchi.

For my mirror, I used my Ronchi tester to guide overall spherical progress as well as parabolization, but used the Foucalt test with a Couder mask to take numerical readings for the final stretch of parabolization.

Parabolizing

The process of parabolizing involves turning an incredibly precise sphere into an incredibly precise parabola. measuring the mirror’s surface

I tried parabolizing three times, since twice I had to restart. Experts at my local astronomy club were able to spot 1/10th wave defects in a surface I thought was spherical, and insist I smooth out the surface.

In total, polishing and parabolizing took 32 hours of cumulative polishing time. This was not a way to save time or money. It is a lot of time spent moving a piece of glass back and forth. As I write this in 2025, an 8" mirror can be bought from Chinese company GSO for $300 on AgenaAstro. If all you wanted was a mirror, the correct answer is to spend 32 hours working a part time job and use that money to buy a mirror. That said, the commercial mirrors would only be smooth to within 1/4th of a wavelength, while mine is smooth to within 1/20th of a wavelength. Plus, I had fun!

Cardboard Couder Mask

I drew circles on cardboard with a ruler and compass traced them out to create this cardboard mask. This isolates parts of the mirror at different points from the center to the edge. The zone radii aren’t regular, but are instead chosen so that each zone focuses light

The spreadsheet

I made a spreadsheet that would take knife-edge measurements of the Couder mask zones and from there integrate to compute the total shape of the mirror. This helped me with the final few steps of the process.