Organize Your Astronomy Filters With a 3D Printed Filter Wheel!

Do you love astronomy, 3D printing, or design? Do you have circular filters you need to organize? Do you like reading random Instructables? Well then this is for you!

A couple of months ago I received some telescope filters when I bought a used Celestron CGEM mount. The filters are a little dusty, but they work perfectly fine. Unfortunately, they are a hassle to screw on an eyepiece, and in the winter, they get all foggy if your warm hands even come close to the glass! I needed some way to indirectly switch between filters. The answer was a filter wheel!

A telescope filter wheel simply holds all the filters behind the eyepiece on a disk, and you rotate the disk to choose a filter. A commercially produced filter wheel can easily set you back a couple hundred dollars. I'm not that keen on spending my hard-earned money on a disk with holes in it, so I decided to make my own! This filter wheel is manual and can hold 3x 2" filters and 4x 1.25" filters.

You will need the following to make a filter wheel:

-An assortment of telescope filters

-A large diameter ball bearing (I used one with a 32mm OD and 25mm ID)

-Some M3 size screws and threaded brass inserts

-A 3D printer

This is where you design your filter wheel. My original design was okay. It works, but it could be improved upon. I won't post the STL's, but I will guide you through what I have learned.

The filter wheel is comprised of the filter holder, the front cover, the back cover, a 2" tube in the front (for telescope connection), and adapters for an eyepiece/camera on the back. Please take note of my drawings to visualize what one might look like.

Securing your filters:

To secure your filters, decide if you want to use the metal rim that the glass disk comes in. I chose to remove the glass filter from the metal case (more on this later) and secure it by means of a "clamping" system. The glass disk fits into a hole, and there are barriers on either side. One of the barriers has to be screwed in once the filter is in place.

For this design to work effectively, you must know the diameter of the glass filter disk, which is different than the diameter of the case it comes in. I chose to sacrifice two orange filters (1.25" and 2") to find their "true" diameter. The 1.25" filter has a glass disk diameter of 27mm, and the 2" filter has a glass disk diameter of 46mm. TAKE NOTE: These measurements may vary by +- 2mm depending on the type of telescope filters you have.

You could also print a circular hole slightly smaller than the filter's threads, and thread the filter into the hole. This does make the filter wheel thicker, but it's easier to secure the filters.

The filter disk:

This is the rotating part that holds the filters. For the rotating contact, add a bearing, don't design a plastic-on-plastic rotating part. Even if it's just a 608, anything will help to make it smoother. Also, the larger the diameter, the less your wheel will wobble. If you filter wheel wobbles a lot, it may distort the image through the eyepiece.

Be sure to include some way to stop the filter wheel once each filter is aligned with the optical path. Example: a lever that falls into a slight cavity every filter position, locking the filter into place.

The housing:

This keeps dust and breath off of your filters. 3D printing this may take a while, so be warned! Or you can use other materials such as wood or sheet metal to perform the same job. While designing this part, be sure that it won't rub against the heads of the screws you use!

Input/Output Adapters:

You'll need some way to connect the filter wheel to the telescope and eyepieces. Make the entire filter wheel as stubby as you can (thickness wise) to minimize the amount of focus space (back focus) your filter wheel takes up; this means not making your adapters super long. I simply designed a 2" ring that screws into the filter wheel and inserts into the 2" telescope focuser.

After designing your filter wheel, begin the printing process. Remember, whenever you are printing a large surface area, ensure that your bed is level, and the print is somewhat easy to remove. You can add a chamfer to the bottom of the print surface to aid in prying it up from the bed.

Chances are, you may have to do some filing on your 3d print. Using ordinary files might work well, but you might have to use a Dremel, Forstner bits, or a belt sander. (I used all three)

Take your time, it's better to spend an hour filing than reprinting a 7-hour part! (But if you have to file that much, it may mean your design is off)

Assemble your parts to build a working filter wheel. . .

First, place the brass inserts into their respective holes with a soldering iron. (That is, if you decided to use them with the M3 screws) It is perfectly fine to thread screws into plastic, but if tightened too much, you could strip the threads!

Build your filter wheel without the filters for the first assembly! Make sure all the pieces fit well before you add any valuable filters.

You will need the filter slots to be relatively close to the actual filter's diameter. When test fitting your filters, I recommend that you use a couple of unused filters to "sacrifice". These will get all grimy and dirty when you check that they fit into their respective slots, so don't test fit your $300 narrowband O-III filter, trying to cram it into its slot!

Some of my slots ended up being too big, so I cut thin sections of thick double-sided tape to act as a buffer. (See pictures)

Insert your filter wheel into your telescope and enjoy the ease of switching filters with a turn of the disk! As long as you didn't make your filter wheel too thick (>30mm), your telescope should have enough extra focus.

I learned a lot from making a filter wheel myself. This version is honestly ok. . .

It works fine, but it could be improved upon. For example, my spring lock system makes it a little rough to turn the disk, and the ball bearing is a little wiggly.

All in all, I'm glad I made one myself! It saved me hundreds of dollars (no joke!), and I got another DIY project under my belt.

Thanks for reading this Instructable, and get ready for more DIY astronomy projects!