Bahtinov Mask for 18-55mm Lenses

Summary of Concept

This is a 3D printable Bahtinov Mask to assist with manually focusing on the night sky. Bahtinov masks work by creating diffraction spikes from a point light source (like a star) spread out across different sections of the lens. When the point is out of focus, the diffraction spikes land on different parts of the sensor, so changing the focus moves the spikes relative to each other. The angled diffraction spikes move together because they are on the same side of the lens, forming an “X” shape that stays together. The horizontal diffraction spike moves to the left or right relative to the “X” depending on the focus, giving feedback to the user on which direction they need to adjust the focus. When the point light source is in perfect focus, the central diffraction spike will be in the center of the “X” pattern, and the Bahtinov mask can be removed to take pictures with the stars in focus. 

The diffraction spikes are more visible at higher focal lengths, but I designed this around still being able to be used at lower focal lengths with bright objects. It becomes more useful with dimmer objects at higher focal lengths. 

This Bahtinov mask has a nominal gap size of 0.45mm for the central spike, and 0.39mm for the angled spikes.

Example Images

This mask works best with bright objects, as the length of the diffraction spikes are related to the brightness of the point light source. Examples shown below.

Venus viewed through the Bahtinov mask at a focal length of 21mm (cropped) while in focus.

Venus in focus with the mask removed at 21mm (cropped).

Vega viewed through the Bahtinov mask at a focal length of 21mm (cropped) and in focus. Note that because Vega is much dimmer than Venus, you have to zoom in much closer to it before the diffraction spikes are obvious. 

Resulting stacked image of Vega and neighboring stars with the mask removed at 21mm (cropped).

Usage

Point the camera at the brightest star (or planet) visible in the sky at the time of shooting. Try to get the star or planet as close to the center of the image as possible, the diffraction spikes are largest for stars close to the center of the field of view. 

To use this Bahtinov mask, first attach it to the front of the camera lens by squeezing the spring loaded sides of the mask and lowering into the front opening on the lens, then releasing. The pressure applied from these printed springs hold the mask to the lens. 

Then, adjust the focal length of the lens to what you plan to be shooting with. 

Now it is time to adjust the focus. I would recommend using a laptop to control the camera so that the focusing motors are engaged and hold the focus in place when you remove the filter. Manually adjust the focus until the diffraction spikes appear. It is easier to see them if you digitally zoom in on the preview window on the star you are using to focus, as well as setting the camera ISO to the max (something like 6400). Adjust the focus until the central spike is perfectly between the two angled diffraction spikes. If you can't see anything in the preview window, you may have to take test images with a few seconds of exposure and compare them. 

Once focus is achieved, take the Bahtinov mask off of the camera with the laptop still connected to keep the lens focusing motors engaged. Once the mask is removed, take another test image with a few seconds of exposure to see if it looks correct. 

If the test image looks good, then turn off autofocus on the lens, disconnect the laptop, and begin taking your exposures with a remote camera trigger. Turning off autofocus before disconnecting the laptop ensures that the camera doesn't try to refocus when the laptop disconnects, which would ruin the manual focusing that was just completed. 

Printing Tips

This model is very unforgiving to print. Printing the diffraction grating requires excellent bed adhesion, a well calibrated z-offset, consistent extrusion rate, good retraction settings, and dry filament. For best results, I recommend double checking your first layer calibration to ensure it is printing 0.2mm tall first layers, cleaning the bed with isopropyl alcohol, drying your filament with a dehydrator prior to printing, and taking a close look at how the first layer will be printed in the slicer. 

Slicer Settings

Layer height: 0.2mm

Perimeters: 2

External perimeters first

Bottom Fill Pattern: Concentric

No supports

Things to Look Out or in the Slicer

Look at the order the lines in the first layer will be printed, if it is printing lots of the grate lines by laying down both the starting and ending lines without connecting to anything, it is likely to have gaps at the start and end of the line. Ideally most lines should print with a connection to the outer perimeter at the start and end. The grate is 3 layers thick, so if there are some gaps on the first layer, the next 2 should fill them. Sometimes rotating the model around the z-axis changes the order the lines are printed, so play around with it some lines will be printed before the outer perimeter, but it should only be a few, not all of them.