There are numerous objects in the night sky. Using a telescope brings them into view but how do you bring them into focus. The simple answer is turn the knob on the focuser until it looks clear to you. I thought this to be effective with astrophotography, until I learned that focused for my eye was not the same as focused for my Google Pixel. That’s when I turned to the Bahtinov Mask.
Bahtinov Mask: What is it?
As you have already gathered a Bahtinov Mask is used to focus a telescope. Invented in 2005, by Russian astrophotographer Pavel Bahtinov, it consists of 3 patterned sections. The pattern is designed to create a diffraction spike to the viewer. Although the pattern makes the spikes, the mask takes advantage of the aperture stop in the optical system to create the view. The two shown here are 3D printed from two different materials. One is softer than the other, but these can be found on many 3D printer sites. However it is your choice to have online companies print for you or print yourself at home. Your local library may also have a printer you can use for a small fee.
Bahtinov Mask: How to Use
My telescope and camera are set up, and now it is time to focus. It is important to focus my scope before doing a drift polar alignment. I will explain polar alignment is a future post. I place the Bahtinov Mask on the front of the telescope. Next I point the telescope at a bright star. Actually, any relatively bright star will do. Once complete I proceed to the step of adjusting. I can then do this next step with the eyepiece, but once the camera is installed, the focus is different. So I begin by opening APT (Astrophotography Tool) on my laptop or Camera FV-5 on my Google Pixel. Using this app I begin taking pictures of the star with the camera. What I see on the screen, I use to adjust the telescope focuser. Now complete, I remove the mask and begin to polar align the telescope.
Bahtinov Mask: Photo Results Explained
Shown above are pictures taken with the Bahtinov Mask installed. It is the same star with different focus. Out of focus are the left and right. Conversely, the center is focused. The pattern on the Bahtinov Mask create the 3 lines or spikes crossing the star. My goal is to adjust the focuser knob to move the center spike equal distant between the other two. This is the achieved focus. Simple enough. Try it and let me know your results.
Bahtinov Mask: Conclusion
The Bahtinov Mask is a great tool to achieve optimum or perfect focus. APT and other software can assist in achieving perfect focus for your photos. Although they are inexpensive, 3D printing them can save more. They expertly help focus on planets, Nebula and start clusters. Unfortunately there is no benefit to use with the Moon and Sun. In fact, using on the Sun is dangerous. So however you acquire one, take your next great photo using the Bahtinov Mask. Enjoy and clear skies.