A Bahtinov mask is a clever tool for precisely focusing an astronomical telescope. The distinctive pattern of the Bahtinov design was created by Russian amateur astrophotographer Pavel Bahtinov in 2005.
To use the mask, you simply need to cover the telescope’s objective and point the telescope at a bright star. The mask creates a pattern that allows you to fine-tune the focus of the telescope to produce the signature “central star-spike” pattern.
I have been using a Bahtinov mask to focus my astrophotography telescopes for nearly a decade now, and I continue to use them for each and every imaging session.
The animation below shows you exactly how the diffraction spike pattern looks while focusing your telescope with a Bahtinov mask.
A simulation of a Bahtinov mask diffraction pattern when focusing. Niels Noordhoek.
Free Bahtinov Mask Generator. Programmed by Satoru Takagi.
Focus Your Telescope with a Bahtinov Mask
A Bahtinov mask can help you achieve a sharp focus for your astrophotography images. This process is an essential step of any deep-sky astrophotography session.
With so much time and effort put into your polar alignment and autoguiding accuracy, it would be a shame to spoil a photo due to poor focus. Over the years, I’ve had my fair share of deep-sky astrophotography focus mishaps.
In many cases, I did not realize how bad the focus of my image was until I attempted to process the final image.
You can avoid this unfortunate circumstance by using a simple tool that effectively confirms that your focus is as sharp as can be, a Bahtinov mask.
In the following video by fellow astrophotographer and friend, Nico Carver, he discusses the process of making your own Bahtinov mask at home using simple materials or a 3D printer:
If you are willing to spend the money, you can save time by purchasing an inexpensive Bahtinov mask on Amazon. Make sure that you order the correct size for your camera lens or telescope objective.
This one has a thickness of 1mm, and is made from aluminum alloy. It uses adjustable screws to secure it to the object of your telescope dew shield.
The Bahtinov mask shown below is a popular choice, as it covers a wide range of entry-level refractor telescopes in the 65-100mm diameter range. A simple tool like this can help you capture sharper astrophotography images when used properly.
A simple Bahtinov Mask design. (SVBONY Fixed Diameter 65-100mm)
If you are not interested in buying a Bahtinov mask, there are several free tools available online to help you make your own Bahtinov mask, including tutorial videos that help you build the correct size for your telescope or lens.
- Free Bahtinov Mask Generator
- 58mm Bahtinov Mask for your DSLR
- It is worth using a Bahtinov mask for visual observation?
How a Bahtinov Mask works
While appointing your telescope at a bright star, you simply place the Bahtinov focusing mask on your telescope’s objective lens and secure it into place using the provided rubber tabs. This process is easier when the telescope is pointed straight up near the Zenith, so try to use a bright star that is high overhead to focus on.
The openings in the mask design create a set of diffraction spikes on the star that will assist in the accuracy of our focus. It will appear on your DSLR display screen or computer as an “X” with a line through the center (when in focus).
The diffraction spike pattern from a bright star using a Bahtinov mask
As you adjust the focuser of your telescope, the central spike will move in and out of the X-pattern. Using a dedicated astronomy camera, loop the image in 2-3 second intervals with your camera control software.
You’ll need to wait for the latest preview (in this case, every 3-4 seconds) to appear before making changes. The ZWO ASIAIR has an excellent focusing tool within the mobile app that works well for this process.
The same process applies using the Live View display on your DSLR or mirrorless camera. The difference is, the image will be much dimmer, as you are essentially looking at a live video feed rather than a short image exposure.
If you use camera control software, such as BackyardEOS, Astro Photography Tool, or the ASIAIR mobile app you’ll have the advantage of taking short test exposures that will show a much larger diffraction spike. You can also magnify the center of the pattern to get the focus just right.
Recommended Article: The Best Astronomy Apps for Stargazing and Astronomy
Focusing my telescope using a Bahtinov mask and the ASIAIR focus tool.
Why Focus is Critical for Astrophotography
In general, the smaller the stars are in your image, the better. One of the reasons I love apochromatic refractor telescopes, is their ability to capture colorful, pinpoint stars.
However, this pleasing characteristic is only present when your focus is spot-on. Refractor telescope owners expect razor-sharp details in their photography, so mastering the art of focusing stars is a must.
Each of these images was focused using a Bahtinov mask.
Different types of telescopes have better ways of achieving a sharp focus than others. For example, a Newtonian reflector creates its own star diffraction spikes by the nature of its design.
These patterns can be dissected and tweaked to diagnose issues with collimation and find a sharper focus for astrophotography.
In this post, however, I’ll be describing tips that are most useful to those shooting with a refractor telescope, that are using a manual focus routine.
Automatic and motorized focuser users have the ability to use software that tells them when a star is as sharp as possible. (Such as the FWHM measurement in BackyardEOS or APT).
If you’re shooting with a DSLR camera or dedicated astronomy camera and you just want quickly confirm your focus is on, using a Bahtinov mask is one of the easiest ways to do this.
Diffraction Spikes Bahtinov Mask by William Optics.
Why I use a Bahtinov Mask
I really enjoy being able to quickly confirm that my focus is as sharp as possible in a moment’s notice. I always lock the focuser into position before capturing the first light frame, but it’s nice to check to see if it’s still dialed-in later on throughout the night.
It takes less than a minute to place the mask on the telescope and check to see if the central spike is still centered in the “X”. If I need to make a slight adjustment, I can easily do so between sub-frames.
I enjoy the simplicity and visual nature of diagnosing and correcting the star diffraction spike pattern. This technique provides instant gratification in real-time, which is rare in this hobby.
Bahtinov masks are inexpensive and easy to use. It’s a nice upgrade to your ongoing list of astrophotography accessories that won’t break the bank but makes a big impact on your images.
If you don’t feel like spending money on a Bahtinov mask, you can always consider making your own: How to make a Bahtinov Mask for your telescope or lens (PDF templates and instructions).
Focus Basics: Using Live View
If you haven’t heard of focusing using live view, I’ll briefly explain how it works. Most DSLR cameras (such as the Canon Rebel XS and beyond) have a feature known as “live view”, where a real-time picture is displayed on-screen.
When you are in focus, bright stars in the night sky will appear on the screen. Even a subtle change in focus can be the difference between a star that is in focus, and one that is so far out that you cannot see it on your live view display.
If you are having trouble focusing your DSLR camera through a telescope (whether you’re using a Bahtinov mask or not), pay attention to the following tips.
Because it is so dark at night, your live-view screen may appear as pure black when looking through your telescope. This could be because:
- You do not have your DSLR’s ISO Sensitivity high enough
- You are so far out of focus that stars are not visible
- You are not on long exposure “bulb mode”
- You are not pointed at a bright enough star!
4 tips to ensure that a star appears on your live view screen:
Aim your telescope at a bright star
Use a high ISO sensitivity
Next, make sure that your camera is set to its highest ISO when focusing via live view. On the Canon T3i that I use, that happens to be ISO 6400.
Set camera to Manual Mode – Bulb Shutter speed
Make sure your DSLR camera is set to Bulb mode – the longest possible exposure – past 30”. Slower shutter speeds will dim the star, and we want it as bright as possible for focusing.
Adjust the focus knob on your telescope
And finally, focus your telescope’s draw tube until you see a bright star appear on screen. You may be way out, so make sure that you check end-to-end. (If you still can’t reach focus, you may need to purchase an extender tube)
A dual-speed Crayford style focuser really helps when adjusting the focus at this level of accuracy.
What about using a camera lens?
The same settings apply if you are using a camera lens in place of a telescope. The only extra step you will need to take is to make sure that the lens is set to its fastest aperture.
A lens set to an aperture of F/4 or faster will allow plenty of starlight to reach the sensor.
Other Ways to Focus Your Camera
If you are not satisfied with your results using a Bahtinov mask, you can improve your focusing accuracy using dedicated astrophotography software like BackyardEOS.
BackyardEOS was built to help astro-imagers improve their acquisition process in the field. There is an excellent function within BackyardEOS that has provided me with a higher level of focus accuracy than ever before: FWHM.
Find BackyardEOS and all of the other software I use for astrophotography on the Resources page.
How to use FWHM (Full Width Half Maximum)
FWHM is a precision focusing aid feature included with BackyardEOS. It is found within the ‘frame and focus’ tab at the top left of the screen. This function associates a value on the star you have selected in a target window. Using the live view mode within BackyardEOS will show you a real-time image of a bright star in your field of view.
Official Description from the creator of BackyardEOS:
“Full-Width Half Maximum is the width of a star’s image at half its peak. Focus is achieved when you get the lowest value for the same star over time, indicating a tighter star. BackyardEOS implements FWHM by calculating the standard deviation (the square root of the variance) of all pixel values in a very small selected area.”
Remember to use the same camera settings as discussed earlier while performing this task. Manual Mode, Bulb Shutter Speed, High ISO, and Pre-Focus.
First, make sure the Live View button is pressed on the lower right-hand side of the BackyardEOS interface. If your camera settings are correct, and your focus is close, you should see a number of stars on the screen.
I prefer to choose a medium-sized star within the frame as my target star. Double-click the target window around your chosen star. This will show a zoomed-in preview window at the top right of the interface.
You will now notice a number below this zoomed star image, and this is the value we will monitor to achieve high accuracy of focus. The Zoom box center can zoom in on the star if desired, but I prefer to use the default 3X zoom.
Adjust the fine focus knob on your telescope and watch as the number associated with that star changes. The goal is to get the star as small as possible, with the lowest number. There is no set number to reach, as stars vary in size. On a medium-sized star, I usually reach a number as low as 3.6, give or take.
Once you have reached the lowest number possible for that star, go ahead and lock your focus into position using the lock screw on your telescope.
Do not use a Bahtinov mask if you are using the FWHM function of BackyardEOS. The photo above is misleading because you will only be adjusting focus and referring to the FWHM number when the mask is off.
The Zoom box is used for an up-close look at the star diffraction pattern only. A real reading of the star size can only be obtained when the mask is off. When it comes to focusing with a Bahtinov mask, or the FWHM function, it’s one or the other.
Test Exposures vs. Live View
For my small refractor, the live view image was not bright enough to get a useful star diffraction pattern. Instead, I used short preview exposures (the Snap Image button) and made adjustments back and forth.
After about 3 slight focus adjustments, I was able to produce the ideal star diffraction pattern to indicate that my focus was as sharp as possible.
The nice thing about the Bahtinov focus mask is that the star diffraction pattern provides you with a useful visual aid. Slight changes in focus are evident in the star pattern right away.
Using a Motorized Focuser
In early 2018, I installed a Pegasus Astro stepper motor kit on my Explore Scientific ED102 refractor. This telescope upgrade allows me to me micro-adjustments to focus without touching the telescope.
Aside from being able to make fine adjustments to the focuser, it also means that I can now adjust the focus remotely from inside the house. This comes in handy on those cold winter nights with the gear running outside.
The Pegasus Astro Stepper Motor Kit installed on my Telescope
It now has this ability to focus on its own, although I have not deployed the autofocus features as of yet.
The dedicated focuser software reads the star size information and communicates with the motor to make small adjustments to the focuser as needed. A temperature probe reads the ambient temperature outside to decide if a focus tweak is required.
As for the rest of the telescopes I use, the Bahtinov mask makes the focusing routine simple and straightforward.
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