How to achieve perfect Focus using a Bahtinov mask in 90 seconds
Updated: Jun 5, 2023
We already briefly showed how to use a Bahtinov mask in one of our previous videos (Our Full Astrophotography Equipment on the Field). We get messages once in a while from beginner astrophotographers asking how we collimate our telescope, so we thought we'd make this quick tutorial to be able to easily link this post and/or video instead of the long equipment one.
For this tutorial, we are going to be using a Bahtinov mask. Those are very cheap (usually under $20) and don't weigh anything!
The Bahtinov mask we are using for this tutorial is our the Fairpoint Bahtinov Focus Mask for 5.5-8.5" Telescope (our telescope is the 8" Newtonian). Make sure you get the right one for your telescope's aperture! You can see a list of many different masks HERE.
This tutorial will be updated with better images and more in-depth descriptions soon.
Step 1 - Aim your Telescope at a Bright Star
The first step is to ensure that your telescope is pointed toward a bright star. Try not to choose a star that is too bright, like Sirius, or it might affect your precision later. Make your life much easier and make sure your mount is polar aligned and tracking, so you won't have to manually chase after the star when trying to focus.
Step 2 - Achieving Rough Focus
With your camera turned ON, go into LiveView so you can see the star easily and in real time. Then, play around with the focusing knobs of your telescope just so you can get a quick, approximate focus.
Step 3 - Zoom In Closer Using Digital Zoom
Third step, use the maximum digital zoom your camera will allow (in our case, it is x10 with our Canon 7D Mark II) to get a closer look at the star. Then, use the arrows/thumb buttons on your DSLR camera to center it perfectly in the middle (This won't really have much of an effect, it just makes it easier if your tracking is not perfect and your star slowly drifts away).
Step 4 - Attach the Bahtinov Mask
Place the Bahtinov mask on your telescope. Depending on which one you own, it should normally come with 3 screws. Those do not need to be screwed into anything, but rather serve to "lock" the mask in place. You should either place those screws on the inside or on the outside of your telescope depending on your aperture size.
As you can see below, we lost one of the screws so we were unable to use them on the inside of the tube. Instead, we used our two remaining screws on the outside of the optical tube to hold the mask. We noticed that it didn't matter how centered the mask was.
Notice how the star in your LiveView display now has spikes of light going through it.
Step 5 - Achieve the Right Focus
Using the precision focusing knob on your telescope (the little one on the right of the image below for Crayford Dual Speed focusers), do your best to get the vertical spike striking the star exactly in the center. It should be fairly easy. A very bright star like Sirius might affect the precision depending on your scope's focal length because the core of the star might look too "fat".
Step 6 - Double Check your Star
If your star looks like the image below, then you are done! See how the vertical spike is striking the star exactly in its center.
You can now take off the mask and put it away! It is a good idea to tighten the lock on your telescope's focuser so that you don't lose focus during the night.
That's it for this very short tutorial! Be sure you don't forget to remove the mask before imaging!
We hope this will help beginners focus their camera using a telescope quickly and easily, so you can spend more time imaging :)
Take a look at the video below to see the entire process from our point of view!
Let us know if you have any questions or comments.
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