Getting sharp focus is a critical and sometimes frustrating part of astrophotography. Even when everything is setup correctly, your equipment has cooled and the skies are steady, it can still be hard to tell if the stars in your frame are sharp or just little a bit off; and the last thing you want is to be disappointed with hours of imaging due to poor focus. Today, we’re going to look at a simple and affordable solution that takes the guess work out of focusing your telescope and lens. This miracle product, that almost feels like it should have its own infomercial, is called a Bahtinov Mask. Invented by Pavel Bahtinov back in 2005 this impressive device has helped me get sharp focus time after time.
Before buying the mask be sure to measure the diameter of your telescope or lens opening to make sure you get the right size. The mask that I bought is adjustable for an outer diameter between 65 and 100 mm which fits the lens hood of my Samyang lens. This specific Bahtinov lens only cost me about 12 dollars.
To get sharp focus with the Bahtinov mask, start by finding a bright star in the night sky, the brighter the star the better. Once you’ve settled on a star, place the mask over the aperture of your telescope or lens. When the mask is properly positioned on your equipment the bright star may appear as the outline of the mask if it is greatly out of focus. As you slowly begin to adjust focus, the three grid patterns that make up the mask will slowly form into a star with three diffraction spikes shooting out of it. Sharp focus has been achieved when the middle spike is perfectly centered between the other two spikes. I’ve found that the 10x digital zoom of my DSLR helps me to determine the positions of the diffraction spikes much easier.
I recently used this mask before imaging the Pleiades star cluster and it helped me to achieve sharp images of the seven sisters and the faint blue reflection nebula surrounding it. An important thing to remember is that even once you’ve achieved perfect focus early on, be sure to recheck it a few times throughout your imaging session as temperatures change during the night. Also, don’t forget to take the mask off before you begin you begin imaging. I may or may not have left it on once which led to some beautifully sharp stars with three spikes shooting through them. It was quite a unique picture, but not exactly what I was going for.
If you’ve used or are looking to buy a Bahtinov mask, please let me know how this brilliantly simple invention has worked for you or any questions you may have about it in the comment section below. Thank you all so much for your support and clear skies from Late Night Astronomy.