Whether you've been doing astrophotography for one week, or 10 years, you've likely had trouble focusing your telescope / lens.  This is especially a problem on some older DSLRs, where you can hardly see the stars on Live View.  Usually, the best you can do is slowly rotate the focus ring until the star looks as small as possible.  Not exactly precise...  Thankfully a Russian astrophotographer - Pavel Bahtinov -  came up with an ingenious solution!  
 

A Bahtinov Mask is a fairly simple design.  It's usually a piece of plastic with some slits cut out.  These slits produce unique diffraction spikes around bright stars.  These diffraction spikes allow you to get a perfect focus every night!  People have come up with some very creative, and inexpensive ways to do this - from 3d printing, to using household items.  However, the most reliable method is to use a purpose-built Bahtinov Mask.  To learn more about Bahtinov masks, check out this great article on NightSkyPix.

When I began using my SpaceCat telescope in 2019, I was happy to find a Bahtinov mask built into the lens hood.  I had never used one before, but it quickly became one of my favorite features of this telescope.  I no longer had to guess where the sharpest focus was, I could clearly see the diffraction spike and be sure!  Unfortunately, I did not have a way to use this mask with my regular telephoto lenses.  

The William Optics RedCat / SpaceCat telescope comes with a Bahtinov mask built into the lens hood.

In late 2020 I was contacted by Kase Filters, who wanted me to try out some of their astro-related filters.  I thought this would be a great opportunity to finally try a Bahtinov mask out on my traditional camera lenses.  I received two different variants - the more traditional circular filter, and a 100mm square filter.  The circular filter can be quickly attached to a normal camera lens, in my case a Tamron 24-70mm.  The 100mm square filter required a special 100mm filter holder, which I had laying around from my early days as a landscape photographer.  Let's talk about which option would work best for you.

Filter Thread Size

In order to choose the correct filter, you'll need to know your lens' filter thread size.  Every lens has a different filter thread, so you'll want to check each one.  You can look on the lens' body itself, or inside the lens cap.  Either way, you should find something like this Ø77 or Ø82.  This is your filter thread in mm.  I'll give a list of my lenses, along with their filter threads below.

• Nikon 14-24 - No filter thread, too big (uses custom 150mm filter holder)
• Nikon 24-70 - 77mm
• Tamron 24-70 - 82mm
• Sigma 35 - 67mm
• Tokina 100 - 55mm
• Tamron 150-600 - 95mm

Now that I know all of my filter thread sizes, I can make an informed decision.  Unfortunately, there's no easy choice.  The Kase Circular Bahtinov Mask is only available in 82mm and 77mm (as of March 2021).  That means I cannot use this filter with my Tamron 150-600mm lens, which requires a much larger filter (95mm).  However, I could adapt the 82mm filter to work with the rest of my lenses.  This will require step-up rings.

Step-up rings allow you to use a larger filter with smaller lenses.  You can find these rings for ~$10, so they are fairly cheap.  On the other hand, you could go with the 100mm square filter instead.  This would allow you to use that same 100mm filter with all of your lenses.  However, this method will ultimately cost more money, and require more gear.  We'll cover that further down in the article.

I'd recommend reading this article on step-up / step-down rings, it should clear up any confusion.

Most Bahtinov Masks will produce this diffraction spike.  You'll want to position the middle line directly in the center to achieve sharp focus.
 

Circular Filter

Most photographers are familiar with circular filters, especially the UV/Protection filters.  When people buy their camera/lenses from camera shops, they are often told they "need" a protection filter (so the camera store can actually make some money on the sale).  This brings up the first consideration.  If you have a UV/protection filter already on your lens, you'll want to take it off at night.  This glass is usually not the best quality, and can cause star distortion and weird flares at night.  Once any UV/protection filters are removed, you can now install the Kase Bahtinov mask.  

One feature I really like about the Kase filter is that it's magnetic.  I've never used a magnetic filter before, but it does make things much simpler.  After you thread the small magnetic ring into your lens filter thread, you can pop on the magnetic Bahtinov mask whenever you want.  For example, maybe you've spent a few minutes finding the perfect composition and dialing-in your camera settings.  You can quickly reach in your pocket, grab the bahtinov mask and clip it on the front of your lens.  If the diffraction spikes look good, you can pull the mask off and begin shooting.  No need to worry about threading the filter in every time!

During my tests of the circular filter, I discovered an unfortunate problem.  As the focal length gets wider, the bahtinov mask becomes less effective.  The diffraction spikes are harder to see, and don't offer much help once you get to about 24mm.  On the other hand, the more zoom you have, the clearer the diffraction spikes will be!  This makes a Bahtinov mask very useful at 200mm+!

Based on these findings, I would only recommend using a Bahtinov mask with lenses 50mm+.  If you are shooting with a wide angle lens, you'll just want to focus the old-fashioned way.  Use Live View, zoom into a bright star, and turn the focus ring until it's as small as possible.  If you don't have a good Live View, then you'll need to take test photos, adjusting the focus after each exposure. 

Kase has an 82mm Bahtinov Mask available, as well as a 77mm Bahtinov Mask.  

The Kase Bahtinov Mask quickly clips to the front of your lens, and allows for precise focusing with lenses 50mm+

100mm Square Filter

As I mentioned earlier, I used to do landscape and long exposure photography.  This was one of my favorite genres of photography, and I had a lot of fun using my 100mm Lee Filters to create beautiful images.  Since I already had a 100mm filter system, the 100mm Bahtinov Mask seemed like the perfect addition!  I could now utilize my filter holder at night!  

If you aren't familiar with 100mm filter systems yet, here's an article that explains the basics.

After you've chosen a filter holder, then you'll need some adapter rings.  These rings will allow you to attach the filter holder to your different lenses.  For example, with my lens selection (listed above), I could buy a 95mm adapter ring, 82mm adapter ring, 77mm adapter ring, 67mm adapter ring, and 55mm adapter ring.  However, this would be very expensive!  Each of those rings can cost up to $60!  This is where the step-up rings come in handy!

I can buy some cheap $5-$10 step-up rings that would attach to a single adapter ring.  For example, I could buy an 82mm adapter ring, then buy a 77-82mm step-up ring, a 67-82mm step-up ring, and a 55-82mm step-up ring.  I would still buy the 95mm adapter ring for my Tamron 150-600mm lens though.  Now I can quickly transfer my 100mm Bahtinov Mask and filter holder to all of my different lenses.

The Kase 100mm Bahtinov Mask does a great job with telephoto lenses!  The diffraction spikes are very crisp and clear, making focusing a breeze!  If you've got a decent Live View on your DSLR, you can even focus in real-time.  However, if you have an older DSLR, you'll still want to take some relatively short test photos to check the diffraction spike.  Then, adjust your focus ring slightly and see how the pattern changed.  Repeat until the center line is dead-center.  Be sure to check out my YouTube video (below) for more information on how to actually use a Bahtinov Mask.

If you have a large telephoto lens with a 95mm filter thread, then I would recommend going for the Kase 100mm Bahtinov Mask.  Just remember, you'll need a filter holder and a 95mm adapter ring.  You can find the Kase 100mm Bahtinov Mask here.

The 100mm Square Bahtinov Mask works perfectly with my old Lee Filter System

Final Word

After using both of the Kase Bahtinov Mask Filters, I'm happy to report they do a good job!  As I mentioned earlier, I never felt the need for a Bahtinov Mask until I began using my SpaceCat telescope and a ZWO ASI 1600MM camera.  With this specialized astro camera, I no longer had a Live View display for quick focusing.  That meant I had to take test photos, zoom in, check the star sharpness, adjust the focus ring slightly, take another test photo, zoom in, see if the focus was better/worse, and so on...  I also wanted to make sure I was getting truly sharp stars.  With the William Optics RedCat's built-in Bahtinov Mask, I can now be 100% sure I'm getting the best results.

For those using a DSLR and telephoto lens, a Bahtinov Mask can really save the day night!  This is especially true if you don't have the best eyesight, or have a hard time seeing your camera's LCD screen.  With the unique diffraction spikes around bright stars, you'll know for sure if you are perfectly focused.  Just remember to double check the focus every ~30 minutes or so during the winter.  This can be done quickly and easily with both the square and circular filter options.  I recommend aiming your lens up to a bright star before you find your object for the night.  After you've got your focus dialed in, then you can aim up towards whatever galaxy/nebula you want to photograph.

You can find the 77mm Circular Bahtinov Mask here, the 82mm Circular Bahtinov Mask here, and the 100mm Square Bahtinov Mask here.

Before we go, there's one other thing I wanted to mention.  I found it odd that the Kase 100mm "Bright Star" filter had many similarities to Lonely Speck's "Sharp Star" filter.  Of course, all Bahtinov Masks are going to look the same, and a 100mm filter is going to look like any other 100mm filter.  However, the SharpStar from Lonely Speck was originally released in 2015, and it seems Kase's Bright Star was released in 2017.  I do find it interesting that the naming scheme is so similar, along with the font placement on the edges of the filter.  I'll let you be the judge of that though.  Either way, I'd recommend reading Lonely Speck's article on Bahtinov Masks, he should explain anything I missed here. 

If you plan on using a telephoto lens for your astrophotography, then a Bahtinov Mask will really help you out!  However, if you are using a wide-angle lens for Milky Way photography, a Bahtinov Mask won't be much help.  Be sure to watch the video below for more information.