A beginner's guide to Bahtinov masks
What Bahtinov masks are used for, and where to buy one online.
A Bahtinov mask is a specially cut visor made from cardboard or plastic that you place on the front of a telescope to help you focus accurately.
The special shape of the cut-outs in the mask produce a set of diffraction spikes in the same manner as the secondary mirror spider vanes on a Newtonian reflector.
However, the Bahtinov mask’s diffraction spikes form a very specific shape comprising a cross with a line passing through it.
Observing a diffracted bright star with this mask in place and adjusting the focus until the line perfectly bisects the cross ensures that accurate focus is achieved."
Using a Bahtinov mask for astrophotography
Focus is the single most critical component of any successful image but it can be remarkably difficult to achieve.
Despite the use of digital cameras and their live view features in astrophotography, determining when a test star is at perfect focus is rather subjective.
However, the use of a Bahtinov Mask makes achieving accurate focus really simple.
Just as a Newtonian Reflector’s spider vane produces diffraction spikes on bright stars, the ‘vanes’ on a Bahtinov Mask produce a complex diffraction resulting in a cross and a horizontal line.
Adjusting focus until the line bisects the cross confirms perfect focus.
Bahtinov masks and imaging deep-sky objects
Accurate focus on deep-sky objects is key to successful imaging, but it’s surprisingly difficult as many objects are too dim to focus on.
However, focusing on a bright star is much easier and if the stars are in focus, deep-sky objects will be too.
A Bahtinov Mask makes it easy to get stars in focus.
As we've noted above, when placed on the front of a telescope aimed at a bright star, you'll see a distinct diffraction pattern comprising an ‘X’ and a horizontal line.
Capturing a series of exposures of the diffraction pattern and checking them allows you to adjust focus between images until the line perfectly bisects the ‘X’, which indicates the best focus position.
Below we've picked out some Bahtinov masks that are available to buy online. Or, you could venture to make your own in our guide how to make a Bahtinov mask.
5 of the best Bahtinov masks
Vbestlife Bahtinov Mask
This Bahtinov mask works with refractors, Dobsonians, Maksutov Cassegrains and Schmidt-Cassegrains, with the outer diameter of the mask supporting objective lenses in the range of 60-90mm.
Three metal posts on the mask enable it to grip your telescope tube firmly, and come with scratch-resistant covers that should protect the surface of your tube. They also keep the mask nicely centred.
This Bahtinov mask is light, meaning it shouldn’t affect the balance of your telescope too much during visual observing or during long exposure astrophotography sessions.
Rother Valley Optics Advanced Bahtinov Mask
- Buy now from Rother Valley Optics
Rother Valley Optics supplies a range of Bahtinov masks, so you should be able to find one to suit your own telescope's specs.
The models in this 'advanced' range are made from thick plastic and have more lines, giving a larger pattern and - in theory - a more precise focus.
They come with 3 metal posts for fixing and a silicone sleeve to protect your telescope tube.
StarSharp Bahtinov Focus Masks
- Buy now from First Light Optics
The StarSharp Bahtinov Focus Masks are available to suit a range of telescope brands, styles and sizes, making them a good option if you need to quickly find out which one is suitable for your telescope, without having to carry out the measurements yourself.
Starwave Bahtinov Masks
- Buy now from Altair Astro
Altair Astro are a well-known seller of astrophotography cameras and telescopes, and they also have their own brand of Starwave Bahtinov masks. The Starwaves are sold in a few different sizes: 65-100mm; 80-110mm 175-220mm; 215-260mm; 250-290mm.
Astromania Bahtinov masks
These Bahtinov masks from Astromania are made from 2mm-thick plexiglass, and are fitted with securing screws that helpfully are capped with soft plastic. This is a precautionary measure to help ensure that the mask does not scratch your telescope tube.
This range is suitable for use with reflectors, refractors, Dobsonians, Maksutov-Cassegrains and Schmidt-Cassegrains.
The use of plexiglass in the manufacturing is a preventative measure against brittleness that can sometimes be brought on when used in very cold conditions.
They come in a range of sizes to suit different types of telescopes, so a bit of measuring will be required before you select which is best for your own tube.
Iain Todd is BBC Sky at Night Magazine's Content Editor. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.