Astrophotography is a fairly technical and challenging kind of photography. Many things have to go right in order to produce decent and high-quality images, and one of these things is focusing on stars.
You can have the best gear and the best tracking results, but if you miss the focus, your image will be ready for the trash. I learned this the hard way myself.
Luckily, focusing devices exist to help you get the focus right, and this article is all about one such device: a Bahtinov Mask.
A Common Problem In Astrophotography – Trying To Focus
If you just started with photographing the night sky, regardless if you are using a simple tripod or a tracker, you may struggle with getting your stars in focus.
If so, welcome to the club, as this is one of the most common challenges we have to face: we all struggled with that.
And this is not surprising: stars are small and faint points of light, autofocus will not lock on them more often than not, and focusing on distant street lights is not always possible or accurate.
So, what to do?
Few Tips To Help You Focusing On Stars
Here is a list of a few simple tips to help you focus on stars.
Do Not Rely On The Lens Hard Stop
Most lenses have a hard stop on the focusing ring. More often than not, the hard stop sets the focus “past” infinity and your stars will be blurred.
Some people like to pull back a little from the hard stop: while this can work with fisheye and wide-angle lenses, for anything longer, this method is sloppy.
The infinite mark on the lens also is not accurate enough.
Finally, some modern lenses focus by wire, meaning the focusing ring has no hard stop nor a distances scale.
Use Wide Angle Lenses
With wide-angle lenses, focusing on stars is rather easy. This is because you can rely on the wide depth of field these lenses have to offer.
Simply set the focus close to the infinity mark or focus ⅓ into the scene and you should have everything in focus.
Use Your Camera Live View
The optical viewfinder is useless for focusing on stars. Instead, rely on the live view to magnify the image of a bright star.
Some cameras allow the image shown in the live view to change accordingly to the camera settings.
To have a better view of the stars, temporarily increase the ISO and set the camera to bulb.
Do not change aperture nor zoom in to try to see the stars better: changing the focal length of the zoom lens and/or the lens aperture may affect the focus.
Check For These Tale-Telling Signs Of Perfect Focus
There are three tale-telling signs for the perfect focus when looking at the live view image:
- The better you focus, the smaller the stars will become;
- The better you focus, the more faint stars will appear;
- The better you focus, the less chromatic aberration you will see around bright stars.
Focus On The Moon
For all practical considerations, the Moon, the planets, and the stars are all at infinity. If you see the Moon in the sky, you can use it to focus on the stars.
This will make the job easier, as our Moon is large, bright, and with surface features that allow you to get a very precise focus.
Use A Focusing Device
All the tips I shared above are rather qualitative and are based on your ability to eyeball the focus quality.
If you need a greater precision or you want to nail the focus in no time, a focusing device like a Bahtinov Mask is what you need.
What is a Bahtinov Mask?
A Bahtinov Mask is a “passive” focusing device that uses diffraction spikes to guide you through the perfect focus.
Who Created It?
Focusing masks have long been used as focusing aid devices.
Bahtinov masks, though, are quite new, being invented in 2005 by the Russian amateur astrophotographer Pavel Bahtinov.
Despite their late appearance in the astrophotography landscape, they quickly gained popularity, becoming one of the favorite tools among astrophotographers.
Their success is due to the fact that they are cheap and easy to make, they are easy to use and allow you to consistently achieve a very precise focus.
Does It Help You Focus Better?
The whole point of a focusing mask is to help you nail the focus on your target.
The Bahtinov mask does this by using a distinctive set of grids with cuts oriented along different directions. Three grids compose a Bahtinov mask.
Considering the largest grid as a reference, the other two grids have cuts that are angled + or – 30º with respect to the direction of the cuts in the grid of reference.
This pattern produces angled diffraction spikes around a star, and these spikes they all cross at the star when the focus is spot on.
How do you use a Bahtinov mask?
The mask is very easy to use.
Firstly, you have to frame a bright star or a planet (Venus works like a charm). The Moon is too large and is not a suitable target for focusing with a Bahtinov mask.
Then magnify the bright star or planet in your live view so as to easily see the spikes the mask will create.
Then place the mask in front of your telescope or lens.
With the mask in place, you will see the diffraction spikes in the live view.
If you start focusing, you will notice that two spikes form an “X”, while the third one is angled with respect to the other two.
All three spikes move with the focus, even if the most noticeable movement is that of the third spike with respect to the two forming the X.
When all spikes cross at one point, then you are in perfect focus.
Note that a Bahtinov mask works the same whether you fix it right on the front element of your instrument or on the lens hood or telescope dew shield.
Does A Bahtinov Mask Fit All Telescopes And Lenses?
So does a Bahtinov mask fit all instruments? Yes and no.
The working principle of the mask is general, and you will have spikes whether you use a refractor or a reflector, a prime lens or a photographic zoom lens.
But you have to consider the mask dimension, to properly fit it in front of your instrument.
Some masks have adjustable pins to allow you to use it with instruments of different diameters, while others are suitable only for a particular lens or telescope.
Some masks, such as the Lonely Speck SharpStar, work like classic square filters used in landscape photography and require a suitable holder.
Another thing to consider when choosing your mask is the focal length of the instrument you will use with it.
Most commercial masks are built for telescopes and advertised for classic photographic lenses only because of their diameter.
But photographic lenses with relatively short focal lengths require masks with finer grids to produce usable spikes.
So be careful when shopping for your mask.
Bahtinov masks are widely available on the market, but they are easy to make, so you can be tempted with some DIY solutions.
Bahtinov Mask Template
Online Bahtinov Mask template generator such as this from Astrojargon allows you to create a mask that is a perfect match for your instrument.
This generator takes into account the diameter of the instrument, its focal length, and other parameters to ensure the best results.
Bahtinov Mask 3D Printing
Particularly if you do not need a very fine grid, Bahtinov masks are easy to 3D print. If you have access to a 3D printed, you can find many projects on specialized websites such as Thingiverse.
3D printed masks can also be purchased directly from people owning a 3D print, such as Astrokraken.
Using Household Products As Alternative Focusing Masks
Every grid or mesh you place on your lens will create diffraction spikes on stars. You probably have a few household items lying around you could use as focusing devices.
One of my favorites is a small colander to filter tea leaves and such.
When popped in front of your lens, the metalling mesh of the colander produces spikes you can use to better guesstimate the perfect focus.
As opposed to the spikes from a Bahtinov mask, the colander produces spikes that do not move with the changing of the focus. All you need to do is to get those spikes as sharp and contrasted as possible.
This makes for a slightly less precise and consistent focus, but it does work with wide and medium telephoto lenses.
A fry pan splatter screen or a mosquito net works similarly to the kitchen colander, but they can be cut and glued on a step-down ring to be screwed on the front of your lens, for an easier focusing experience.
List Of The Best Bahtinov Masks
Bahtinov masks are available on Amazon and specialized webshops and many brands produce them.
While the design does not change, material quality and specs can make a difference. Some of the best masks you can buy off Amazon are:
There are no ways around this: your stars must be in focus to have acceptable astrophotography images, except if you are after some artistic results.
When it comes to stars, missing the focus is way too easy, and all we said here about the use of a Bahtinov mask barely scratches the surface of what there is to know about focusing.
For instance, did you know zoom lenses can creep and change your focus during the session? Or that temperature changes cause your instrument to shrink or expand, therefore shifting the focus?
If you want to know more about focusing on stars, this article we have recently published is for you.