How To Focus On Stars : Quick Guide To Getting Your Star Focusing Sharp

“Help! It seems I cannot get the stars in focus. How Can I focus on the stars?”

This cry for help is far too common on astrophotography forums and groups.

Indeed, one of the conundrums in astrophotography is to get things in focus, and we have all struggled with this.

With the exception of a few artistic images, stars that are out of focus are a deal-breaker.

In this article, I will teach you how to easily nail the whole focusing process.

blurry stars under the night sky
A selfie under the stars. The blurred sky helps to separate myself and my gear from the starry sky, giving a pleasant effect to the image.

Is Focusing On Stars Easy?

The short answer to this question is, “it depends.”

If you do not have a live view, if you don’t have a lens that can be used to efficiently do manual focus or you do insist on relying on your camera auto-focus for this, then… No, Focusing on stars is difficult.

Else, with a bit of practice and using the tips I’ll share in this article, you will discover that the difficult things in astrophotography are others 🙂

Manual Or Auto Focus For Stars?

Stars are small and faint points of light in perpetual motion. Most often than not, autofocus will hunt for the focus forever. 

You will have a slightly better chance for autofocus to work if you use fisheye and ultra-wide lenses: even if the autofocus messes up a little, because of the huge depth of field typical of these lenses, chances are your stars will look sharp anyway.

If there is the Moon or a (very) distant bright light, you can try to use the autofocus on it and then switch back to manual focus before recomposing the image for the stars, without altering the focus.

If you are serious about astrophotography, though, it is time to learn how to manually focus on stars. This allows you to have the best accuracy and results.

How To Manual Focus On The Stars

Using Infinity On Your Camera Lens

Some modern lenses and high-end bridge and compact cameras have no hard stop for the focusing ring. 

In these cases, you are “focusing by wire,” so there is not a scale for the distance or the infinite mark on your lens.

Sigma and Olympus lenses are sharp and affordable
My Sigma ART DN 60 f/2.8 is a great lens, sharp and affordable, but has focus-by-wire. The old Olympus Zuiko OM 200 f/4 legacy lens is built for manual focus: hard stop and distance scales are present, the focus ring is large and smooth.

But even if you have a regular lens, you should not rely on the mark for infinity or the hard stop (which is often past infinity).

When manual focusing, it is better to look for the three tale-tell signs of a good focus.

The 3 Tale-Telling Signs Of Good Focus On Stars

When focusing on stars, you should look for these three signs to help you estimate the quality of your focus:

  1. The Size of the Stars
  2. The Number of Visible Stars on the camera LCD screen
  3. The Amount (and color) of Chromatic Aberration (CA)

The Role of Stars Size And Star Number

This is the first sign you should pay attention to when focusing on stars. The better you focus, the smaller the stars look on your LCD screen.

If you are way out of focus, you may not see any stars at all, as their light is blurred and dimmed by spreading it over a larger area.

Because when focusing, you concentrate the star’s light to a point. Thus, the better you focus, the more faint stars become visible on your LCD screen.

out-of-focus Vs in-focus stars
The image shows the comparison between out-of-focus Vs in-focus: stars shrink, and more stars become visible (on the right image, some new stars are circled).

The Role of Chromatic Aberration

Stars are bright and the night sky is dark. Therefore the high contrast at the edges of stars creates a certain amount of chromatic aberration, CA.

One way to reduce it is to stop down your lens, but the focus also affects the amount (and color) of the CA. The better you focus, the less CA you can see.

Chromatic Aberration in Jupiter image
The image shows how the amount of CA in this image of Jupiter varies with the focus. On the left, the image is slightly blurred, and CA is very visible. On the right, CA is almost gone as the image is in perfect focus. Lens: Canon FD 300 f/5.6 legacy lens used at @ f/5.6.

Because the CA color often changes when moving from short focus to long focus, you can use the CA color to know in which direction you should focus.

Sometimes, at the exact focus point, you have CA of a color (say Purple) on one side of the star and of the other color (say, Cyano) on the opposite side of the star.

A Simple Workflow For Eyeballing The Focus On Star

Here is a simple workflow for eyeballing the focus as best as one can.

  1. Disable any form of image stabilization, as you are on a tripod. If you are using lenses with focus by wire, make sure to disable the option to reset the lens when the camera powers down, to avoid losing the focus if the camera goes in stand-by.
  2. Use a red dot star finder to frame a bright star or a planet such as Venus or Jupiter.
  3. Turn on the Live View. In some cameras, the brightness of the image will change with the camera settings. If so, to brighten the image, increase the ISO and set the camera to the longest shutter speed or Bulb.
  4. Magnify the bright star on the Live View.
  5. Begin focusing looking for the minimum star size, for faint stars coming into view, and for minimizing the CA.
  6. Optional: you can use some gaff tape to tape down the focusing ring so that the focus will not move when installing the anti-fog heated strip on the lens.

The Moon Is Up There: Can’t I Focus On The Moon Instead? And If I Can, How To Focus On The Moon?

We tend not to do deep sky astrophotography when the Moon is out, as the light it reflects back, washes out the sky.

But there is no reason why you cannot use it to focus on the stars: for all practical considerations, the Moon, the Planet, and the Stars are all at infinity.

To focus on the Moon, use the Live View and magnify an area near the Lunar Terminator, the division between the side of the Moon in the shadows and that in the light. 

focus on craters on edge of moon
Here the terminator was close to the Moon edge, yet craters were visible and detailed enough to help me focus.

Here, in fact, the sideway light reveals a great deal of details and craters, you can use to help you focus: try to get those features as sharp as possible.

With the Full Moon, the frontal light flattens most of the details on the lunar surface. 

Luckily, Montes Teneriffe, just south of the famous Plato crater, at the northern edge of Mare Imbrium, does provide quite a lot of contrast even during the Full Moon. 

This, my suggestion is to use them to focus, rather than trying to get a sharp lunar edge.

location of Plato on the Moon
Images of the Montes Teneriffe and the location of Plato on the Moon. This simulation of the view of a full Moon and the kind of contrast you can expect was done with the Moon Globe app for iOS.

Focusing Masks: A More Accurate Way To Focus On The Stars

You may ask if there is not a more accurate way to achieve focus other than eyeballing the 3 tale-telling signs I discussed before.

The answer is… Yes! Just get a focusing mask

A very popular focusing mask is the Bahtinov Mask. This mask is a simple disc of plastic to place in front of your telescope, with cuts that are oriented in three different directions, as shown in the photo below.

A Bahtinov mask helps focusing
A Bahtinov mask and how it is placed on the lens. It does not matter if you put it on the lens or on the lens hood for it to work.

The cuts create three diffraction spikes: two are fixed and cross at the star. While focusing, the third spike moves with respect to the other two, and when they all meet at the stars, you are in focus.

diffraction spikes visible while focusing on venus
The brighter the star or planet, the better. Here I was focusing on Venus, and the diffraction spikes are very visible. The two short lines are fixed and always cross at the star. The long line moves, and you are in focus when it crosses the fixed spikes at the star.

To have good spikes, you need to have a bright star in your instrument’s field of view, but also the width of the cuts must be appropriate to the focal length of your telescope/lens. The rule of thumb is that the shorter the focal length, the narrow the cuts.

DIY Focusing Mask For Photographic Lenses

With the increased popularity of astrophotography among amateur photographers, we have on the market an ever-increasing number of Bahtinov masks advertised for photographic lenses.

Problem is most of those masks work only with very long telephoto lenses, from 200 mm on. For shorter lenses, the masks may fit them as diameter, but the width of the cuts will be too large to create useful spikes.

If you want to create a custom mask, you can use this Bahtinov Mask template generator to make your own mask for the desired focal length.

If you, like me, are not afraid to experiment, you may find simpler solutions. 

One boring evening, I discovered I could build a focusing mask pretty much out of anything having a fine mesh: a small kitchen sieve, a splatter screen for my fry pan, or a net to keep flies and mosquitos out my window.

use a kitchen sieve as a Bahtinov Mask
Comparing the fine mesh of a kitchen sieve to the large cuts of my Bahtinov Mask.

All these items, because of the fine mesh, can produce spikes also with short focal length. The difference with the Bahtinov mask in the diffraction patterns these devices create.

diffraction spikes with a diy mask
The diffraction spikes created by popping a kitchen sieve, or splatter shield, in front of a 70mm lens.

Here, you do not have cuts, but small squares: the spikes are a series of dots irradiating from the star and are fixed. All you have to do is try to get the pattern as strong and sharp as possible.

If you use a mesh from a mosquito net or a splatter shield, you can glue it on a step down ring, so you can screw the mask on your lens rather than holding it in place by hand.

custom made Bahtinov mask
A step down ring to adapt filters smaller than your lens has wide edges you can use to glue on it a mesh or your custom made Bahtinov mask. This way, you can screw your mask on the lens like a normal filter. Remember to remove it from your lens once you have done focusing!

What Are The Best Lenses For Manual Focusing?

While they may have subpar optical performances, old legacy lenses often offer a better manual focusing experience than many modern ones.

Legacy lenses were built for focusing manually, so they have large and smooth focus rings with a large focus throw

The focus throw is the amount of rotation needed to go from close focus to infinity: a large focus throw makes it easier to make finer focus adjustments.

High-end modern lenses are also good at the “manual focus” game, but kit and budget lenses may be more difficult to focus and less precise.

My Focus Was Perfect On The Test Shot, But Now My Stars Are Blurry. What is Happening?

There are many reasons why you may get blurry stars during your imaging session: a sudden gust of wind, a passing car creating vibrations that are not dumped quickly enough from your tripod, etc.

But if you have all (or most) of the images blurry, your focus is changed and here are the most common reasons why this happens:

  1. Large changes in ambient temperature can cause focus shift, as the metal barrel of the lens or the telescope tube slightly shrinks due to the cold temperature. A headed strip used to fight lens fogging is a good way to mitigate the effect of temperature changes; Also, break down your imaging session in shorter sequences, so that you can still check your focus and adjust it if necessary.
  2. If you are using a zoom lens, you may have zoomed it to make focusing easier, took a test photo to confirm focus, and then zoomed out to recompose your shot. I found most zoom lenses have a slight focus shift when zooming in and out. Avoid zooming in and/or out if you want to make sure your focus stays consistent.
  3. Similarly to the previous case, if you have a heavy zoom lens and you point it at the sky, the zoom can retract (creep) slightly under its own weight. Do not forget to lock it with the proper switch (if present) or by taping down the zoom ring with a bit of gaff tape.
  4. With small lenses, your heating strip to avoid lens fogging can go around the focusing ring: by installing the strip, you risk to rotate the focus ring. To prevent this, tape the focus ring down with a small piece of gaff tape.

Final Tips For A Smooth Focusing Experience

Here are a couple of tips for improving your manual focus.

Use A sturdy Tripod

A sturdy tripod will not only help to keep your gear as stable as possible, but it also avoids a lot of camera shake when focusing.

As I said, to focus, you should use the live view and magnify your target star many times, so that you can appreciate changes in its size and chromatic aberration.

But by doing so, every time you touch the camera, the image will shake, sometimes to the point you cannot understand if you are focusing or defocusing. 

As this video shows, a good tripod will ensure a smooth and enjoyable focusing experience.

Use A Ring For Guiding Scopes For Easier And Smoother Focusing Adjustments

Video Makers are used to applying ring brackets on their focusing ring. This has the effect of increasing the “apparent” focus throw, as the rings are larger than the lens, and you need to rotate more to move from close focus to infinity.

DIY focusing tube ring
My DIY focusing tube ring device to achieve a smoother and more precise focusing experience.

This, together with the increased size of the bracket, makes for a very smooth focusing experience.

If you are using photographic lenses, you could replicate this effect by fitting a ring tube for guiding scopes on the focus ring, as in the photo. 

You can buy those rings in different sizes from any astrophotography related webshop.

Before You Go

Before you go, have a look at this short animated video I made to recap all the info written in this article.

Conclusion

Manual focusing on stars does not need to be a nightmare, and with the few tips and tricks presented in this article, you won’t have to worry about blurry stars anymore. 

All you need is to go out there and practice.

About Andrea Minoia

Andrea Minoia works as a researcher in a Belgian university by day and is a keen amateur astrophotographer by night. He is most interested in deep sky photography with low budget equipment and in helping beginners along their journey under the stars.