How To Easily Focus On The Stars

With the exception of artistic shots, nail the focus on stars is crucial in astronomic images and starry landscapes.

Stars Bokeh. Image Credit: Andres Nieto Porras.

Stars Bokeh. Image Credit: Andres Nieto Porras.

Why Is Focusing On Stars DIfficult?

At the beginning of our journey to the stars, we all struggle with getting the focus spot-on on stars, particular if we use standard photographic equipment.

  • Stars are faint;

  • Stars are small;

  • Stars move;

  • Autofocus will not lock on the stars 99% of the time and will hunt endlessly for the focus;

  • …

What can you do, then, to improve your focus?

The Pre-focusing Myth

When asked on forums and groups, one recurrent advice is: “just prefocus your camera during daylight on a distant object and lock the focus.”

While it seems like a piece of sound advice at first, this pre-focusing technique never really works, for at least 3 reasons:

  1. You may be using a lens, or bridge camera, that resets itself when your camera is turned off;

  2. Temperature dropping between daytime and nighttime can introduce a noticeable focus shift;

  3. With zoom lenses, the focus can change with the focal length used;

Killer Focusing Tools: Bathinov Masks

A classic Bathinov mask.

A classic Bathinov mask.

Bathinov Masks are the goto focusing tools for astrophotographers around the globe. They work wonderfully by creating diffraction spikes around the stars, which guide you to the perfect focus.

Image Credit: Lonely Speck

Image Credit: Lonely Speck

Precise and fast. Job done, problem solved… or is it?

Not quite: with photographic lenses, the gaps in the mask creating the diffraction spikes are often too wide to produce the spikes.

So a Bathinov mask is a killer tool for telescopes, but a waste of money if you are using short(-ish) focal lengths lenses.

The only way to have a working Bathinov mask is to 3D print one using a Bathinov mask generator or to buy the SharStar2 mask from Lonely Speck, which, though, requires a filter holder for 100mm squared filters.

Perfect Focus: Telltale Signs

So we are back to square one: time to learn the telltale signs of perfect focus.

First of all, turn off the autofocus and image stabilization (you don’t need it as you are on a tripod). Then don’t consider the infinite symbol engraved on your lens or the focus hard stop to be a reliable indication for focusing.

These are the only three signs you have to look for when manual focusing on stars:

  • Stars shrink to a minimum size when are in focus;

  • Fainter stars become visible when your focus is correct;

  • Chromatic aberration (purple/blue fringing) is reduced to a minimum when your focus is correct

Telltale signs for good focus.

Telltale signs for good focus.

To make easy to see these signs, use the (tiltable) LCD screen and live view. Then frame and magnify a bright star (or planet). With some cameras, you can get a brighter live view image by increasing the exposure time to BULB and increasing the ISO to 3200.

Just remember to dial in the proper ISO and exposure time when you are done focusing.


DIY Focusing Device For Short(-ish) Focal Lengths.

Wouldn’t it be nice to have a focusing device that works with wider lenses than classic telescopes? You are in luck, as you can build one yourself for a few dollars.

I noticed a kitchen sieve like the one to filter your tea, produces diffraction spikes around stars when popped in front of a lens. The spikes created are not mobile and are often dotted (depends on the focal length used).

The Sigma Art DN 60 f/2.8 is nice for astrophotography with an MFT camera (EFL 120mm), but focusing is a pain. On the right, the diffraction spikes produced by placing the kitchen sieve in front of the lens.

The Sigma Art DN 60 f/2.8 is nice for astrophotography with an MFT camera (EFL 120mm), but focusing is a pain. On the right, the diffraction spikes produced by placing the kitchen sieve in front of the lens.

What you do is aiming to get those spikes in view and as defined as possible. When you get sharp spikes, your focus is good.

While this works well, it is quite a messy job and you may risk to scratch your lens, but we can easily improve the design.

Instead of the kitchen sieve, take a frying pan splatter screen and a step down ring fitting your lens diameter.

With the net from a single splatter screen, you will have enough material to to create focusing masks for all your lenses :)

With the net from a single splatter screen, you will have enough material to to create focusing masks for all your lenses :)

Cut the metallic net of the screen to fit the size of the step-down ring and glue it on its flat edge: now you have your very own focusing device allowing you to quickly and consistently focus on stars with your photographic gear.

Building a focusing mask out from an old splatter screen. Ignore the Bathinov mask… it does not work for focusing but was the only step-down ring I had laying around in the house :)

Building a focusing mask out from an old splatter screen. Ignore the Bathinov mask… it does not work for focusing but was the only step-down ring I had laying around in the house :)

This is what you should see when focusing using the kitchen sieve or the custom made mask from the splatter screen.

Focusing on a star using high magnification in liveview and with the DIY focusing mask.

Focusing on a star using high magnification in liveview and with the DIY focusing mask.

Infograpix

I have created the Infograpix project, a series of infographics you can take with you in the field as a reminder and a guide.

Click on the image below to download your Infograpix about Focusing on Stars.

Right-Click on the image to download your Infograpix.

Before You Go

If you want to know more about focusing on stars, watch my video on the AstroPills’ YouTube Channel.