How Often Should You Refocus During an Imaging Session?

Here's a fun fact nobody tells beginners: perfect focus doesn't stay perfect. You can nail it at sunset, walk away for three hours, and come back to a memory card full of slightly soft stars. Your gear didn't betray you. Physics did.

The good news: focus drift is predictable. Here's when to check.

1. When the temperature drops.

This is the big one. As the night cools, your lens barrel or telescope tube contracts, and the focus point moves with it. The biggest drop usually comes in the first few hours after sunset, which is also when you're most excited and least likely to check.

A common rule of thumb: check focus every time the temperature drops 1 to 2°C (about 2 to 4°F), or roughly once an hour if you don't have a thermometer handy. Fast optics (low f-numbers, like f/2 lenses or f/4 telescopes) are the most sensitive, because their zone of sharp focus is very thin.

2. When you change filters.

Different filters can bend light slightly differently, which shifts focus. Unless your filters are sold as parfocal (meaning they focus at the same point), refocus after every swap.

3. After a meridian flip.

If your mount flips sides when your target crosses the meridian, everything on your scope just turned upside down. Focusers can slip, and Schmidt-Cassegrain mirrors can shift. Check focus before you start shooting again.

4. When you move to a very different part of the sky.

Pointing from near the horizon to straight overhead changes how gravity pulls on your focuser and mirror. A quick check after a big slew is cheap insurance.

5. When you touch anything.

Changed lenses, zoomed, reattached the camera, added an adapter, or bumped the focus ring while fumbling for your coffee? Refocus. You know what you did.

6. When your stars start looking puffy.

If later frames show stars that are bigger or softer than earlier ones, focus has drifted. Many capture programs report a star size number (often called HFR); if it's creeping up over the night, that's your cue.

What drift is NOT

If stars look fuzzy and the image is getting hazier overall, check for dew on your lens or corrector plate before blaming focus. And if stars are sharp in the center but stretched at the edges, that's tilt or a field flattener spacing issue, not focus.

The one-minute refocus check

Clip on your Bahtinov mask.
Take a 3 to 5 second exposure of a bright star near your target.
Center the middle spike.
Remove the mask.
Repeat step 4.

That's it. A minute of checking beats a night of soft stars.

Telescope owners: get a two-for-one.

If you use an SCT, Newtonian, or RC, a Tri-Bahtinov mask checks focus and collimation in the same minute. Temperature swings can nudge collimation too, so you might as well catch both. See How to Collimate with a Tri-Bahtinov Mask.