Ask any veteran astrophotographer what the most common beginner mistake is, and most will say the same thing: out-of-focus images. Unlike daytime photography, you can't rely on autofocus in the dark. Stars are tiny pinpoints that are easy to think look sharp when they're not. This guide covers every method — from a $15 Bahtinov mask to a fully automated electronic focuser — so you never struggle with focus again.
Why Focusing a Telescope Is Hard
Two things make astrophotography focus uniquely difficult. First, stars are essentially infinitely far away — you're always focusing at infinity, but "infinity" on a telescope focuser isn't a single marked position, it varies slightly with temperature and equipment flex. Second, the human eye is very good at compensating for soft focus — a star that looks reasonably sharp on a phone screen at night often turns out to be blurry when you zoom in at home. You need a method, not just your eyes.
The #1 rule: Always focus on a real star through your actual imaging setup — camera attached, filter in place if you're using one. Focusing without the camera or with a different eyepiece will give you a different focus point. Narrowband filters shift the focus point slightly from broadband, so always focus with the filter you plan to image with.
All the Focusing Methods, Ranked
Bahtinov Mask
A diffraction mask that creates three spikes on any star. Focus is achieved when the central spike is perfectly centred. Accurate and cheap — under $20. The most popular first-step for beginners.
Live View + Zoom
On a DSLR or mirrorless: enable live view, zoom to 10× on a bright star, and manually adjust focus until the star is the smallest point. Slow but works for casual sessions.
HFR / FWHM Software
Software like SharpCap or N.I.N.A. measures the Half-Flux Radius of stars in real time. Smaller HFR = better focus. Move the focuser to minimise this number. More accurate than visual methods.
V-Curve / U-Curve
Move the focuser through a range of positions while software measures HFR at each point. The minimum of the V-curve is perfect focus. Used by SharpCap's Smart Focus and N.I.N.A.'s autofocus.
Electronic Auto Focuser (EAF)
A motorised focuser that runs the V-curve automatically. Set it to re-focus every 30 minutes or when temperature drops 2°C. Completely hands-free. The ZWO EAF is the most recommended.
Temperature Compensation
As temperature drops overnight, telescope tubes contract and focus drifts. An EAF with temperature compensation moves the focuser automatically by a pre-calculated number of steps per degree change.
How to Use a Bahtinov Mask
A Bahtinov mask is a circular plate with a pattern of slots that creates three diffraction spikes around any star. It's named after Pavel Bahtinov who invented it in 2005 and it remains the gold standard cheap focusing tool for telescopes up to about 200mm aperture.
- Fit the mask over the front of your telescope (they're sized by aperture — make sure you order the right size)
- Point at a moderately bright star — Magnitude 2–4 works well. Not too bright, not too dim
- Zoom your live view or use your planetarium software to centre the star
- You'll see three diffraction spikes forming an asterisk pattern — one central spike and two angled spikes either side
- Adjust focus until the central spike bisects the gap between the two outer spikes exactly — perfectly centred
- Remove the mask and begin imaging — don't touch the focuser again
Bahtinov mask tip: Zoom in as much as possible when using a Bahtinov mask — the difference between "close" and "perfect" can look tiny at normal zoom but is clearly visible at full zoom. On a DSLR, use 10× live view zoom. In SharpCap or N.I.N.A., zoom the preview to 200%+.
Focus Drift: Why You Lose Focus During the Night
Even if you nail perfect focus at the start of the session, you may find images getting soft an hour later. This is almost always thermal focus drift — as the night gets colder, your telescope tube contracts slightly, changing the focus position. Metal tubes drift more than carbon fibre tubes. A 5°C temperature drop can shift focus by 50–200 focuser steps depending on the telescope design.
Solutions in order of complexity:
- Re-focus manually every 45–60 minutes using your Bahtinov mask or HFR check
- Use N.I.N.A.'s trigger-based autofocus to re-run the V-curve automatically when temperature drops a set amount
- Add an EAF with temperature compensation — it moves the focuser a calculated number of steps per degree change without needing a full V-curve run
Focusing with Narrowband Filters
Narrowband filters (Ha, OIII, SII) shift the focus point slightly compared to broadband because they pass a different wavelength of light. The shift is small but measurable — typically 30–100 focuser steps. Always focus through the filter you plan to image with. If you use N.I.N.A. with a filter wheel, you can set a focus offset per filter so NINA automatically compensates when switching between filters mid-session.
ZWO EAF Pro — Electronic Auto Focuser
Motorised · temperature compensation · works with N.I.N.A. and ASIAIR
Focus Method by Experience Level
Just starting out: Buy a Bahtinov mask for your aperture (~$15–20). Use it every session. It takes 3 minutes and is accurate enough for most imaging work.
Using capture software (SharpCap, N.I.N.A.): Add HFR monitoring to your workflow. Watch the number go down as you approach focus, then stop at the minimum.
Serious about automation: Add a ZWO EAF. Set N.I.N.A. to autofocus at session start, every 30 minutes, and on filter change. Focus will never be the reason your images are soft again.