Troubleshooting: My Bahtinov Pattern Looks Wrong

A Bahtinov mask is simple: three spikes, center the middle one, done. Until it isn't. Here's what's going on when your pattern looks less like a diffraction pattern and more like modern art.

"I don't see any spikes at all."

You're way out of focus. When focus is far off, a star is a big blurry donut and the spikes are smeared into nothing. Remove the mask, get the star as small as you can by eye, then try again.
The star is too dim. Pick the brightest star in the sky, or a planet.
Your exposure is too short. On a camera, try a 2 to 5 second exposure or raise the ISO for focusing.

"The spikes are faint and fuzzy."

Brighter star, longer exposure, higher ISO. Wide-angle lenses make small patterns, so zoom in as far as live view allows. If you're shooting through thick atmosphere near the horizon, pick a star higher in the sky.

"The pattern won't sit still."

That's atmospheric seeing, the same thing that makes stars twinkle. Use several short exposures and judge where the middle spike sits on average. If the whole pattern jiggles every time you touch the focuser, wait a second or two for the vibration to settle before judging.

"I keep overshooting the center."

Small turns. Smaller than that. If you're using a telescope focuser, use the fine-focus knob if you have one. If the middle spike crosses the center, you went too far; come back halfway.

"I see extra spikes."

Newtonian and Ritchey-Chrétien telescopes have support vanes holding the secondary mirror, and those vanes make their own diffraction spikes. Rotate the mask until its spikes are clearly separated from the vane spikes.

"The middle spike won't center, no matter how far I turn."

Your lens or telescope may not be able to reach focus at all. On a camera, a lens adapter with the wrong spacing can cause this. On a telescope, you may need more or less inward or outward focuser travel, or an extension tube. Try focusing without the mask; if you can't get a sharp star either, it's not the mask.

"The middle spike is centered, but my stars are still soft."

You bumped focus removing the mask. Remove it gently, and tape the focus ring.
Focus drifted. Temperature drops move focus over the night. Here's when to recheck.
Dew. Check your lens or corrector plate.
Collimation. On an SCT, Newtonian, or RC, misaligned mirrors make soft stars even at perfect focus. A Tri-Bahtinov mask checks both at once. See the collimation guide.
It wasn't focus at all. Stars that are sharp in the middle but stretched at the edges point to tilt or flattener spacing. Stars that are all streaked the same direction point to tracking or wind.

"My mask won't stay on."

Check that your outside diameter is within the mask's range (sizing guide), and snug the soft-tip screws evenly. Lost a screw in the grass? Replacement soft-tip screws come in packs of three.