I finally spotted why so many stacked astro photos look blurry in the corners

Everyone stacks their frames but nobody checks if their mount's polar alignment drifted mid session, which ruined 3 hours of my Andromeda data before I realized the stars were trailing just 2 pixels per frame over 200 lights.

2 comments

2 Comments

brookep6215h ago

Hmm I gotta push back on that a little. A 2 pixel drift over 200 lights is actually tiny, like 0.01 pixels per frame on average, so that’s not really ruining anything in a stacked image unless you’re shooting at super long focal lengths. Polar alignment drift is real but usually shows up as a gradual trend across the whole frame, not just the corners. Blurry corners in stacked astro photos are way more often from optical issues like coma, field curvature, or the focuser sagging slightly when the camera shifts. I’d check if your flattener or reducer is spaced right before blaming the mount, especially since 2 pixels of drift over 3 hours is actually a really good alignment for most setups.

-1

wood.john13h ago

Respectfully, 2 pixels is 2 pixels no matter how you slice it. Stacking software might average it out, but that drift still eats into star sharpness across the whole frame, not just the edges. Optical stuff like coma can definitely mess up corners, but I've seen perfect spacing still get soft corners from slight polar misalignment over long sessions.