Care
A session that ends abruptly with a blank eyepiece usually ends because the corrector or the objective has gone below the dew point. A dew shield buys an hour or two; a heater strip removes the problem, and the only figure that decides whether it works is whether it reaches round the tube.
Optics facing open sky lose heat to it and drop below the surrounding air. That is why dew forms on a telescope on a night when nothing else is wet, and why a shield — which restricts the view of open sky — delays it.
A dew shield extending in front of the objective narrows the cone of sky the glass radiates to. It buys time. A heater strip holds the glass a degree or two above the dew point and removes the problem for as long as there is power.
A strip has to reach around the tube at the point it heats. This site prints the published length and the tube sizes it suits, because “fits most telescopes” cannot be checked against anything.
A telescope brought indoors wet and covered will stay wet. Where a cover publishes whether it breathes, this site says so — a waterproof cover over damp optics keeps the damp in.
30 cm strip · 12 V supplies · even heat distribution
A 30 cm dew heater strip that wraps around the tube and runs from common 12 V power sources, for long observing and imaging sessions.
Best for: Imaging sessions where a shield alone does not keep the corrector clear
≈ £91 / €103
Tube outside diameter 225–245 mm · velvet lined · rolls flat
A flexible plastic dew shield with a velvet inner finish for tubes of 225 to 245 mm outside diameter, closed with hook-and-loop.
Best for: An 8 in Schmidt-Cassegrain or a 200 mm reflector on an autumn evening
≈ £88 / €96
Aluminium heater ring · for 6 in Schmidt-Cassegrain
An aluminium dew heater ring specified for the 6 inch Schmidt-Cassegrain, with no description published at all.
Best for: A 6 in Schmidt-Cassegrain whose corrector plate dews on every clear night
≈ $39 / £44 / €55
For the Telrad finder · impact-resistant plastic · doubles as a cover
A dew cap for the Telrad projection finder, preventing the projection window fogging and covering it when the finder is not in use.
Best for: A Telrad owner whose finder fogs before the telescope does
≈ €29
420D Oxford · PU coated · buckled straps and drawcord · storage bag
A PU-coated 420D Oxford cover with buckled straps and an adjustable drawcord, cut larger than most and supplied with its own storage bag.
Best for: A telescope left assembled outdoors or in an unheated outbuilding
≈ £40 / €38
Because the glass at the front of the tube faces open sky and radiates heat away to it. A Newtonian’s primary sits at the bottom of a tube and is shielded from most of that.
Long enough to wrap the tube at the point being heated, with the ends meeting. A strip that leaves a gap leaves a cold arc, and dew forms there first.
It clears dew that has formed, briefly, and it puts warm air across the optics which disturbs the image for several minutes. A heater that prevents the dew is the better answer.
Any figure written into the body of this page was true on the day it was written. The amount shown next to each instrument is requested from the retailer as the page loads, and it is that amount, not the text, that stands.
Telescopes and mounts are discounted over and over through the year — hardest of all in the weeks before Christmas, which is exactly when most of them are bought — and a struck-through figure on its own says nothing whatever about what the same model was charged at four weeks ago. UniverTrack keeps a week-by-week record for each retailer, so a discount is measured against a figure somebody genuinely paid instead of one invented for the sale — and it says when the moment to buy has genuinely arrived.
