Dew, frost and the hairdryer of last resort
The best skies of the year arrive with a catch. On a still, clear December night the temperature falls fast, and anything that radiates heat to the open sky gets colder than the air around it. A telescope lens pointing straight up is very good at this. The air reaches its dew point, and the front of the lens fogs. Below freezing, it frosts. Either way the stars turn into soft blobs and then disappear.
Last Tuesday it took thirty-five minutes. Air temperature −2 °C, no wind, and a crust of frost on the objective that looked quite pretty under a torch.
What works
- A longer dew shield. The cereal-box shield I made in July worked well in summer, but winter frost turned it soft and wavy within a fortnight. Its replacement is the same idea in black closed-cell foam cut from an old camping mat, about 20 cm long and held with two rubber bands. It shrugs off frost and roughly doubles the time before fogging, because the lens sees less open sky.
- Heat. A disposable hand warmer held against the tube just behind the lens with a third rubber band. Crude, but it keeps the glass a degree or two above ambient, and that is all it takes.
- Caps on between targets. When I go inside for tea, the cap goes on. Five minutes uncovered while I am indoors is enough to start it.
- Binoculars under the coat. Keeping them inside my jacket between looks works better than anything else I have tried with them.
What does not
- Wiping. A cloth just smears the dew into streaks and puts fibres on the coating. I did it once in a temper and regretted it.
- Breathing near the eyepiece. Obvious when written down. My eyepieces fog from my own breath long before the sky gets them.
- Bringing cold gear straight into a warm kitchen. Everything mists over at once, inside and out. Now it all goes in a closed bag, sits in the hall for an hour, and comes out dry.
The hairdryer of the title is the last resort. On low heat, from about half a metre away, for twenty seconds, it clears a frosted lens without anything dramatic happening. It needs an extension cable through the back door, and the neighbours have now seen me using a hairdryer on a telescope at midnight. That cost has been paid.