A small telescope on a cold island: dew, frost and low targets

Cooling down takes longer than you think

A mirror carried out of a warm cabin into minus five is not usable for a while. The glass holds its heat, the air inside the tube moves in slow currents, and the star image at high power boils in a way that no amount of careful focusing will fix. With a six-inch mirror I allow thirty minutes, and on the coldest nights closer to forty-five. The practical answer is to carry the telescope out first and do everything else afterwards: dress, fill a flask, sort out charts, find the red torch.

Storing the thing somewhere cold solves most of it. Mine lives in an unheated boathouse from October, which means it begins the evening within a few degrees of the outside air and is ready almost at once. The cost of that arrangement is condensation whenever it comes back indoors, so it does not come back indoors until spring. I have had no trouble with the scheme beyond a tendency for spiders to take a serious interest in the focuser over the winter months.

Dew, frost and the things that end a session

Dew is the archipelago's speciality. Surrounded by water, with air that cools quickly under a clear sky, the eyepiece fogs before midnight on perhaps half the nights I go out. A long dew shield on the front of the tube buys hours by shielding the optics from the open sky, and it costs almost nothing to make from a rolled sheet of plastic foam. Eyepieces stay in a closed box with only the one in use exposed, which sounds excessively fussy and works better than anything else I have tried.

Frost is the harder version, and there is no defeating it, only delaying it. Once a mirror drops far enough below the dew point of the air, a fine white bloom forms across it and the night is over; breathing anywhere near the optics accelerates that dramatically. I have learned to stand to one side, to keep the eyepiece I am about to use inside my jacket, and to accept that a session ending at one in the morning with frosted glass is an ordinary outcome rather than a failure.

• Carry the telescope out first, then get dressed
• Dew shield on the tube; it buys two hours for the price of nothing
• One eyepiece out at a time, the rest in a closed box
• Breathe away from the optics, without exception
• Never wipe a frosted mirror; bring it in and let it dry itself

Low in the south, and what the latitude costs

The latitude takes a large piece of the sky away permanently. Anything south of about minus thirty degrees of declination never rises here at all, and a good deal above that clears the horizon by so little that it sits in haze and treetops. The bright southern showpieces of the summer Milky Way are effectively unavailable: the centre of the galaxy crawls a few degrees up at best, through the thickest air and the worst of the low glow, and there is nothing whatever to be done about that.

Even Orion, which feels like a winter birthright, culminates at only about twenty-five degrees from here. The great nebula is still worth every minute of the attempt, since the trapezium splits cleanly and the wings are obvious in a six-inch, but it is never the object that observers further south describe, and it is best caught in the narrow window on either side of due south. I plan winter evenings around that window rather than assuming the constellation is available all night.

What the aperture actually delivers here

What six inches does deliver is detail on things that are high, which at this latitude means an enormous amount of the northern sky. Globular clusters resolve to grainy edges rather than staying cotton wool. Planetary nebulae become small and definite instead of merely stellar. Open clusters in Auriga and Cassiopeia fill the field with stars the binoculars only hinted at, and the brighter spring galaxies overhead show an elongated core and, on the best nights, a suggestion of the dust lane I know is there.

Against a genuinely dark sky further from the city this instrument would show noticeably more, and I have seen that for myself on two trips north. The lesson I took from those nights was not that I needed a bigger mirror but that sky quality is the dominant term in the whole equation. From the jetty, the honest summary is that a modest telescope under this sky beats a large one left in a cupboard, and that the limiting factor is almost always the city to the west.