A dew shield made from a cereal box
The problem with summer observing, apart from the lack of night, is dew. Once the air cools after midnight the grass goes wet, the garden table goes wet, and the front lens of the refractor goes misty in about forty minutes. The view turns soft, then milky, then useless. Bright stars grow haloes. I used to think the seeing had collapsed until I shone a torch down the tube and saw the glass beaded with water.
The 70 mm has a short built-in dew cap, about five centimetres, which is not enough. The lens radiates heat to the open sky, cools below the dew point, and collects water. A longer tube in front of it shields the glass from most of the sky and slows that cooling.
What I made
- One cereal box, opened out flat. The unprinted grey inside becomes the outside, and I covered it in black insulating tape anyway.
- The inside lined with a sheet of black craft foam, which cuts reflections better than bare card.
- Rolled into a tube about 20 cm long, sized to slide over the existing dew cap with a little friction. The rule I found is a length of roughly two to three times the aperture, so 14 to 21 cm for a 70 mm lens. I went to the top of that range.
- Held with two strips of tape and a rubber band. Total cost: nothing I did not already have.
Does it work
On Thursday night, clear, 13 °C at midnight and falling, grass soaked by 23:30, I spent from 23:15 until 01:40 on Albireo, Mizar and Alcor, and a slow sweep of the Cygnus star fields. The lens stayed clear for the full two hours and twenty-five minutes. The finder, which has no shield, fogged at about the one-hour mark, so I now know the control experiment works too.
Albireo, by the way, is still the best thing in a small telescope on a summer night: a gold star and a blue one, close enough to share a field at 28× and different enough in colour that it looks staged. Mizar splits easily at 70×, and Alcor sits nearby as a separate star you can see without any optics.
The shield is not elegant. It looks like what it is. It will not survive heavy rain, and the tape will eventually give up. When it does I will make another one from the next cereal box, which is a replacement schedule I can live with.