Darkness is what matters in Astronomy. Stars shining brightly, painted on the black canvas of space.
And there’s nothing spoils the view more than having a grey or washed out view through your telescope.
One form of light pollution is of course the overhead washing out of the sky due to street lights en-masse shining up into the atmosphere, which is then reflected back down to earth by water-vapour in the air. Filters are used to help astronomers with this kind of light pollution, and when it is impractical to visit a location that is away from the source of the lights. When the full Moon shines high in the sky, you instead choose brighter objects to observe, such as the planets and more dominant stars and constellations. Of course you can take time instead to appreciate the beauty of the Moon herself.
The other form of light pollution that causes bad contrast in telescopes which we do have a little more control over, is stray, unwanted light coming into the end of the telescope at an angle from nearby domestic lighting; be that from streetlights, decorative garden or patio lighting, house lights, the next door neighbours pesky football pitch floodlight, or the gentler but still invasive light from the Moon.
Well at least the Moon has every right to be there.
You can close curtains or turn off un-necessary house lights, garden lights, and have a friendly word with the neighbour. You can even write to the local council to see if they will do something about the nearest streetlamps. Or you can purchase a “dark-cap” or “dew-cap” to wrap around the end of the telescope to help prevent strong side-lighting. I often use a large patio-table umbrella opened up and laid on its side to shield me from a particularly annoying streetlight.
The main issue here is that the interior surface of the telescope tube, even when painted matt black by the manufacturer, still has a degree of reflectivity that causes the light to scatter and bounce its way down the tube, and into the eyepiece, where it adds a subtle lightening of the background, and this reduces the resolution and contrast, and therefore the enjoyment of observing stars, and can make viewing deep-sky objects far more difficult.
A very effective and permanent solution to employ with open tube telescopes such as Newtonians, is the addition of materials with low light reflectivity to the inside surface of the telescope tube, and this procedure is known as “flocking”.
Read my latest article on How to flock the interior of the Orion Skyquest XT10 newtonian reflector OTA tube.
In this article I describe how to add black flocking material (ProtoStar Hi-Tack Flocking Sheets) to the interior of my Orion XT10 newtonian reflector telescope tube to increase the contrast when observing the night sky.