Page 55 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

WHAT TO USE

FOR DEEP-SKY
& LIGHT
POLLUTION

The difference is that these
BROADBAND
particular bits of glass are Also known as light-pollution
contained in a metal frame, reduction fi lters. By fi ltering
which has a screw thread. out the light from certain types
You use this to screw the of street lamp they darken the
sky, allowing nebulae,
fi lter onto the back of your
comets, galaxies and star
eyepiece before you pop it into
clusters to stand out. Cost:
your telescope. This is where From £45
fi lters go, as a rule – all except
solar fi lters, which for reasons
that are explained on the
opposite page, fi t onto
your telescope’s front lens.

FILTER SELECTION
It can be quite easy to forget NARROWBAND
These fi lters for nebulae are
when you’ve put a fi lter on
designed to let through light
because, once the eyepiece is
from glowing hydrogen or
in the telescope, the fi lter is oxygen from the nebulae,
out of view. Many times, even while eliminating sky glow
experienced observers have and street lamp pollution.
scratched their heads because Cost: From £70
the telescope view was not
as they expected, only to
eventually fi nd a fi lter left
The Hubble Space lurking on the eyepiece. You
Telescope employs can actually get adventurous
sophisticated fi lters and combine fi lters, but it’s
not recommended to use more
than two at a time because
ULTRA-HIGH
light loss becomes a problem. CONTRAST
Perhaps the easiest fi lter
These fi lters are for serious
to begin with is the kind
nebula observers who have
known as a neutral density a good scope and dark skies.
fi lter, which is a lot like the They let in only very selective
sunglasses we’ve talked light from these clouds and
FILTERS about. This dims the view of have various coatings to
IN SPACE all colours from entering your blacken the sky.
Cost: From £75
All the fi lters mentioned here are designed to remove elements eye and is great for cutting
of visible light to enhance viewing. However, there is more to down on the bright light from
the electromagnetic spectrum than visible light. Many space the Moon, especially around
telescopes employ fi lters to pick up emissions in X-rays, gamma the time of its full phase.
rays and radio waves. Two very important fi lters that are used There are, however, many
by the Hubble Space Telescope allow it to see in ultraviolet and other kinds of fi lters and
infrared. These fi lters isolate the particular kind of light, and in so
doing reveal workings of the Universe that are simply not visible we’ve listed a selection on
to our eyes. these pages.
Happy fi ltering!
skyatnightmagazine.com 2012 55