Helios 9877 User Manual page 18

special filters that can almost magically
remove the orange cast to the sky created by
sodium streetlights revealing a dark, star-
studded sky beyond. These filters are not
cheap – you can spend £100 quite easily for
two for a pair of binoculars – but to the tele-
scope user they are almost mandatory for any
serious work concerned with hunting for
galaxies and faint nebulae.
For the lunar and planetary observer, howev-
er, it is a very different story. The Moon is
never impaired by the streetlight's glow
(though the delicacy of earthshine on a very
young crescent moon to the west shortly after
sunset may be lost) and the bright planets
such as Venus, Mars, Jupiter and Saturn may
all be comfortably observed even from the
heart of a city. One word of warning, though –
if you have to set your telescope up on tarmac
or concrete, especially after a warm day, the
ground can reradiate absorbed heat after dark
that causes a shimmering effect in the tele-
scope, rather like looking through running
water.
This rippling effect is caused by air currents
circulating in and around the telescope that
distorts the image. Always position a tele-
scope outside for half an hour or so (security
willing, of course) prior to commencing
observing so that optics and conditions can
settle down such that you will get the best
views. Once you have a powerful telescope
you will notice that the air is very unsettled
on some nights, even after standing the tele-
scope out for its cooling-off period. The fault
then lies in the upper atmosphere and you will
have to leave detailed planetary viewing to
another night!
Photography: Sooner of later you will feel the
desire to capture some of the beauty of the
night sky on film to show to friends and rela-
tives. You may be surprised to learn that an
expensive camera and sophisticated drive
system to follow the stars in the diurnal
courses is not necessary in order to get very
attractive pictures of the constellations: all
that you need is a 35mm camera with a manual
(or 'B' setting) setting, a standard 50mm lens,
a fast colour (or B/W, it's up to you) film, tri-
pod and cable release.
The film you will need can be print or trans-
parency (slide), but you need to buy one with
a speed of about ISO 400 (ASA). With a
50mm lens set to its widest aperture of f/1.4
or f/2 and focused at infinity we can capture
stars fainter than those we can see with the
naked eye over an area of sky that encom-
passes two spans of the outstretched hand at
arms length, or about 40 degrees.
Since the camera is more sensitive than the
eye to faint light, we have to very careful to set
our camera and tripod well away from any
direct street or security lighting - standing in
the shade of a fence or wall will usually do the
trick. It is also a good idea to tell members or
your household that you are going out to do
some astrophotography, so that the bathroom
light does not come on unexpectedly, flooding
your back garden with light and ruining your
carefully obtained exposure!
One accessory that you will find immensely
useful is a small flashlight fitted with a dim
red bulb or a red filter over the lamp. The pre-
cious dark adaption that you gain through
being in pitch blackness for a period of 15
minutes or more is less affected by red light
than any other, so it is excellent for making
notes or ensuring that you camera has been
c o rrectly set without ruining your night
vision.
Taking the exposures: With your subject suit-
ably framed within the viewfinder (all but the
brighter stars will be difficult to see with the
screen of some SLR cameras) make sure that
the lens of the camera is wide open, the dis-
tance scale set to infinity and the cable release
is firmly screwed in. With the palm of your
hand held covering the lens (but NOT touch-
ing the camera), open the shutter with the
cable release and lock it. Now briskly move
your hand to one side to start the exposure
without jogging the camera - this technique is
18