28 • Telescope Basics
When working with any optical instrument, there are a few things to remember
to ensure you get the best possible image.
•
Never look through window glass. Glass found in household windows is
optically imperfect and, as a result, may vary in thickness from one part of a
window to the next. This inconsistency can and will affect the ability to
focus your telescope. In most cases you will not be able to achieve a truly
sharp image. In some cases, you may actually see a double image.
•
Never look across or over objects that are producing heat waves. This
includes asphalt parking lots on hot summer days or building rooftops.
•
Hazy skies, fog, and mist can also make it difficult to focus when viewing
terrestrially. The amount of detail seen under these conditions is greatly
reduced. Also, when photographing under these conditions, the processed
film may come out a little grainier than normal with lower contrast.
•
When using your C8 as a telephoto lens, the split screen or microprism
focuser of the 35mm SLR camera may “black out.” This is common with all
long focal length lenses. If this happens, use the ground glass portion of
your focusing screen. To achieve a very sharp focus, consider using a
focusing magnifier. These are readily available from your local camera
store.
•
If you wear corrective lenses (specifically glasses), you may want to remove
them when observing with an eyepiece attached to your C8. When using a
camera, however, you should always wear corrective lenses to ensure the
sharpest possible focus. If you have astigmatism, corrective lenses should
be worn at all times.
Your Celestron telescope can be used for both terrestrial and astronomical
photography. Your G-8 has a fixed aperture and, as a result, a fixed f/ratio. To
properly expose your subjects photographically, you need to set your shutter
speed accordingly. Most 35mm single lens reflex (SLR) cameras offer
through-the-lens metering that lets you know if your picture is under or overex-
posed. This is important for terrestrial photography where exposure times are
measured in fractions of a second. In astrophotography, the exposures are
much longer, requiring that you use the “B” setting on your camera. The actual
exposure time is determined by how long you keep the shutter open. More on
this in the section on “Celestial Photography.”
To reduce vibration when tripping the shutter, use a cable release. Releasing
the shutter manually can cause vibration, something that produces blurred
photos. A cable release allows you to keep your hands clear of the camera
and telescope, thus reducing the possibility of shaking the telescope. Me-
chanical shutter releases can be used, though air-type releases are best.
General Observing Hints
General Photography
Hints
