51
Using the example we started with above, we can determine the field of view using the same 10mm eyepiece. The 10mm
eyepiece has an apparent field of view of 46°. Divide the 46° by the magnification, which is 200 power. This yields an
actual field of .2°, or a little less than a quarter of a degree.
For terrestrial viewing, field size is often referred to as feet at a thousand yards. To convert this to feet at one thousand
yards, multiply the actual field of .2° by 52.5. This produces a field width of 10.5 feet at one thousand yards.
The apparent field of each eyepiece that Celestron manufacturers is found in the Celestron Accessory Catalog (#93685).
General Observing Hints
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 telescope 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 eyeglasses), you may want to remove them when observing with an
eyepiece attached to your telescope. 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.
General Photography Hints
Your Ultima 2000 can be used for both terrestrial and astronomical photography. It 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
overexposed. 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. Mechanical shutter releases can be
used, though air type releases are best.
