Telescope Maintenance 56
Effective f# or f-ratio
The effective
f# can be found by this equation:
EF
EFL
Dp
..#
...
=
where
E.F.L. is the effective focal length, Dp is the diameter of the primary mirror and E.F.# is the effective
f-ratio. Make sure you are consistent with units. Use millimeters or inches, not both.
LONG EXPOSURE PRIME FOCUS PHOTOGRAPHY
This is the last form of celestial photography to be attempted after others have been mastered. It is intended
primarily for deep sky objects, that is objects outside our solar system which includes star clusters, nebulae,
and galaxies. While it may seem that high magnification is required for these objects, just the opposite is
true. Most of these objects cover large angular areas and fit nicely into the prime focus field of your
Celestron Telescope. The brightness of these objects, however, requires long exposure times and, as a
result, are rather difficult.
There are several techniques for this type of photography, and the one chosen will determine the standard
accessories needed. If, for example, you use a separate guidescope, the camera attaches to the telescope
with a T-Adapter (#93633-A) and a T-Ring for your specific camera. However, the best method for long
exposure deep sky astrophotography is with an off-axis guider. This device allows you to photograph and
guide through the telescope simultaneously. Celestron offers a very special and advanced off-axis guider,
called the Radial Guider (#94176). In addition, you will need a T-Ring to attach your camera to the Radial
Guider.
Other equipment needs include a guiding eyepiece. Unlike piggyback photography which allows for fairly
loose guiding, prime focus requires meticulous guiding for long periods. To accomplish this you need a
guiding ocular with an illuminated reticle to monitor your guide star. For this purpose, Celestron offers the
Micro Guide Eyepiece (#94171). Here is a brief summary of the technique.
1. Polar align the telescope using the declination drift method.
2. Remove all visual accessories.
3. Thread the Radial Guider onto your Celestron Ultima 9
1
/
4
. or Ultima 11.
4. Thread the T-Ring onto the Radial Guider.
5. Mount your camera body onto the T-Ring the same as you would any other lens.
6. Set the shutter speed to the "B" setting.
7. Focus the telescope on a star.
8. Center your subject in the field of your camera.
9. Find a suitable guide star in the telescope field. This can be the most time consuming process.
10. Open the shutter using a cable release.
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