Celestron CR-150 HD - Manual (Page 46)

46 • Celestial Photography
The easiest way to enter the realm of deep-sky, long exposure astrophotography is
via the piggyback method. Piggyback photography is done with a camera and its
normal lens riding on top of the telescope. Through piggyback photography you can
capture entire constellations and record large scale nebulae that are too big for prime
focus photography. Because you are photographing with a low power lens and
guiding with a high power telescope, the margin for error is very large. Small mistakes
made while guiding the telescope will not show up on film. To attach the camera to
the top of the telescope, thread the bottom of the camera body onto the 1/4"- 20
photographic adapter bolt located on the top of the telescope tube clamping ring. In
order to guide the exposure, you will also need an optional motor drive (#93518 or
#93523).
As with any form of deep-sky photography, it should be done from a dark sky
observing site. Light pollution around major urban areas washes out the faint
light of deep-sky objects. You can still practice from less ideal skies.
1. Polar align the telescope (using one of the methods described earlier) and
start the motor drive.
2. Load your camera with slide film, ISO 100 or faster, or print film, ISO 400
or faster!
3. Set the f/ratio of your camera lens so that it is a half stop to one full stop
down from completely open.
4. Set the shutter speed to the “B” setting and focus the lens to the infinity
setting.
5. Locate the area of the sky that you want to photograph and move the
telescope so that it points in that direction.
6. Find a suitable guide star in the telescope eyepiece field of view. This is
relatively easy since you can search a wide area without affecting the area
covered by your camera lens. If you do not have an illuminated cross hair
eyepiece for guiding, simply defocus your guide star until it fills most of the
field of view. This makes it easy to detect any drift.
7. Release the shutter using a cable release.
8. Monitor your guide star for the duration of the exposure making the necessary
corrections needed to keep the star centered.
9. Close the camera’s shutter.
As for lenses, use good ones that produce sharp images near the edge of the field.
The lenses should have a resolving power of at least 40 lines per millimeter. A good
focal length range is 50 to 500mm for lenses designed for 35mm cameras.
Piggyback Piggyback Piggyback Piggyback Piggyback