79
The exposure times listed in Table 9-1 should be used as a starting point. Always make exposures that are longer and
shorter than the recommended time. Also, take a few photos at each shutter speed. This will ensure that you will get a
good photo. If using black and white film, try a yellow filter to reduce the light intensity and to increase contrast.
Keep accurate records of your exposures. This information is useful if you want to repeat your results or if you want to
submit some of your photos to various astronomy magazines for possible publication!
This technique is also used for photographing the Sun with the proper Celestron solar filter.
Piggyback 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. The telescope is mounted on a
wedge and is polar aligned. 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 telescope, use the piggyback mount. This can be purchased as an optional
accessory.
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.
1.
Polar align the telescope (using one of the methods described earlier) and start the clock drive.
2.
Load your camera with slide or print film, ISO 400 or faster!
3.
Attach the camera with a normal or wide angle lens to the piggyback mount.
4.
Set the f/ratio of your camera lens so that it is a half stop to one full stop down from completely open.
5.
Set the shutter speed to the "B" setting and focus the lens to the infinity setting.
6.
Locate the area of the sky that you want to photograph and move the telescope so that it points in that direction.
7.
Find a suitable guide star in the telescope field. 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.
8.
Release the shutter using a cable release.
9.
Monitor your guide star for the duration of the exposure making all corrections using the hand controller.
10.
Close the camera's shutter.
As for lenses, use only those 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 35 to 200mm for lenses designed for 35mm
cameras.
The exposure time depends on the film being used. However, five minutes is usually a good starting point. With slower
films, like ISO 100, you can expose as long as 45 minutes. With faster films, like ISO 1600, you really shouldn't expose
more than 5 to 10 minutes. When getting started, use fast films to record as much detail in the shortest possible time.
Here are proven recommendations:
•
Ektar 1000 (color print)
•
T-Max 3200 (black and white print)
