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Celestron CGEM 800 Manual

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Because of the high magnifications during eyepiece projection, the field of view is quite small which makes it difficult to find
and center objects. To make the job a little easier, align the finder as accurately as possible. This allows you to get the object
in the telescope's field based on the finder's view alone.
Another problem introduced by the high magnification is vibration. Simply tripping the shutter
even with a cable release
produces enough vibration to smear the image. To get around this, use the camera's self-timer if the exposure time is less
than one second
a common occurrence when photographing the Moon. For exposures over one second, use the "hat trick."
This technique incorporates a hand-held black card placed over the aperture of the telescope to act as a shutter. The card
prevents light from entering the telescope while the shutter is released. Once the shutter has been released and the vibration
has diminished (a few seconds), move the black card out of the way to expose the film. After the exposure is complete, place
the card over the front of the telescope and close the shutter. Advance the film and you're ready for your next shot. Keep in
mind that the card should be held a few inches in front of the telescope, and not touching it. It is easier if you use two people
for this process; one to release the camera shutter and one to hold the card. Here's the process for making the exposure.
1. Find and center the desired target in the viewfinder of your camera.
2. Turn the focus knob until the image is as sharp as possible.
3. Place the black card over the front of the telescope.
4. Release the shutter using a cable release.
5. Wait for the vibration caused by releasing the shutter to diminish. Also, wait for a moment of good seeing.
6. Remove the black card from in front of the telescope for the duration of the exposure (see accompanying table).
7. Replace the black card over the front of the telescope.
8. Close the camera's shutter.
Advance the film and you are ready for your next exposure. Don't forget to take photos of varying duration and keep accurate
records of what you have done. Record the date, telescope, exposure duration, eyepiece, f/ratio, film, and some comments on
the seeing conditions.
The following table lists exposures for eyepiece projection with a 10mm eyepiece. All exposure times are listed in seconds or
fractions of a second.
Planet ISO 50 ISO 100 ISO 200 ISO 400
Moon
4 2 1 1/2
Mercury
16 8 4 2
Venus
1/2 1/4 1/8 1/15
Mars
16 8 4 2
Jupiter
8 4 2 1
Saturn
16 8 4 2
The exposure times listed here 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 get a good photo. It is not
uncommon to go through an entire roll of 36 exposures and have only one good shot.
NOTE: Don't expect to record more detail than you can see visually in the eyepiece at the time you are photographing.
Table 7-2
Recommended exposure time for photographing planets.
… 45 …


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