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| My Telescope An 8-inch f/6 Newtonian on a Dobson Mount |
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Photons from distant astronomical objects are
falling on the ground like rain all around us. With something bigger than the lens of our
naked eye to collect and focus them, we can see many thousands of things in the night sky
that are otherwise too dim or too small. This is what telescopes do. More than anything else, the aperture (diameter) of a telescope's objective (the primary mirror or lens) determines how many photons it can collect. The sturdy and inexpensive altazimuth mount popularized by John Dobson makes Dobsonian reflectors the cheapest per inch of aperture of any design, which is why Dobs are affectionately known as light buckets. My Celestron light bucket has an 8-inch primary
mirror at the bottom of the tube, which means that it can gather about 800 times as many
photons as my dark- The surface of the mirror is curved, so that the light reflected from it converges toward a small secondary mirror, which then bounces it out of the tube and into the eyepiece. The length of the light path from the primary to the eyepiece is called the focal length, and it depends on how fast the curvature of the primary mirror makes the light converge. My Dob's f/6 focal ratio means that the focal length is 6 times the aperture, or 48 inches. The top end of the tube is normally open to the air, but since I wanted to take these
pictures in daylight, the end is covered here with a full- |
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