Afocal Moon

Another easy way to take an astronomical photograph with a digital camera (DSLR or not) is to aim the telescope at the Moon, hold the camera up to the eyepiece, and snap away.

This sounds silly, but as you can see from Figure 4.3, it works. The telescope should be set up for low power (x 50 or less, down to x 10 or even x 5; you can use a spotting scope or one side of a pair of binoculars). Use a 28-, 35-, or 50-mm fixed-focal-length camera lens if you have one; the bulky design of a zoom lens may make it impossible to get close enough to the eyepiece.

Set the camera to aperture-priority autoexposure (A on Nikons, Av on Canons) and set the lens wide open (lowest-numbered f-stop). Focus the telescope by eye and let the camera autofocus on the image. What could be simpler?

This technique also works well with non-SLR digital cameras (see Appendix A). Many kinds of brackets exist for coupling the camera to the telescope, but (don't laugh) I get better results by putting the camera on a separate tripod so that it cannot transmit any vibration.

Figure 4.2. The Moon passing in front of the Pleiades star cluster. Half-second exposure at ISO 200 with Canon Digital Rebel (300D) and old Soligor 400-mm telephoto lens at f /8. Processed with Photoshop to brighten the starry background relative to the Moon.

This setup is called afocal coupling. The effective focal length is that of the camera lens multiplied by the magnification of the telescope - in Figure 4.3, 28 x 50 = 1400 mm.

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