Parfocal eyepiece

For rough focusing only, and for centering deep-sky objects that are too faint to show up on the SLR screen, a parfocalized eyepiece is handy (Figure 8.6 c). That is an eyepiece that you have adjusted to focus at the same position as your camera. Once you've found an eyepiece that is approximately correct, you can fit a parfocalizing ring around it to control how far it goes into the eyepiece tube. To match a DSLR, you'll probably need to fit the eyepiece with an extension tube, easily made from a 14-inch (35-mm) sink trap extension from the hardware store.

A parfocalized eyepiece is particularly handy for moving quickly to the correct focus after a big change in optical configuration, such as adding or removing a focal reducer. Many flip-mirror systems and other elaborate camera adapters include one.

But parfocal eyepieces don't give exact results. The focusing mechanism of your eye affects how they focus. The error is usually small but significant.

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