How to use a camera lens as an Eyepiece?
Cameras such as SLR have removable or interchangeable lenses. The best lens for this purpose is a 28mm, 35mm or 50mm.
WARNING the end of the lens that attaches to the camera can have sharp metal parts that can damage your eye, so cover these parts with something soft. Often lens come with plastic end covers, you could drill a 16mm hole through the centre of the cap and attach this.
We must be able to attach this lens to the telescope by the thread in the front of the lens. This thread is normally used to attach filters. Threaded rings to suit this thread can be bought for about $9.00 and glued to a film canister with the bottom end removed, as this will fit neatly inside a 1 ¼” focusers.
Before we can use the lens we must adjust the settings. Set the lens to the maxim F ratio (say 2.8) and set the distance focusing to maximum distance (Marked 00)
Now attach the lens to the telescope by the front end and view through the back of the lens with eye protection in place. (See warning above)
Adjust the focuser, as if it were a normal eyepiece, you will need to adjust the position of your eye to eliminate the dark stop in the centre, which is created by the secondary mirror in a Newtonian or Cassegrain type telescope, practice on the Moon. The eye will be about 25mm back from the lens, which will suit people who wear glasses. See diagram below.