It's called a Canon Angle Finder C.It has two magnifications 1.25 and 2.5.I find it invaluable for focusing (x2.5).I used to use a stepping motor ( See picture) for focusing the lens.This involved taking a series of short exposures and seeing where focus was.Then it was a simple matter to go back as the focus position is known.Of course the Angle Finder C works with any lens or telescope.It also saves on neck strain etc as it can be rotated.
Bert
The Angle Finder C from here
http://shop.centre.net.au/index.html...5CX&it=product
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