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I bought one of these from Bintel a long time ago and it works well.
It has a peephole to ensure the secondary is centred in the focuser draw-tube.
It has cross-hairs so you can adjust the secondary collimation screws and get the primary donut centred.
It has the window to see the primary donut that is reflected back and you can tweak the primary collimation screws to centre the reflected donut.
I'm quite happy with it and its not expensive.
The best device to buy is the Cats Eyes Auto-Collimator but expensive.
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