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if you wear glasses, the focus point with and without your glasses will change.

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I don't understand this statement...I thought the point of the mask was that you use the position of the spike to determine focus. The position shouldn't change just because the eye focuses differently--should it?

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Yes, it does. As you probably know, nearsighted individuals can use telescopes without their glasses by changing the focus slightly from the position when they are wearing their glasses. The perfect focus point for when they have their glasses on will be different from the point when they take them off. The diffraction spikes with the mask are centered when the perfect focus is achieved for whatever imaging device is at the eyepiece- whether a CCD or your eyeball- with or without glasses. So when you look through the scope and have the spikes centered with your glasses on, then take them off, the center spike will move. Kind of strange, but true!

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John Wunderlin
Vendor - Spike-a Bahtinov Focusing Mask
Empire Ranch Observatory
Iowa County Astronomers

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