Why is Window Glass So Bad to Shoot Through?

Started 3 months ago | Questions thread
Leonard Migliore
Leonard Migliore Forum Pro • Posts: 18,334
Re: Why is Window Glass So Bad to Shoot Through?

Because it's not flat.

Windows are made out of float glass: https://en.wikipedia.org/wiki/Float_glass

where molten glass is poured onto a metal bed and slowly cooled. This makes it pretty flat. But in optics, we like things even flatter. Your customary optical surface is flat to 1/4 wave of light. So that's about a tenth of a micron. If it's not that flat, it messes up the wavefront of the light going through it. Now if you're stopped down to f/11 or so, window glass doesn't cause a lot of problems but if you're using some aperture, you get unsharpness.

As an aside, the Hubble telescope primary was accurate to 1/64 wave. But it was accurate to the wrong shape.

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Leonard Migliore

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