The glass windows are made from the best quality BK7. These material is very homogeneous and free from internal defects. The windows are precision polished on both surfaces and help parallel to 3arcmin. λ/10 transmitted wavefront distortion is usually preferred but λ/4 is offered as an option when this is not an issue. Ar coatings can applied to these windows .
　

Material: BK7 grade A
Refractive index: 1.52 @500nm
Diameter: +0 -0.2mm
Substrate flatness: λ/10 or λ/4
Parallelism: 5arcses or 3arcmin
Bevel: 0.2mm X 45º
Surface Quality: 40-20
Both side polished
Square or Rectangle is available