Torr Scientific offer high-flatness fused silica viewports in a range of sizes, ranging from λ/10 to λ/20 @ 632.8nm, using a bonding process as an alternative to vacuum brazing.
Vacuum brazing viewports at high temperatures creates stress in the optic during the cooling down process. This stress is caused by the different expansion / contraction rates of the optical material and the metal carrier. Materials are chosen to try and match their expansion coefficients as much as possible, but there are limits on what can be used.
The stress causes the optic to change shape in a variety of ways, which affects the overall flatness of the optic. The overall effect on the transmitted wavefront error (TWE) is not as bad as expected as the parallelism is preserved between the two optic faces.
Torr Scientific offer bonded viewports as part of their standard product range. The optics are sealed into the flange assembly at room temperature, so there is no residual stress occurring as part of the sealing process. The optic retains the original flatness as manufactured.
The downside to this bonding process is a maximum bake-out temperature of +120°C compared to the +200°C for a brazed equivalent.
It should also be noted that polishing fused silica optics to a very high flatness requires thicker optics in the case of the λ/20 versions, particularly with the smaller optics CF16, CF38, & CF64 (1.33″, 2.75″, 4.5″ Conflats).