CCD window specifications (Vik Dhillon, Feb 2001)





The CCD head will require a window to allow light through to the chip whilst retaining the vacuum seal of the camera head. Because the window is so close the focal plane, the surface flatness and parellelism of the substrate is not as important as the surface roughness (characterised by the S/D number). We were about to order these windows from CVI Laser corporation (product code: PW1-2025-UV), via their UK distributors Elliot Scientific (the specifications can be found here). The cost of three such uncoated windows is £357+VAT (uncoated). However, Specac offered us six such windows as part of our lens contract at no extra cost. The specifications of the Specac windows are slightly lower - as listed below - but they are still acceptable and they are free, so we have decided to go with them. The advantage of having three spare windows is that we can use them in place of the filters to obtain the highest throughput to each of the arms, without having to refocus the instrument.

The internal transmission of UV-grade fused silica is greater than 99.9% across the entire region 300-1100nm. These losses are in addition to reflection losses, of course, which we will minimize by coating the windows with the same broadband AR coating used for the lenses at the same time as the rest of our optics (CVI are happy to do this).

Using the window as part of a vacuum seal means that the window will flex slightly due to the difference in pressure on its two sides. This flexure effectively turns the window into a meniscus lens. We have calculated the flexure under these conditions and find it is 0.5 microns for a 5mm thick window of diameter 50mm. We have calculated that such an enormous radius of curvature gives a negligible change in the effective focal length of the camera and hence will have a negligible effect on the image quality.