Coatings were deposited within a circular aperture to mm of the edges of the square blank. The filter was easily accommodated inside an existing UKST plateholder with only minor modification. The filter is actually 1 mm closer to the focal surface than existing filters, maximising image quality. Results integrated from 2732 points measured over the clear aperture using a He-Ne laser at 6330 with aperture size of 51 mm. A measure of the number of minor surface imperfections per unit area.
Any sag is in the same sense as the existing curvature of the focal surface.
Barr Associates have not provided any details but indicate that any minor sagging present will not create any problems. Certainly the image quality across the entire filter surface is excellent. The interference lines are quite parallel with little distortion. The H focal surface filter mounted in a dedicated plateholder only becomes uncovered inside the protected telescope tube of the UKST and is thus less prone to external environmental effects such as high humidity since dry nitrogen is purged over both emulsion and filter at the focal surface. Off the telescope the filter is stored in a dry inert atmosphere which affords further protection ensuring filter longevity.