Ultraviolet Spectrograph Lens

AbstractA 700 -mm f/4.7 spectrograph camera lens was designed for imaging spectral lines in the200- to 400 -nm region on a 120 -mm flat image field. Lens elements of fused silica andcrystalline calcium fluoride have so little secondary spectrum that raytracing calculationspredict a monochromatic resolution limit of 30 lines /mm without refocusing in the 238- to 365 -nm region. Light scattering at the polished calcium -fluoride surfaces is avoided bysandwiching the fluoride elements between fused silica and cementing with silicone fluid.The constructed lens makes good spectrograms.IntroductionThe ultraviolet spectrograph lens to be described was designed to re$lace the visible -light camera lens in the Los Alamos MOD -40 Sweeping Image Spectrograph, which sweeps veryshort spectral lines along the film to get data for the study of rapidly changing events.The problem was to design a lens that would focus light in the 200- to 400 -nm region witha resolution of about 15 lines /mm and good contrast throughout the 10- degree image fieldneeded for recording on a 102- by 127 -mm photographic plate. For wide ranging surveys, thecorrection of longitudinal chromatism should be so complete that, without refocusing, thedesired resolution would be obtained for all wavelengths at all parts of the image field.However, complete adherence to this difficult performance specification is not essentialbecause recording at the optimum focus is usual when studying a particular spectral line.Fortunately, the lens- design calculations indicate that most of the desired performancewill be obtained without refocusing. The first test films show sharp recordings of afew mercury lines, thus leading to the expectation that the desired performance will beachieved. Because quantitative evaluations are not yet available, we can only say thatperformance of the new lens seems to exceed the 10- line /mm photographic resolution thatwas obtained with the visible -light lens, which was a quadruplet made with ordinary opticalglasses for imaging in the 400- to 700 -nm region.DiscussionThere were several problems to be overcome before a successful special -purpose ultravioletlens could be constructed. The first problem was the discovery of optical glasses with goodlight transmission when a 200 -mm thickness is used in the 200- to 400 -nm spectral region.The large thickness is needed for construction of the large camera lens, an objective lens,and an explosion -proof window. The glasses must have dispersion characteristics suitablefor use as crown and flint glasses. Fortunately, these glasses had already been discoveredby earlier workers. Large artificially -grown single crystals of optical -grade calciumfluoride are available for use as the crown elements. Ultraviolet -grade optical fusedsilica is available for use as flint elements and windows.The second problem is the relatively poor polish taken by the soft crystalline calciumfluoride. When this lens design was initiated in 1977, the residual roughness of the bestavailable polished surface scattered enough light to degrade the image contrast an intoler-able amount. To overcome this difficulty, the fluoride elements were cemented betweenfused silica elements. To eliminate the light scattering by surface roughness, the opticalcoupling compound used as a cementing agent must have a refractive index close to that ofcalcium fluoride. Also, it must transmit the light efficiently and remain pliable duringthermal expansion changes. The lengthy search for a suitable cement turned up siliconefluid (20,000 centistoke viscosity), which has very good transmission to 200 nm when usedin thin films, is quite inert, has a very low evaporation rate at room temperature, andfreezes at a low temperature. Although the polish now available on calcium fluoride is muchbetter than formerly, lens redesign is not justified because it is not known how much thelight scattered at the air -fluoride surfaces will degrade image contrast.The third problem is the procurement of an antireflection coating that has low reflectionand high transmission characteristics in the 200- to 400 -nm region. To obtain good durabil-ity and wide -band antireflection characteristics, a multilayer coating is probably needed.Unfortunately, an effective antireflection coating for such a wide region does not seem tobe available. However, the uncoated lens makes satisfactory images.