Reflectance of Aluminum Overcoated with MgF_2 and LiF in the Wavelength Region from 1600 Å to 300 Å at Various Angles of Incidence

Abstract

Since the discovery that Al overcoated with MgF(2) or LiF produces high reflectances to wavelengths as short as 1150 A and 1000 A, respectively, these coatings have been used extensively in vacuum ultraviolet instruments in the wavelength region where their reflectance is high. If the instrument is intended to cover wavelengths shorter than the two given above, usually either Pt or Ir is used, with a loss of speed at the longer wavelengths. This paper presents reflectance data showing that fluoride-overcoated Al can be useful to wavelengths as short as 500 A. Measurements were made from 1600 A to about 300 A at normal, 35 degrees , and 85 degrees angles of incidence, angles used in normal, Seya, and grazing incidence spectrometers, respectively. These measurements show that from the boundary of the high reflectance region to 500 A, the reflectance at normal and 35 degrees depends on the thickness of the fluoride coating and can be as high as 24% at 800 A for a MgF(2) thickness of 150 A. For shorter wavelengths, the reflectance shows a decreasing thickness dependence and at 304 A is very low-about 1%. At grazing incidence, the reflectance shows some thickness dependence from 1500 A to about 1000 A, but toward shorter wavelengths the dependence disappears and the reflectance increases slowly to about 80% at 500 A. In addition to the reflectance measurements, polarization effects are discussed.

Far-Ultraviolet Sensitivity of the Cosmic Origins Spectrograph

“…We use the component efficiencies displayed in Figure 3. The triangles are the measured MgF 2 /Al reflectance (250 Å of MgF 2 ) from Hunter et al (1971). The squares are the COS Segment B detected quantum efficiencies measured by the instrument team (Vallerga et al 2001) for the CsI-coated detector.…”

“…The over plotted statistical error bars are also small. G140L absolute efficiency measurements were not available for the full far-UV wavelength range, so we used the available measurements to constrain a model computed as the product of the MgF 2 /Al reflectance and a groove efficiency for a trapezoidal-shaped Hunter et al (1971). No error bars were cited by those authors.…”

“…From the inception of the Cosmic Origins Spectrograph (COS) (Morse et al 1998;Green et al 2003) as an instrument for the Hubble Space Telescope (HST), it was appreciated that the combination of a single bounce reflection grating with the large geometric collecting area (∼40,000 cm 2 ) and the ∼15% reflectivity at 1000 Å of the magnesium fluoride over-coated aluminum (MgF 2 /Al) mirrors (Hunter et al 1971) could provide useful sensitivity in the far-UV bandpass between the rather sharp MgF 2 /Al discontinuity at ∼1150 Å and the Lyman edge at 912 Å. The possibility that COS could work below 1150 Å was intriguing because of the potential for new science.…”

The Apollo 17 far ultraviolet spectrometer experiment

“…The design of The Ebert mirror and the diffraction grating are coated in vacuum with Al and overcoated'with Mg F 2 (Hunter et al 1969) to provide a high optical efficiency.…”

Comparison of different strategies to realize highly reflective thin film coatings at 1064nm

“…To prevent metallic coatings from oxidization and maintain mechanical durability, these coatings usually have dielectric overcoat [6,7]. Protected metallic coatings are not sensitive to wavelength, angle of incidence and polarization, and they are useful for a wide range of low power monochromatic and polychromatic applications.…”