A 4-layer protected silver coating was chosen for the Gemini telescopes due to its high reflectivity in visible and infrared wavelengths. Due to the reflective and absorptive properties of silver, the Gemini 4-layer coating suffers from poor reflectivity in wavelengths shorter than 400 nm. By utilizing different dielectric materials for protective layers it is possible to enhance the reflectance of silver to 350 nm, without compromising infrared performance or coating durability. Several different recipes were found that achieve a goal reflectivity of 80% at 350 nm and 90% at 400 nm. Five recipes were chosen for testing based on their modeled UV enhancement, the protective properties of their materials, and their simplicity. These recipes consist of variations of aluminum oxide, as well as silicon dioxide or silicon nitride. The aluminum oxide replaces the nickel chromium interlayer in the current Gemini coating and provides a receptive surface for the other protective layers. These materials were chosen because they can be produced in the Gemini coating facility without requiring significant and costly upgrades of the sputtering cathodes.
A new technique has been developed to collimate the Gemini telescopes using the Peripheral Wavefront Sensors (PWFS) to measure focal plane offset and tilt. For several years prior to 2014, observers at Gemini North noticed a variation in the focus Zernike term of about ±30 μm when guiding with the PWFS. It was speculated that variation was due to a tilt of the PWFS rotary table. Further testing revealed that it was actually due to an incorrect tilt of the secondary mirror (M2), causing the focal plane to be offset and tilted relative to the PWFS axis. Due to the Ritchey- Chrétien design of the telescopes there is no Seidel comatic field pattern typical of an aligned telescope. Instead a constant comatic field pattern occurs from either tilt or decenter of M2, and patterns arising from tilt can be eliminated with the appropriate decenter. For the Gemini telescopes, proper alignment is not guaranteed from a zero-coma condition. The new technique measures PWFS focus variation around the periphery of the imaging field, 6 arcminutes off-axis, by programming the telescope pointing to move in a circle while PWFS tracks a guide star, completing a full circle. The measured focus variation is then used to calculate M2 tilt. The tilt and decenter offset are then adjusted to zero both focus variation and coma and achieve collimation. The technique permits correction of the erroneous M2 tilt to <~ 30 arcseconds, corresponding to a wavefront error <~ 3 μm, but is limited by short-period focus variations.
Since 2004 the Gemini telescopes have used a protected 4-layer silver coating on their 8-meter diameter primary mirror and other smaller optics. Protected silver was chosen for the twin telescopes due to its high reflectivity and low emissivity properties. For over 10 years the protected 4-layer silver coating at Gemini exceeded the science requirements for reflectivity of 88% between 0.4-0.7 μm and 84% between 0.7-1.1 μm. Initial durability requirements that the coating should last at least two years have been also been surpassed. All mirrors have met the durability requirement, with most outlasting it significantly. Provided is a ten year retrospective on the progress in the use and maintenance of 4-layer silver coatings on large astronomical optics.