Pier Optical Path
The Pier Optical Path (POP) transfers the light from an F/12.65 beam at F2 to a telecentric F/50 beam at F3, the scientific focal planes in the Coudé rooms, where the the Scientific Instrumentation Suite (SIS) and the Coudé Sensing Assembly (CSA) are located.
The POP is an on-axis, fully spherical refractive system housed within the pier in a vacuum vessel to prevent local air turbulence along the optical path down to F3 (see Figure 1).
Figure 1. Layout of the Pier Optical Path with the various optical elements forming it.
Several optical design options based on one, two, and three magnification stages were evaluated before finalising the design. Constraints such as the limited vacuum vessel space, available optical path length, and the sizes of the optical elements led to the selection of a two-stage magnification relay with a field lens that limits the diametre of the final stage (camera doublets).
The first magnification stage consists of an achromatic F/75 triplet, covering the entire spectral range and serving as the entrance window to the vacuum vessel. After this stage, a long-pass dichroic filter with a 680 nm cutoff wavelength, tilted at 19°, splits the optical path. This beam splitting helps minimise the number of optical elements needed to manage chromatic focal shift (and, in addition, facilitates the distribution of the SIS across different the different Coudé rooms, organised by spectral range). The infrared (IR) beam is transmitted by the dichroic filter, while the visible (VIS) beam is reflected. An intermediate pupil plane with a diametre of 120 mm is generated in both beams. A deformable mirror (M7) is located in the pupil plane of the visible arm. This deformable mirror plays a key role in mitigating the variation of the wavefront distortion with wavelengths due to atmospheric turbulence. This effect is particularly pronounced during early morning observations when atmospheric conditions may lead to noticeable distortions. By adjusting its surface in real-time, M7 compensates for these distortions, improving image quality. Like the dichroic filter, M7 is tilted by 19°.
Each arm includes its own field lens, a highly transmissive singlet across all wavelengths, placed near the intermediate focal plane but far enough to prevent image degradation from dust or imperfections on the lens surface. Finally, each spectral arm features an achromatic doublet (referred to as camera VIS and IR doublet) that focuses the beam at each Coudé room, delivering a diffraction-limited focal plane. The second lens of each doublet acts as the exit window of the vacuum vessel.
The POP has been designed to minimise chromatic focal shift across the working spectral range, reducing the need for instrument refocusing when changing the observed spectral line.