During the last 6 months, work has been done on the design of a lens-based Pier Optical Path System. It will relay the light beam from the focal plane to the instrument room, 30 metres below.
Introduction. The European Solar Telescope (EST) will have an onaxis Gregorian configuration with an entrance pupil diameter of 4.2 metres. The telescope aims to observe the Sun with unprecedented spatial and spectral resolution. Following standard practice, the main telescope will be placed on top of a high building, to keep the telescope aperture away from image distortions generated by the ground heat. At the same time, the scientific instruments will be installed at the base of the building where there is more room to accommodate them. The base of the telescope has more stability, so vibrations and local seeing degradation are smaller there. In the case of EST, the vertical distance between the telescope mount and the Coudé room containing the scientific instruments is around 30 metres. This long distance requires a complementary optical system to transfer the focal plane from the top of the building to the scientific laboratory at the bottom of the structure.
The current version of this EST Pier Optical Path (POP) system uses a lensbased relay consisting of a collimatorcamera set up transferring the F2 Gregorian focus to a new F3 science focus at the instrument location (see the conceptual design in Figure 1). The option of a mirror-based system was evaluated but discarded due to mechanical and space limitations; the required off-axis configuration and apertures of the mirrors are not feasible.
The main challenge of a lens-based POP system is to overcome the chromatic aberration introduced by the optical glass. This phenomenon implies that lenses produce an image plane whose spatial location varies with wavelength. In order words, a given instrument can be focused for a certain wavelength range but defocused for other spectral ranges. Thus, the EST POP system is being designed to deliver diffractionlimit performance over the entire wavelength range to be observed with the telescope.
Figure 1. EST optical layout including the current version of the POP system. Distances between optical elements as well as their sizes are not to scale.
System requirements. The design of the optical system should fulfill the requirements presented in the EST Science Requirement Document (SRD). The most relevant related to the POP system are: i) the working spectral range goes from 380 nm to 2200 nm, ii) the image quality shall be limited by diffraction over a field of view (FoV) of 90” x 90”, and iii) the POP system shall be optimised to deliver the highest possible throughput (in order to reach outstanding polarimetric accuracy at the highest spatial resolution and be able to follow the fast evolution of solar phenomena), which calls for a minimum number of optical surfaces in the system.
Pier Optical Path design. The original concept of the POP design was an optical system composed of a set of two achromatic doublets, as shown in Figure 1. Several glass combinations were examined and analysed by means of simulations and analytical methods. Their performance over the working spectral range was compared and studied. Nevertheless, owing to the great extension of the spectral range, none of the examined glass combinations was able to produce a residual chromatic aberration that guaranteed the required diffractionlimited performance.
Those results prompted us to think of an alternative design to compensate the strong chromatic aberration. Having in mind that the current lightdistribution system divides the light coming from the telescope into four different optical instrument arms (as described in the the previous issue of the newsletter), we wondered whether one of those spectral divisions could be done inside the POP system. By splitting the beam after the collimation stage with a dichroic beamsplitter, we would introduce an extra camera doublet, but each doublet would deal with a significantly reduced spectrum. The division was made at 780 nm so that one camera doublet operates in the red and near-infrared range and the other in the blue and visible range. Further analyses were carried out seeking the best glass combinations for both lenses, and a noticeable improvement in the performance was achieved. Nonetheless, the capability of this design was still slightly out of the SRD image resolution requirements and we considered substituting the collimator doublet by a triplet. The results obtained with this triplet-based collimator coupled with two doublet camera lenses via a beamsplitter are highly satisfactory and predict that the POP system shown in Figure 2 should achieve the required diffraction-limited performance for the entire EST FoV and the entire spectral range.
We will now conduct an analysis of optical glasses and their combination in chromatically-corrected optical systems, taking into account material hardness, thermal properties, and birefringence. Detailed ray-tracing simulations will also be carried out with Zeemax to verify the performance of the design.
Figure 2. EST optical layout including the new design of the POP system.