Design of a large, low-scattered light telescope for solar observations

This paper is an interim report of a feasibility study which is in progress for a large 400 cm aperture solar telescope ("CLEAR"). Unlike other large solar telescopes constructed in the last three decades, CLEAR does not use the concept of evacuated telescopes to eliminate internal seeing. The requirement for full access to the far infrared spectral region (>2.5 mu m), and for low scattered light, eliminates the use of the entrance window which evacuated telescopes require. Instead, CLEAR avoids internal seeing by carefully controlling the internal thermal environment of the telescope by a number of means: (i) thermal control of the primary mirror; (ii) now of ambient air over the primary mirror surface and in the telescope; (iii) locating the primary focus outside the telescope beam and enclosure where the heating resulting in concentrated sunlight can be managed better (this requires the use of an off-axis primary mirror); and (iv) the use of a prime focus heat stop/absorber. In addition to controlling the internal seeing, such a configuration produces a telescope with very low scattered light characteristics, allowing quality observation of regions outside the solar limb and of sunspots. By eliminating the need for a large entrance window, the CLEAR concept therefore opens up the possibility of larger aperture solar telescopes. Notwithstanding its off-axis configuration, the Gregorian telescope produces excellent images (< 0.1 arcsec) over a 5 arcminute diameter field-of-view at the f/30 Gregorian focus. In addition to the four instrumentation stations near the Gregorian focus (i.e., direct Gregorian, Nasmyth, two "folded Gregorian"), the design provides for extensive instrumentation locations in a coude area. By means of a 3-level rotating coude platform, large instruments can be located at respectively f/30, f/45 and f/60 foci.