The optical design for the E-ELT is that of a folded three-mirror anastigmat, folding being provided by two flat mirrors
sending the beam to either Nasmyth foci along the elevation axis of the telescope (see Figure 1).
The optics are mounted on an altitude azimuth telescope
The present concept
features as a baseline for the primary mirror (M1) an elliptical f/0.93 segmented mirror of 39-m diameter and a 11.1-m central obstruction.
The 4.2-m secondary mirror (M2) is convex and
returns the beam, through a hole in the quaternary mirror (M4), to the
3.8-m mildly aspheric concave tertiary mirror (M3) located at the vertex of
the primary. The beam is reflected by M3 to the adaptive optics system, the 2380x2340mm quaternary flat
adaptive mirror M4, supported by
up to 8000 actuators, and the fifth mirror in the train (M5) that allows for the final image correction.
M4 is inclined at 7.75 degrees to the beam
direction. M5 is a flat mirror, elliptical in
contour, defines the altitude axis of the telescope and steers the
beam towards the Nasmyth focus. The output beam at f/17.48 is very
nearly diffraction limited over the entire 10-arcminute field of view.
The total Nasmyth field of view is limited to 10 arcmin by the dimensions of the way-through hole in M4.
The rather large Nasmyth focal ratio is constrained by the backfocal distance, and the location and size of mirrors M4 to M5.
Additionally, the design allows for the f/17.48 beam to be redirected through
relay optics to a Coudé focus (see Figure 2) envisaged within telescope foundations at
the ground level. The need for a Coudé focus is driven by the narrow field high-resolution ultra stable spectrograph top level
requirement. While this is not an absolute
requirement and alternate solutions may be feasible for such an
instrument, all necessary provisions have been included in
the telescope design for implementing a Coudé train.