Broad-band Imagers

The broad-band imagers (BBI) are responsible for photospheric and chromospheric observations at selected (non-tunable) continuum wavelengths and spectral line cores. Some of the main requirements that have led to the proposed design are:

• Different resolution modes: a high resolution mode (1 x 1 arcmin², 0.015 arcsec/pix, 4k x 4k detectors), to exploit the diffraction limited quality of the telescope+MCAO system, and a large field of view mode (2 x 2 arcmin², 0.03 arcsec/pix, 4k x 4k detectors), to exploit the full telescope field of view, are requested. A single mode covering the full FoV at the maximum resolution is not feasible right now because either the detector and/or the filters are not manufacturable with the required size and properties. To ensure the maximum, the two modes will have to be available independently to the user. This way, the observer can decide the appropriate image scale on each channel. It may turn out that some channel is more useful at high resolution mode while another is most appropriate in maximum FoV mode, or any combination.
• Image reconstruction techniques: Even if no final decision about the post facto image reconstruction techniques has been taken, it may be foreseen that MOMFBD (Multi-Object Multi-Frame Blind Deconvolution) will be used for chromospheric filters and phase diversity for continuum filters. These solutions imply that for each chromospheric channel one will need a continuum channel for reference which, in turn, will be split into two (in and out of focus) for phase diversity reconstruction.
• Telecentric configuration: continuum windows will be used as reference for speckle reconstruction. To that aim, the aberrations of the narrow-band channel must be the same as those of the corresponding continuum channel (at each spatial point), implying that they need to share the same optics. This means that the filters have to be located after the beam separation, and given that this separation is done near the instrument focal plane, the filters must be located near the detector.

The BBI has three channels. Two channels are placed on the “blue” (λ ≤ 550 nm) arm of EST while the third one is on the “red” (λ ≥ 550 nm) arm. The blue and red channels are fed through beam splitters (BS2 and BS4) that are not part of the BBI but of the common path of the light distribution system. The number of channels has been defined on the basis of the observational programs that require simultaneous observations in different bands. These requirements have led also to the distribution of filters among the various channels (see Table).

Each of the three channels has two different resolution modes: a high resolution mode (small FoV) to exploit the diffraction-limited quality of the telescope + MCAO system and a large field of view mode to exploit the full telescope field of view.

Each channel is split into three sub-channels. The three sub-channels share the same optics to have the same aberrations. Out of the three sub–channels of each channel, the first one hosts narrow band filters for chromospheric observations, the second one hosts wide band filters used as reference for speckle reconstruction and photospheric observations and the third one generates an out-of-focus wide-band image for phase diversity reconstruction.