Abstract
The generation of Extremely Large Telescopes (ELTs) with mirror diameters up to 40 m has thick secondary mirror support structures (also known as spider legs), which cause difficulties in the wavefront reconstruction process. These spider legs create areas where the information of the phase is disconnected on the wavefront sensor detector, leading to pupil fragmentation and a loss of data on selected subapertures. The effects on wavefront reconstruction are differential pistons between segmented areas, leading to poor wavefront reconstruction. The resulting errors make the majority of existing control algorithms unfeasible for telescope systems having spider legs incorporated. A solution, named the split approach, is presented, which suggests to separate reconstruction of segment piston modes from the rest of the wavefront. Further, two methods are introduced for the direct reconstruction of the segment pistons. Due to the separate handling of the piston offsets on the segments, the split approach makes any of the existing phase reconstruction algorithms developed for nonsegmented pupils suitable for wavefront control in the presence of telescope spiders. We present end-to-end simulation results showing accurate, stable, and extremely fast wavefront reconstruction for the first light instrument mid-infrared ELT imager and spectograph of the ELT that is currently under construction.
Highlights
The pupils on the Extremely Large Telescope (ELT)-generation telescopes are inevitably segmented and partially shaded by thick support structures due to their large sizes
We introduce two methods for direct segment piston reconstruction denoted by Π providing the piston information on the disjoint segments ðpiÞi1⁄41;2;: : : k divided by k spider legs
Each table demonstrates the results obtained with the preprocessed cumulative reconstructor with domain decomposition (P-CuReD) for a spider-free simulation as benchmark and the spider effected results obtained within the split approach using each of the direct segment piston reconstructor (DSPR)
Summary
The pupils on the Extremely Large Telescope (ELT)-generation telescopes are inevitably segmented and partially shaded by thick support structures due to their large sizes. In contrast to the SH sensor, the pyramid sensor does have a footprint of the segmented piston in its data (see Sec. 5.2.2 and Fig. 5). This suggests that the reconstruction of such modes is possible, if the reconstructor is able to extract this information from the sensor data and efficiently use it
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