Optimized optical system with a 1.65 m wide-field corrector for the 6.5 m high-performance MUltiplexed Survey Telescope.

The development of modern large-scale spectroscopic survey telescopes responds to the urgent demand for spectral information in astronomical research. Tsinghua University has previously proposed a 6.5 m MUltiplexed Survey Telescope consisting of a Ritchey-Chretien configuration and a 1.8 m multi-element wide-field corrector, achieving excellent performance and world-leading survey efficiency. However, an optimized 1.65 m multi-element corrector with five lenses is proposed to overcome the constraints on glass uniformity and verification in fabrication of the previous corrector design. It maintains outstanding image quality, with the 80% enclosed energy diameter not more than 0.559 arcsec within 3° FoV over up to a 55° zenith angle. The optimized optical system does not revise the working mode of the ADC or the curvature of the primary mirror while ensuring the reasonability and accuracy of manufacturing of large corrector elements. It provides a more feasible reference optical design for the MUltiplexed Survey Telescope in subsequent iterations and communications with manufacturers.

A distortion of very-high-redshift galaxy number counts by gravitational lensing.

The observed number counts of high-redshift galaxy candidates have been used to build up a statistical description of star-forming activity at redshift z ≳ 7, when galaxies reionized the Universe. Standard models predict that a high incidence of gravitational lensing will probably distort measurements of flux and number of these earliest galaxies. The raw probability of this happening has been estimated to be ∼0.5 per cent (refs 11, 12), but can be larger owing to observational biases. Here we report that gravitational lensing is likely to dominate the observed properties of galaxies with redshifts of z ≳ 12, when the instrumental limiting magnitude is expected to be brighter than the characteristic magnitude of the galaxy sample. The number counts could be modified by an order of magnitude, with most galaxies being part of multiply imaged systems, located less than 1 arcsec from brighter foreground galaxies at z ≈ 2. This lens-induced association of high-redshift and foreground galaxies has perhaps already been observed among a sample of galaxy candidates identified at z ≈ 10.6. Future surveys will need to be designed to account for a significant gravitational lensing bias in high-redshift galaxy samples.