Two new correctors are needed to accommodate the wide field of the 8K x 8K CCD Mosaic (see accompanying article). The 0.9-m corrector, a relatively simple doublet of fused silica, is now complete and has been tested successfully on the telescope. The 4-m corrector is more ambitious and complex and should be nearing completion as you read this. This article provides a brief overview of the features of these optical systems, both designed by Ming Liang (NOAO).
The Mosaic field of view at the 0.9-m telescope will be 59' x 59' with 0.43" pixels, compared to the existing imager having a field of view of 23' x 23' with 0.68" pixels. The corrector must accommodate the diagonal of 83'. Predicted spot sizes are much smaller than a pixel over the entire field, and actual imaging tests with a "small" CCD (the T2KA 2048 x 2048) placed off-axis confirm that there is no significant image degradation from the corrector.
Special attention was given to broad band optical coatings to ensure that the four glass-air interfaces do not adversely impact the telescope efficiency. The existing small field corrector introduces a 35% loss in the U band--we wanted to recover that deficiency with the new corrector. We contracted to Continental Optics for a coating that appears to work well; preliminary indications show the surfaces to be nearly lossless in the U and B bands.
Low surface brightness workers will enjoy the low scattering properties of the system. We saw no evidence of any scattering except close to saturated bright stars. We are working to reduce vignetting introduced by the telescope baffles, but we place the low scattered light properties at a higher priority.
The 4-m corrector layout was motivated by the atmospheric dispersion compensated (ADC) optical design by Richard Bingham (UCL) for the CTIO 4-m. Changes over the CTIO design were made to improve U-band transmission, provide a wider field, and reduce weight and cost. We expect all optical elements to be completed shortly and placed into the 4-m prime focus over the summer shutdown.
The new corrector is designed to be used with the Mosaic to provide a field of view of 36' x 36' (diagonal of 51') with 0.26" per pixel. The corrector also works with the standard T2KB imager to offer superior image quality, even over the small field of T2KB, and an ADC to enhance the image quality, especially in U and B at zenith distances greater than 45 degrees.
Again, the new corrector design is intended to protect the U band transmission and will use a broad band coating for the fused silica elements. We are investigating options for special coatings on the UBK7 and LLF6 glasses as well. As with the 0.9-m corrector, attention is being given to minimizing scattering, and like the 0.9-m corrector, we expect the 4-m corrector to be excellent, too.
George Jacoby, Taft Armandroff,
Todd Boroson, David Vaughnn, Ming Liang