Significant improvements in image quality, and resulting sensitivity, at both optical and infrared wavelengths can be achieved by first order wavefront stabilization. When implemented in hardware with high speed, small amplitude motion of an optical element, this is known as tip-tilt correction. Tip-tilt retrofits are improving the performance of many existing large telescopes. Under the leadership of Richard Elston, the Blanco 4-m has now joined the club.
A complete tip-tilt system requires a moving optic, a guiding sensor, servo system, and an appropriate science instrument. Fortuitously, we had available a rough-ground secondary and complete cell and mechanical structure, originally built 25 years ago for a 4-m f/13 widefield Cass focus but never implemented. Gary Poczulp of the Tucson optical shop trimmed, lightweighted, and figured the glass for IR use at f/14. The La Serena engineering group reworked the cell and mount, using commercial piezoelectric transducers to provide fully controlled motions of order an arcsecond at tens of Hertz. At the focal plane, we have built a fast guider interface, which uses a dichroic to feed both a science instrument and a CCD TV. The sampling area of the CCD is limited to a few arcseconds and provides centroid information from a guide star at up to 240 Hz. Mounted on an extremely rigid XY stage, it may access a suitable guide star anywhere within a 100 x 100 mm field.
For the initial science instrument, the IR imager COB has been permanently relocated from KPNO to CTIO. The KPNO IR group had previously modified it to provide 0.1" per pixel over the field of a 10242 Aladdin InSb array. During July and August 1996, Tucson engineers installed a 5122 Aladdin quadrant and a new preamplifier. Ron Probst and Esteban Parkes (from the Tololo electronics group) spent September in Tucson participating in the final stages of installation and testing. Following successful laboratory and telescope tests, the instrument was crated and shipped to CTIO. This is the largest instrument we have ever transported between sites; the crate had to be carefully specified not to exceed the size capacity of airplane cargo hatches!
A significant milestone was achieved last 18 January on the Blanco telescope. We closed the tip-tilt guiding loop between secondary and sensor, providing stabilized images at 2µm to COB. Further engineering tests and usage over several nights were encouraging. Power spectra of the centroid error signals are flat beyond 10 Hz, indicating that all low frequency sources of image motion are compensated - seeing, wind shake, servo errors, etc. Flexure tests show that the system, including optical coupling to a science sensor deep inside its large dewar, is adequately rigid. Centroid stabilization shows a broad maximum in performance with respect to system gain, and useful correction is achieved at frequencies an order of magnitude less than the system capability. Unfortunately a low quality folding flat, pressed into service due to delayed delivery of the final mirror, degraded the delivered image quality on the IR array. However, image cores were still significantly tightened compared to non-tip-tilt corrected images. Finally - and very important for a facility instrument - the mechanical hardware, electronics, and software were robust when used by astronomers, not engineers.
Over the next several months we will be further testing and refining the system to improve sensitivity, defining its performance, and developing a user interface for the telescope operators. In addition to IR imaging with COB, we will be using the system for IR spectroscopy with the IRS beginning in June 1997 (see separate article). In the longer term, we will upgrade the IR array in COB to a full 10242 Aladdin array, providing 100" x 100" of corrected field of view, as soon as a suitable device is provided by the Tucson array development effort. Users of CTIO optical instruments may eventually benefit as well. The tip-tilt guider box is designed for easy, rapid switching of dichroics, which could enable use of the system for CCD imaging with appropriate blue/red dichroic splits.
Nick Buchholz, Richard Elston, John Filhaber, Brooke Gregory,
Paul McIntyre, Gabriel Perez, Ron Probst, Nic Roddier,
Ricardo Schmidt, German Schumacher, and a host of colleagues
in La Serena and Tucson