As a necessary adjunct to the CTIO Hydra project, a new corrector was constructed for the Ritchey-Chretien focus of the Blanco 4-m Telescope. It is now functioning to specifications. Without a corrector, a telescope of this type has a strongly curved field and the image quality at the edge of the field is poor.
The new corrector was designed by Charles Harmer (NOAO Tucson) and built in the NOAO Optical Shop. The mechanical mount and control system were designed and fabricated at CTIO. The Harmer corrector reduces the curvature of the field to 8.7m radius and makes the telescope pupil lie at the center of curvature, optimal for use with optical fibers. With a perfectly stable atmosphere, it should be capable of producing images smaller than 0.5" in size everywhere in the field at all wavelengths between 330nm-1.5µm. Under real conditions the images will almost always be limited by seeing. So far, the smallest images seen have been 0.75" FWHM. There is no significant variation in image quality over the field.
Fabrication, testing and quality control of the corrector were excellent. The field distortion modeling parameters produced from the "as built" measurements were so accurate that Hydra was able to position its fibers to sub-arcsecond precision without any tweaking the first time it was installed.
The Harmer corrector incorporates Atmospheric Dispersion Compensation (ADC), which makes it especially useful for broad band spectroscopy. It consists of two curved elements of fused silica, which perform the basic aberration correction, and a pair of flat, rotating zero deviation prisms in between providing the ADC function. When the dispersions of the prisms are pointed in opposite directions, their net effect is zero and the system behaves as a conventional corrector. By rotating the dispersion vectors such that the two prisms add partially or entirely in the proper direction, an artificial dispersion of any magnitude and position angle can be generated to cancel the atmospheric dispersion. The corrector automatically brings the light from all wavelengths within the design range to a single focus anywhere in the sky up to a zenith angle of slightly over 70º (air mass 3). Its effect can be best seen with the Hydra gripper camera using a filter which transmits only UV and IR. At high air masses, one sees two well-separated images when the corrector is set in the neutral position. When the ADC is switched on, the images merge dramatically and gracefully into a single round spot anywhere in the sky.
Harmer's design is unique and ingenious in that the prisms are made from cemented fused silica and a light flint glass instead of two dissimilar glasses. The elements are made with non-parallel surfaces in such a way that they give zero deviation. Because four of the six elements are then made of silica, the UV throughput is good. All eight surfaces are coated with UV optimized sol-gel over MgF2, making the corrector's transmission nearly 100% at all wavelengths from 1µm to 380nm, falling gradually to approximately 70% at 350nm and 50% at 340nm. There is little useful transmission below 330nm.
The Harmer corrector is now a permanently installed component of the Blanco Telescope. It can be used with any Cass Focus instrument, not just with Hydra. For example, when used with the R-C spectrograph, it will permit efficient broad band long slit spectra to be taken at arbitrary zenith angles and high air masses. The entire corrector assembly is motorized and can be flipped out of the beam in less than a minute to allow observations with other instruments or farther into the UV or IR.
Tom Ingerson (tingerson@noao.edu)