The WIYN Consortium has initiated the construction of the WIYN Tip-Tilt Module (WTTM), which is an optical-NIR reimaging system that utilizes fast tip-tilt compensation. The WTTM will replace the Calibration Illumination Assembly (CIA) in the WIYN Instrument Adapter System (IAS) as shown in Figure 1. The CIA functions are maintained by relocating it under the WIYN azimuth skirt and bringing its calibration light to the IAS via a fiber-optic. When complete, the WTTM will provide the IAS with a second tilt-compensated imaging port to complement the standard port feeding the Mini-Mosaic CCD camera. Figure 1 shows the IAS pick-off mirror in position to feed the WTTM. Because both imagers are independent and can run simultaneously, selection between the two can be accomplished quickly by simply moving the pick-off mirror. Following the pick-off mirror, the original WIYN focus is located at the input of the WTTM (Figure 2), still inside the IAS structure. WTTM optical design re-images this focus through a series of off-axis mirrors, allowing the WTTM to be used over a very wide wavelength range. With enhanced silver coatings, the WTTM maintains high efficiency from 0.4 µm through 1.7 µm.
The first mirror (M1) images the telescope pupil at the tip-tilt mirror and is followed by a two mirror (M2 and M3) camera. The science beam is fed off the first surface of the beam-splitter, where the FOV is 4' × 4' at a plate a scale of 0.12" per 15 µm pixel. The image quality produced over this field is seeing limited to below 0.3" at 0.67 µm. The quadrant error sensor is fed through the beam-splitter. Some of the astigmatism produced by passing through the beam-splitter is used to sense focus drift as seen by the WTTM. The improvement in image quality from tilt compensation alone should result in 0.1" improvement to FWHM during median seeing conditions at WIYN and 0.15" during the 10% best nights. By including focus drift compensation we expect the median and 10 percentile FWHM improvements are 0.15" and 0.25" respectively. With the WTTM users can expect to have stable images on the order of 0.7" FWHM under median conditions and 0.35" in 10 percentile case.
A substantial hurdle in the WIYN tip-tilt project was passed during this past quarter. On 19-20 March 1999 the WIYN Tip-Tilt Module (WTTM) project held its Preliminary Design Review (PDR) before a distinguished committee and other interested parties. The committee comprised Rene Racine (Chair), Lonnie Cole, Don McCarthy, Bob Parks, Harvey Richardson, and Steve Ridgway. A prompt written report by the committee was distributed to the WTTM team in early April. Briefly, the committee viewed the WTTM project favorably and urged its completion with "all possible dispatch." The report in its complements and criticisms has been and will continue to be a valuable resource as the WTTM design is finalized and the project moves into fabrication and testing. The WTTM team has produced a written response to specific issues raised by the PDR Committee and is ready for distribution at press time.
At the 9 April 1999 WIYN SAC meeting, WTTM Team leaders responded to the Committee's written report and questions from SAC members. The timely arrival of the written report prior to the SAC meeting resulted in prompt approval by the SAC for the project to move forward.
The WTTM is currently undergoing final design and detailing in preparation for cutting metal. To this end, a substantially more detailed finite element analysis model of the WTTM is being developed. This model is being used in our efforts to prevent flexure and resonance problems that have compromised other similar systems. The control system has undergone some minor rearranging as a result of some short comings noted at the PDR and is nearing its final design.
With the approval to proceed, the WTTM project has begun the process for procuring items with long lead time. The purchase order for the error sensor detectors, EG&G avalanche photodiode modules, has already gone out. The bid package for the WTTM optics is expected to be finished and released for competition by 15 May 1999. Recently the WTTM project took delivery of its computer system for developing the Linux-RT + LabView software system. In addition, the tip-tilt mirror subsystem is also expected to purchased in the next month or so. Other key milestones for the WTTM project are:
Place Order for WTTM Mirrors June 1999 Delivery of WTTM Mirrors September 1999 Error Sensor Assembly Complete March 2000 Optical Housing 1st Assembly March 2000 IAS Downtown for WTTM integration (6-8wks) May 2000 Testing at the Telescope November 2000 Shared Risk Observing April 2001
Chuck Claver, WTTM Project Scientist