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Refurbishing the Drives on the Blanco 4-m Telescope (1Dec95) (from CTIO, NOAO Newsletter No. 44, December 1995) The next step in CTIO's program to modernize the Blanco telescope involves upgrading its drives and control software. This will be carried out during two extended shutdown periods in 1996 and 1997. The planned hardware changes should result in significantly improved pointing and offsetting accuracy. They will also lead to better open-loop and closed loop (guided) tracking performance, which is essential if we are to reap the fully benefits of the recent image quality improvements. Other major motivators for this effort are to improve reliability and maintenance. The present servo system, encoders, and sequencing logic are more than 20 years old. Thus, like a well-maintained but aging car, the Blanco telescope is beginning to suffer from creaks and rattles and the kind of idiosyncratic behavior that presages an increase in both down time and the effort that must be expended to keep it going. The changes to the telescope control software are aimed at improving efficiency in the operation of the telescope. For instance, we plan that a suitable guide star will be selected and the guide probe moved to the correct position while the telescope is slewing, eliminating the time currently lost in manually searching for guide stars. The same software will also allow the automatic selection of wavefront reference stars when working with the f/14 tip-tilt secondary. This will also make the control software for the Blanco and Mayall telescopes fully compatible, simplifying the planned interchange of instruments between North and South. In carrying out this project we will follow closely the steps taken at KPNO in the successful effort to upgrade the drives of the Mayall telescope. We thus economize on the engineering effort required and benefit from the experience gained at KPNO. We have already received considerable help from Scott Bulau and his team in the planning stages of this project. Nonetheless, completely replacing the drive electronics remains a major effort that will occupy a significant fraction of CTIO's resources over the next two years. It also inevitably involves shutting down the telescope for substantial periods so that the installation, testing, and tuning of the new system can take place. We are planning for two shutdown periods. The first will occupy the last six weeks of the fall 1996 semester and the first few weeks of the spring 1996 semester (mid-June through August 1996). The total duration of the shutdown will be about eleven weeks; as of mid-October the exact schedule is not yet defined. This first shutdown will be used to carry out the following activities: o Repair the declination gear box. Two of the gears in the declination drive suffered severe damage when a fragment of metal found its way into the gear box and got trapped between their teeth. The damage was repaired as well as possible during the last shutdown in August 1994. However, the damage still shows up as periodic spikes in the drive current (one every 2.5d) as the telescope is moved in declination. Generally, the present 4-m servos ride out these fluctuations, although occasionally they cause the declination brakes to trip in during a slew. The new servos we plan to install, however, have much higher frequency response, and are hence highly sensitive to such perturbations. Fixing this problem is thus a prerequisite if the new drive electronics are to function properly. Unfortunately, disassembly and reassembly of the gear box is a complex operation that requires the whole assembly to be returned to the US to carry out the repair. The time to ship the gearbox to and from the US, and to effect the repair, determines the duration of the shutdown. The exact time required is uncertain, subject to final negotiations with the manufacturer. o Replace the 20 year old relay-based "ladder logic," which provides for sequencing, enabling, interlocks, etc., for any activity performed on the telescope with the solid state equivalent--Programmable Logic Controllers. o Install new, more reliable, absolute encoders. o Carry out normal maintenance activities such as realuminising the mirrors. The second shutdown, of similar length, will be during the fall semester of 1997. By blocking out different months, we hope to spread the scientific impact of these closures on our user community. During this second period we will replace all the telescope drive electronics, including the servo controllers and drive amplifiers. Unfortunately, the activities in the first and second shutdown cannot occur in parallel. Most of the time during the second shutdown will be dedicated to tuning the servos, which requires that the telescope be moveable. Steve Heathcote, Mark Phillips
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