Winter '96 Shutdown of the 4-m Blanco Telescope

A major shutdown of the 4-m Blanco Telescope is scheduled for this (southern hemisphere) winter to carry out the first phase of a program to upgrade the telescope drives and associated systems. It is hoped that this work will result in significant improvements in the tracking and guiding accuracy of the telescope that will yield an immediate payback in terms of improved image quality. By replacing major parts of the telescope's more than 20-year-old control logic, we also expect to reap benefits in terms of improved reliability, less downtime, and reduced maintenance costs. The shutdown will occupy a total of 11 weeks from 24 June to 6 September. During this period we will do the following:

• Implement measures to improve the tracking and guiding performance of the telescope. As described elsewhere in this Newsletter, while the telescope has delivered images as good as 0.5" in short, V-band frames, longer guided exposures taken on the same night had FWHM no better than 0.7". Seeing measurements taken on a large number of nights (which include the effects of tracking and guiding) show that the best images obtained in routine operation are also around 0.7" FWHM. This and other evidence convinces us that the telescope's closed-loop tracking performance now sets the floor on the best image quality that can be obtained. Quite apart from the degradation of the current image quality, this problem, if not corrected, will also limit the usefulness of the f/14 tip-tilt system. Correction of tracking errors would consume a large fraction of that system's bandwidth, leaving little headroom for following atmospheric effects. Prior to and during the first week of the shutdown we will be gathering the information needed to understand this problem in detail and correct it. The steps to be taken include a complete revision of the hardware and software used to generate the telescope drive pulses, and a complete re-tune of the servos and guider. Our goal is to obtain the very best performance that can be achieved with the present servo hardware. Replacement of the servo system in its entirety is a longer term activity slated for a future project.

• 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. It is primarily this activity that dictates the duration of the shutdown, since the telescope will be completely immobile while this work is carried out.

• Install new, more reliable, absolute encoders.

• Implement computer control of the instrument rotator. This will benefit spectroscopists by allowing the spectrograph slit to be set to the parallactic angle or parallel to some feature in an extended object, while the telescope is slewing to the target position. The guide probe will also be moved to the position of a pre-selected guide star at the same time. Currently these operations are performed manually reducing observing efficiency.

• Carry out "normal" maintenance activities such as realuminizing the mirrors, recollimating the optics, generating new AO look up tables and pointing maps, etc. We will also carry out preventive maintenance activities, such as inspecting the gear box for the shutter drive to forestall failures of the kind recently suffered by the Mayall Telescope.

We had planned earlier to repair the declination gearbox during this shutdown. However, the cost of this repair was found to be prohibitive, and we are now planning for a more cost-effective "software solution" to the problems caused by the damaged gears. Budget and manpower constraints have also forced us to lengthen the schedule for complete replacement of the servo system. As a result we are now looking at more modern, commercially available servo systems such as the Delta-Tau controllers proposed for the Gemini telescopes, rather than copying the upgraded KPNO servos.

Steve Heathcote