NAIC/AO Newsletter, July 97
This has been an extremely active and exciting period for Upgrade activities.
After completion of the cable tensioning in February, the platform is, by design, higher than required for proper focusing. In operation, it will be held at the proper height by the tiedown system, consisting of three jacks, each pulling on cables attached to the mast and boom at the end of an apex of the triangle structure. The jacks themselves were installed by the Facilities Department, and the electronics control units were put in place at the end of May by Electronics Department personnel. In relatively short order, the drives were tested, and the fiber optics communications link to the control room checked out. While it will still be some time before the system works under computer control, the platform was pulled down to approximately its correct height in time for a 430 MHz run early in June. There have been problems with the DC motors used to operate the jacks, but these have now been resolved. There is considerable work to be carried out calibrating the load cells and their associated amplifiers, which is necessary for accurate control of the platform, but this should be fitted in with other projects during the next couple of months.
Another major effort in June/early July was the first part of the commissioning of the Vertex drive systems for the carriage house and Gregorian (in elevation) and the telescope azimuth. These are all completely new electric drive systems. The actual motors, amplifiers, etc. have been in place for some time, thanks to work by NAIC staff, but the recent two week period was intended to iron out the bugs in the system, demonstrate its ability to make the various drives follow a commanded position input, and to have NAIC officially accept the systems.
A very complex system (in terms both of hardware and software) such as this was bound to have childhood problems. The Gregorian and azimuth drives each have 8 motors and associated electronics, while the carriage house has two sets. There is a control computer, a "control room" PC for monitoring, and of course the system also has to accept commands from the (NAIC) telescope control computer. All of these systems communicate by various fiber optics links. NAIC personnel have been finding various minor problems, and Vertex personnel started dealing with them immediately upon their arrival. They made considerable progress, although the detailed testing turned up a slew of other minor problems. The test period was also slowed by some problems with motor amplifiers, which had to be repaired. With all of this, the commissioning period ended with a great many problems solved, but a list of quite a few outstanding problems remaining. Because of these, we did not undertake a final acceptance procedure, and Vertex will be coming back to Arecibo in September both to deal with the "punch list" and to go through the final acceptance.
The excellent result of this part of the drive system commissioning effort was that the systems show terrific performance potential. The CH and Gregorian drive systems met their tracking specifications with no problems, and in fact the Carriage House exceeded its specifications by a large factor. The azimuth system just about met specifications, despite the fact that the azimuth rail has not been aligned, and the system was clearly suffering from variable loading, which would have a negative effect on tracking. We expect all axes to pass with flying colors in the final acceptance procedures in September.
The work of the major contractor, COMSAT/RSI has been completed on the site. They spent several months working on alignment of the azimuth rails, but were not able to reach an acceptable level of accuracy. They also indicated that they would prefer not to tackle the elevation alignment issues. In consequence, NAIC decided to terminate their efforts, since the alignment tasks did not appear to be making satisfactory progress. This was a considerable disappointment and disruption to schedule, as it meant that NAIC's already overstressed Facilities Department had to take over the very demanding tasks of aligning the whole system. However, they have thrown themselves into this with accustomed determination and based on success in previous alignment efforts, the only question is the pace at which these activities can be completed.
The first task was to adjust the elevation rail spacing, as this was necessary to be able to move the Gregorian and the carriage house. This was done prior to the Vertex visit in June. Next, the curvature of the Gregorian and Carriage House rails will be addressed. Following this, the Facilities crew will move to the azimuth rails, which are likely to need significant attention, both in terms of radial and height alignment. This effort will certainly take several months, but we hope to be able to carry out test observations to an increasing extent as this phase proceeds.
We also will be continuing receiver installation. We have had some problems with cryogenics system reliability, which we hope are now behind us. The 21cm receiver waveguide window gave signs of mechanical weakness, and is being replaced with a thicker unit to avoid any possibility of problems in the future. We have put in place the old 21cm receiver which uses a turnstile junction and is consequently quite narrow band. It should be perfectly usable for test observations until the broadband system is again available. Next, we will get the 4-6 GHz receiver cooled down and ready, although it may be a while before the optics and pointing really justify its use.
Software work is continuing in various areas. The monitor system for the operators/observers was brought up in June, and will be refined in the month ahead. The tracking software is probably the next major module to get working. As the drive system and rails permit, we will be trying to verify actual beam tracking and pointing on the sky. The user interface (ADAM) is gradually being expanded, and should be available at the time of the start of the Commissioning Phase.
A variety of other systems have also been brought on line. One of these is a video camera monitoring system for safety in the Gregorian. This lets the operator verify that all major subsystems (rotary floor, cable wrap, tertiary) are in a safe state, and that there is no equipment (or people) that could make problems. This system was installed in early June and works well, but it turns out that it generates a disagreeable amount of interference in the 430 MHz range. This is being run down and corrected at this time, but illustrates the system interactions we are bound to have as we bring the complex, Upgraded telescope into operation.