The initial setting of the panels on the GBT will consist of setting the panel to panel heights at each actuator. In order to avoid repeated setting of the surface, it is highly desirable that this setting be done just once, with sufficient precision for high frequency operation of the GBT. Due to the fact that we are using the same mold for several tiers of panels, panel edges along a radius will be offset by up to 200 µm. In order not to degrade the overall surface precision, this initial panel to panel setting should be accurate to around 25 µm. In this note we describe a suitable instrument for measuring and recording the relative panel heights at each actuator.
A sketch of the proposed tool is shown in Figure 1.
A shaft, screwed onto the actuator, provides a reference for a jig that slides over the shaft. A step in the shaft permits location of the jig. A tapered shaft (in the manner of a screwdriver blade) fits between the panel gaps in order to locate the jig in rotation around the shaft. Four electronic dial indicators mounted to the jig make contact with the reference points on the panel. A suitable indicator is a Mitutoyo 534-182-1. This indicator has a range of 12.7 mm, a resolution of 1 µm, a visible display and an SPC output. The indicator is battery powered, with a battery life of 500 hours. Assuming a separation between the indicators of approximately 15 cm, the angle of the actuator extension shaft with respect to the surface tangent needs to be known to an accuracy of around one arc minute to limit errors in setting to less than 25 µm. Gravity is a convenient reference to use here. In the radial direction, the angle of the surface tangent with respect to gravity is know for each actuator position. In the circumferencial direction, horizontal is a convenient reference plane.
The accuracy required of the tilt measurement (better than 1 arc minute) precludes the use of a large dynamic range "digital protractor." A suitable inclinometer is manufactured by Schaevitz, a LSRP-14.5, which has a range of ±14.5 degrees. A series of wedges will be needed to maintain the radial inclinometer within its range for the outer parts of the reflector. This simple option has been chosen over a servo controlled platform which, while more elegant, would be more bulky and complex.
A block diagram illustrating the various components is shown in Figure 2. All components are battery operated. A convenient (but certainly not necessary) means of identifying each actuator would be a bar code transfer on the reflector surface. The operator would read the bar code after installing the tool. The correct actuator tilts would be stored in the computer, along with the correct digimatic indicator readings. Corrections to these readings would then be computed on the basis of the tilt deviations. A reading for "adjustment screw #1" could then be displayed on the screen. The operator would adjust #1 until the reading is zero and moved on to #2. After the setting process is complete, all four digimatic readings are stored, along with the tilt readings to be recovered later in a lab-based PC. It should be noted that with this procedure not only are the panels set with high precision with respect to one another, but the relationship between the corner cube (when installed) and the panels in now known to a high precision. This, theoretically at least, permits the setting of the surface using the laser rangefinders.
One issue that needs to be settled is the deformations resulting from the weight of the operator(s) in the vicinity of the actuator. We can include a "strobe" option that will permit the operator to stand some distance away and remotely initiate readings.
This page was last modified on September 11, 1997.
