Procedures for Adjusting Secondary Mirror
The secondary mirror is carried in a cell that bolts into the top end of the telescope. This cell is a modification of a design from the University of Arizona, with the mirror carried on a tripod which is positioned with six motors. Three of these motors (linear actuators) press on the ends of the tripod parallel to the optical axis and provide axial positioning and tilt. The other three motors are attached to levers that restrain the tripod laterally and provide lateral positioning of the mirror. The mirror is balanced by counterweights which provide rather limited translations, so these motions, especially laterally, should be used with care. Axial home positions for the legs of the tripod are determined by proximity sensors, which should be adjusted to about 0.12 inches above their mounts. Unfortunately, the only way to get at these sensors is to take the tripod out of the cell. To get it out, you must remove the six springs tensioning the locating elements, then remove the three lateral links by unscrewing the 5/16-in adjusting screws through the arms of the tripod. Before doing this, use a caliper to record how far each screw sticks out so you can put it back the same way. After adjusing the sensors, reassemble the cell. Attaching the lateral springs is easier with the proper grip on them. In attaching the axial springs, it helps to hold the links with long-nosed pliers, as shown. The link for the axial counterweight is reattached next, and it can be tempremental. Hold id in tension with pliers as with the axial springs, and make sure the threads on the link are not being caught on the threads (unfortunately) on the lever body.
The mirror must be adjusted in its cell to colimate the telescope and give adequately sharp images for acquisition and guiding. This is done by using the coma of a Cassegrain system to determine compensating tilts and possibly translations of the secondary mirror to give good images. To a great extent, errors in centering may be compensated by appropriate tilts, as this calculation in which a 3-mm centering error is compensated by a 0.435-degree tilt.
Take an image far enough from focus to see a well developed shadow of the secondary mirror. Tilt the secondary mirror while recentering the image in the guide camera to make the extrafocal image as symmetrical as possible. Focus the telescope and take an image of a globular cluster or of a bright star near the center of the field to assess the remaining coma. Activate the centering routine and tilt the secondary until the remaining coma is removed as much as possible. Record the positions of the three axial actuators and use them to determine the zero points for the focusing routine in the control system.
The telescope may be used as a prime focus camera by removing the secondary mirror and replacing it with a special fixture and set of counterweights. To change the configuration, put the telescope in its maintenance position with the 44-inch stay par as explained above, remove the secondary mirror and its electonics package, install the counterweights with the special crane, place the prime-focus fixture on the top of the secondary support structure and tighten the screw holding it on, hook up the control line to the focusing motor and wire to the camera, and add one counterweight to the bottom of the tube. Next, remove the stay bar, push the telescope up past its limit switches, and balance the tub with the procedure described below.
The telescope tube must be rebalanced for the two different configurations of the top end used in maintenance and operations. There are essentially two such configurations to consider 1) a setup with a CCD camera at the top end for tests at prime focus of the primary mirror and 2) the Cassegrain system with the secondary mirror in place. The prime-focus setup requires a group of four lead weights to be strapped to the top end of the telescope to compensate for the missing weight of the secondary mirror and its cell. The secondary and cell together weigh about 40 pounds, and we have assumed this weight in designing the telescope.
To balance the telescope, add counterweights to the bottom of the tube. These are held on with two 1/2-inch bolts through holes in the tilt drive sector. Run the telescope tube up and down under the terminal program galilterm, reading the demand on the drive. Add weights to the drive sector until the demand is roughly the same going up and down. Secure the weights with lock washers.
Run these tests over the network, logged into the telescope-control computer (t13a) from the control room. Commands for the Galil controller are given in a manual kept in the desk in the control room. A terminal session for balancing the telescope might look like the following (Note that the tilt drive is in the second, Y, position):
t13b>telnet t13a --> connect to computer in the telescope (logging in etc) over the ethernet t13a>cd /home/eaton/ast/src/galil --> switch to proper directory t13a>galilterm --> start terminal program on computer in the telescope KP ,5 --> set proportional PID constant for axis Y KD ,35 --> set derivative (velocity) PID constant KI ,0 --> set integral PID constant = 0 SH Y --> servo here, Y (tilt) axis JG ,25000 --> sets jog speed for Y to 25,000 cps BG Y --> begin motion in Y axis TT --> tell torque (in volts) repeat several times . . . JG ,-25000 --> reverses motion TT --> tell torque (in volts) repeat . . . ST Y --> stop motion
