TopBox User Manual

Filter Wheel Module

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Fig. 1. The TopBox filter wheel module. Motors (hidden in this view) are mounted on the rear panel as seen in Figs. 3 and 4 below.

Mechanical System:
The filter wheel assembly is a removable module designed to give access to the filters without removing the entire TopBox from the telescope. Two stepper motors mounted on the outside of rear panel (S side panel of the TopBox) operate two six-position filter wheels independently on concentric shafts by drive belts passing through cowled slots in the panel. The upper wheel (FW1) is attached to the larger diameter shaft held in the two middle bearing plates and driven by the lower belt and pulley from the motor on the W side (far side as seen in Fig. 2). The space below this wheel allows it to carry filters up to 20mm thick, including the polarization analyzers (note Fig. 3). The lower wheel (FW2) is attached to the smaller diameter shaft held in the top and bottom bearing plates and driven by the upper belt and pulley from the motor on the E side (near side as seen in Fig. 2). It can carry filters up to 6mm thick.

Filter Mounting:
In Fig. 1 both wheels are shown in Position 0 ("Empty", "Filter Wheel 1 Position 0 = F1P0", "Filter Wheel 2 Position 0 = F2P0"), with both wheels rotating clockwise as filter position numbers increase from 0 to 5. Thus polarizer P1 can be seen in F1P1, and polarizer P2 in F1P2 is at the right rear, hidden by the two middle bearing plates. The I and R bandpass filters can be seen in F2P5 and F2P4, while the V and B filters in F2P3 and F2P2 are hidden from view. (The U filter in F2P1 can be seen through polarizer P1.)

Fig. 2. Filter wheel module side view showing belt drives, bearing plates, concentric shafts, and filter holder mounting detail.

The filters are installed in machined metal casings which can be custom made following a basic template to fit a wide range of thickness, apertures up to ∼2", and shapes both round and square. Each casing has a mounting frame which slides radially onto a recessed shelf on the top surface of FW1 or on the bottom surface of FW2. The frame is then secured to the wheel by its "ears" with two radial bolts. Some attention is required to verify that the filter casing is level with the horizontal filter wheel surface after mounting, although leveling is automatic if the bolts are tightened carefully and evenly. Figs. 1, 2, and 3 show examples of the recessed shelves on the top surface of FW1, the pattern of four countersunk flat head screws attaching the frames with their "ears" to the filter casings, and the pairs of radial mounting bolts holding the frames to the filter wheels by their "ears".

Fig. 3. Filter wheel module showing motors, mounting bulkhead for alignment of bearing plates, and home sensor roller switches, actuators (also visible attached to filter wheels at right front in Fig. 1), and wiring.

Module Installation and Removal:
The filter wheel module makes a tight sliding fit into the S end of the TopBox guided by the lower bearing plate, which slides along a recessed channel in the TopBox base plate until the three upper bearing plates engage slots in the filter wheel bulkhead, which is a fixed wall inside the TopBox (Fig. 3). The filter wheels themselves pass through the opening at the bottom of the bulkhead. The entire sliding assembly is secured to the framework of the TopBox by nine captive screws around the edges of the rear panel (Fig. 4). These screws can be tightened by hand; however, a screwdriver may be used to loosen them if they have been overtightened. The short motor cables are labeled and can be quickly connected or disconnected at the white molded plugs, which will only snap together properly if they are correctly aligned.

Fig. 4. Filter wheel module showing TopBox rear panel mounting.

The filter wheel module is most easily installed with the telescope balanced and pointed at the zenith. It weighs about 25 pounds, so the procedure requires some strength and you will probably move the telescope during the installation effort, which will invalidate the coordinates of the HOME position set in the telescope control system. The resulting pointing error may be corrected by a "Sync into TPoint Model (least accurate)" operation at the beginning of the next observing session.

Before carrying the module up the rolling platform ladder to install it, note that the free end of the upper bearing plate can wobble slightly in the vertical direction. This plate is more likely to engage its bulkhead slot if it is free to wobble — not stuck in an upward-angled position. Ready? Then carefully climb the ladder with the filter wheel module in hand.

Hold the module by one motor with one hand, support it from underneath the lower bearing plate with the other hand, fit the lower bearing plate so that it rests into the channel in the TopBox base plate, remove the supporting hand and use it to hold the telescope by the side handle on the tailpiece, and slide the module into the TopBox. It will slide in smoothly until about 1/4" remains, then the bearing plates must be lined up to engage the slots in the bulkhead before it will move the final 1/4" and fit into place. If excessive force seems necessary to engage the slots, STOP! The alignment is incorrect! Resist the urge to hammer or beat on the back of the rear plate the close the final gap! STOP! Slide the module out a few inches, wiggle it gently up and down, and try again. If you can't push evenly from a side position, roll the platform around to the N side, grasp the box by the E and W sides with your palms, and pull in on the rear plate by wrapping your fingers around the S end of the box. If repeated attempts fail, remove the filter wheel module, take it back to a work table, and check the bearing plate spacings again.

Removing the filter wheel module is usually an easier task than installing it. After disconnecting the short motor cables and releasing the captive screws, pull the module free from the bulkhead slots with one hand on one motor and the other hand holding the telescope by a side handle on the tailpiece. If this seems to require too much force, use a small, narrow screwdriver blade in several places around the edges of the rear panel to carefully wedge it about half an inch away from the edges of the box until you can pull the module out by hand. Use one hand under the lower bearing plate to support the weight as the plate slides out of its channel.


Last modified: Sep 13, 2012
David McDavid

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