Focal Plane Unit (FPU)

Figure 2.11.: Postscript file of the mask as it is output from the mask preparation soft-ware. A laser-cutter readable machine file (’gerber file’) is also generated and is used to cut the mask.

The FPU has three operating states: OPEN, HOLD and LOCKED:

In OPEN position, a mask is either inserted into or removed from the FPU. When a mask is put into the focal plane, it is first rotated into the FPU by the Mask Handling Unit (MHU, see next section) into a gap between the fully retracted mask clamping-arm tips and the mask centering pins of the FPU. In this state, a mask is always grabbed and held by the MHU grabber.

The HOLD position is the mask-hand-over position between the MHU and the FPU. After the mask’s has reached the OPEN position, the MHU moves the mask∼5mm out of the open position and into the HOLD position in the direction of the focal plane. The mask frame touches four spring loaded pads after this move. Also, the tips of the two centering pins extend into the alignment holes in the mask frame as they have a smaller diameter at their tip. In this position the centering pins serve as support pins only. Figure 2.13 shows a close up of this FPU part. From the other side (direction of the inside of the instrument), the two clamp arms now approach the mask frame. In the hold-position, these arms press only slightly against the frame so it is held between the sping-loaded-pads on one side and the spring-loaded tips of the clamping arms on the other side. In

2.3. Focal Plane Unit (FPU)

Figure 2.12.: Focal Plane Unit inside its housing mounted in the base plate of LUCIFER’s inner structure.

The two stepper motors which drive the clamping arms can be seen in the upper part, their electrical connections leaving the image on the top.

Arm-’X’ is to the left and arm-’Y’ to the right. The stepper motors drive two ball screws which in turn move a knee joint (attached to the ball screw’s nut) to which the arms are bolted. The open black square in the center is the border of the 4x4arcmin² FOV. The spring loaded pads and the alignment pads that are used for the mask alignment are located in the left and right corner. Their corresponding clamping arms are visible above them. Small reed contact switches next to the arms sense the mask frame position in open, hold and locked state.

this configuration the MHU grabber can now securely release the mask frame. The slight push of the clamping arms against the pads and the tips of the centering pins prevent the mask from falling out of place.

The LOCKED position is the final position where the mask is in the focal plane and can be used for observations. After the grabber has released the mask in the HOLD position, the clamp arms move further down, pushing the mask frame onto the centering pins. The motion stops when the mask frame has been fully pushed onto the centering alignment

pins and the spring loaded pads are fully pushed into the FPUs base plate. Now the alignment-surfaces, which center the mask, are in contact and align the mask correctly inside the FPU.

The FPU clamps are elbow joints driven by ball screws. The mechanism provides self-locking of the clamps in LOCKED position without the need for a hold current on the motors which would generate additional heat over long periods of observation time.

Extensive tests and usage have shown that the ballscrews’ dimensions are to small and abrasion can block the motion. Thus the ballscrews were revised during the thesis’ work and now have a bigger diameter. The motion’s error rate has dropped significantly.

Clamp motion control and mask position detection

Both clamp arm drives have three micro switches attached to them which indicate the OPEN, HOLD, and LOCKED position. They act as motion limit switches in OPEN and LOCKED position. In HOLD position, a position-reference switch is activated. Reed con-tacts are installed in the FPU to monitor the actual mask movement. The concon-tacts are po-sitioned next to the mask fame and are actuated by two 3mm diameter Nd-magnets that are glued into the mask frames. Each reed contact has two switchover points, switching from ’on’ to ’off’ and back ’on’ when a magnet is moved along the contact. The contacts are aligned such that they are ’on’ in ’OPEN’, ’off’ in ’HOLD’ and ’on’ again in ’LOCKED’

position. With this arrangement, the mask position can be determined to an accuracy of 1mm in z-direction. This is sufficient to reliably judge if the mask is in the proper position for grabbing. This information, together with the arms’ state switches, is used to evaluate whether it is safe to grab or release the mask in HOLD position.

In an earlier hardware version, the mask frame position check was done using a lever-actuated switch. This setup proved to be prone to errors. As the solution the magnet actuated reed-contacts were introduced in this thesis work.

Alignment of the masks in the focal plane

Proper and reproducible lateral alignment of the masks in the focal plane is crucial for successful science observations. Masks need to be inserted and taken out of the FPU smoothly and reliably to not wear out or damage the mechanical components. During MOS science field acquisition and pointing correction, the mask needs to be (repeatedly) put in and out of the focal plane. A reproducible positioning is of utmost importance in this case.

In earlier hardware versions described in Hofmann et al. 2004, lateral mask alignment was done by moving two hollow centering cylinders located in diagonal positions inside

2.3. Focal Plane Unit (FPU)

Figure 2.13.: Close up view of the clamping mechanism. The left view is from top. The blackened clamping arm and the pin/pad array below are visible. They protrude from the mask position reference surface. The arrow points at the reed-contact that is used for mask position detection. Right: Clamping arm and pin/pad viewed from side in FPU ’OPEN’ position. During mask-insertion and -removal, the mask is rotated in and out of the gap between the arm and the pin, perpendicular to the paper plane.

the mask frames onto centering pins with fitting diameter anchored in the FPU base-plate. The fitting travel range was a few millimeter. All parts were hardened and coated with the dry-lubricant DICRONITE^®. During extended cold tests, canting between the frame and the pins was observed, mostly when a mask was locked in the focal plane for an extended period of time. Differential thermal expansion of the mask (which is fully exposed to thermal radiation through the entrance window of the instrument) was suspected to be an issue in these cases because of the small thermal contact (the pins are the only thermal contact between the mask and the FPU). Also, we observed that in some cases the push-springs that are used to push the mask out of the locked position did not move sufficiently in sync which lead to canting of the frames on the alignment pins, causing damage to the pins (see figure 2.14).

Consequently, a new design emerged from this thesis: Now the clamping arms drive inside-cones in the mask frames against positioning ball-cups in the FPU base plate.

Figure 2.15 shows a CAD drawing of the new pin design. This setup makes canting impossible. Extended cold test proved this setup to be very reliable and accurate.

Figure 2.14.: Old version of the alignment pin with scratch marks due to canting and off-axis insertion of the mask. The spring loaded pads are located to the left and right of the pin. The three elements protrude from the mask position reference surface. In ’LOCKED’ position the mask rests on this surface.

Figure 2.15.: Old version of the alignment pin (left) and new version (right) as CAD draw-ing. In the new version canting between pin and mask can no longer occur.