Multi Object Slitmask Unit (MOS)
2.2. Focal plane masks
The focal plane slit masks are the essential components of the MOS-unit. Up to 33 masks can be stored inside LUCIFER at a time. The masks are distributed over two cabinets.
One cabinet is fixed inside LUCIFER. It houses 10 permanent masks. These are longslit masks with slit widths of 0.25, 0.50. 0.75, 1.0, 1.5, and 2.0 arcsec on sky and masks for tests and optical and spectral alignment. The second cabinet is exchangeable and has 23 storage slots for multi object ’MOS’ masks that are laser cut to the specifications given by the observer. A cabinet exchange is usually done monthly prior to the science nights block.
Figure 2.6.: An empty mask frame as one would see it from the outside of the instrument looking towards the focal plane. The handle where the MHU-head grabs the mask is on the left. The mask-fpu-alignment holes are in the top right and bottom left of the frame. These are coated stainless steel insets mounted into the aluminum frame. The frame identification number ’37’ is at the bottom. Above it, the gap between the base-frame and the cover-frame is visible in which the mask sheet is inserted. The gap’s curvature follows the telescopes focal plane in one direction, giving the mask sheet a cylindrical shape. The metal insets on the right side and top left of the frames align the frame inside the storage cabinet. This ensures that the frame is always at the same position in the cabinet, i.e. the grabber position can be the same irrespective of telescope and instrument orientation. The recess in the frame on the left side is for securing the frame in the cabinet together with the retainer when the instrument is rotated upside down.
2.2. Focal plane masks
A mask consists of the mask frame and the mask sheet which is held therein. The side facing the instrument (i.e the side ’seen’ by the detector) of both the mask frame and the mask sheet are blackened. This is to reduce and diffuse light originating from reflec-tions on optical surfaces. The mask frame has interfaces to the Mask Handling Unit (a
’handle’), to the Focal Plane Unit (two inside cones for centering the mask on the FPU’s alignment pins) and to the storage unit (hardened surfaces for guiding and locking). The frames are 180x180mm2 in size, having a clear aperture of 144x144mm2, which corre-sponds to 4x4arcmin2 on sky. All masks are cylindrically curved perpendicular to the dispersion direction following the telescopes field curvature in one dimension. The ra-dius of curvature of 1030mm. A full 2D spherical correction requires spherical masks.
These are expensive to manufacture and it is difficult to maintain their shape under cryogenic conditions given the thickness of the stainless steel mask sheet of only 120 microns, see section 2.8. The cylindrical curvature is defined and ensured by the shape of the frames which are machined accordingly. We limit the slit distance from the optical axis to +/- 1.25 arcmin in dispersion direction and place the mask center 0.5mm behind the telescope focal point to reduce the defocus on the edge of the mask and to get an overall more uniform focus. Following the field curvature in only one direction reduces the usable FOV from 40×40 to 40×2.50 in dispersion direction. This limitation is not too severe for practical applications on sky as the spectrum of slits near to the mask’s edge is clipped by the detector area. This is because the optical layout is such that the spectrum extends equally to both sides measured from the slit position on the detector. First year operation has shown that no science program suffered from this limitation.
Figure 2.7.: Mask sheet with multi object slits. Four alignment holes are visible on all four sides along the outer edge. The mask ID number can be seen on the front left. It identifies the mask and can be checked during on sky acquisi-tion. The side of the mask which is facing the instrument is blackened to reduce stray light. The curvature of the mask sheet follows the telescopes focal plane curvature in one direction. The slits are laser cut to the spec-ifications of the science user. A special mask planning software is used to design the mask.
During labtests, stainless steel material of various thickness and surface properties was tested for their performance in the cold. The final material has been tested in the lab and showed only diffuse reflections in the near infrared.
Figure 2.8.: Close up on the mask sheet with mask ID number and mechanical alignment hole on top. The little holes in line below the ID number are for spectral alignment. With the help of these holes one can check the masks orientation with respect to the detector. Three MOS slits are visible on the sheet. The one square hole on the top right is one of several used for alignment of the mask during on-sky acquisition. Field stars are placed in them for alignment.
Figure 2.9.: Comparison of cuts with different cutting parameters (e.g. laser power and shielding gas pressure). Top row: Parameters adjusted. The cut edges are clean, Bottom Row: Laser power and gas flow not yet optimized. Molten ma-terial has re-solified in an irregular shape on the edges, making it unusable as an entrance slit for a spectrometer. Inset: Close up of the recast material.