IMCS (Image Motion Compensation System)
The MODS image motion compensation system (IMCS) makes use of an IR (1550 nm) laser which shines through approximately the same optical path as light from the science target and is detected by a quad cell near the science detector (see Figure 1 below, taken from Marshall et al. 2004, SPIE 5492). Feedback from the quad cell drives three actuators on the collimator mirror, first, to align the collimator, and then to maintain the alignment throughout the observation. The quad cell counts are read out every second and updates are made every 3 seconds, based on an average of 3 measurements. The IMCS runs closed-loop only for exposures that are 10-seconds or longer, to average over instrument “seeing” along the ~10 meter long optical path traversed by the IR laser.
Figure 1: A schematic showing the components of the IMCS for the blue channel of MODS. Taken from Marshall et al. 2004, SPIE 5492.
The IMCS operates in all 9 modes in which MODS is used: direct and dual imaging, grating and prism. The bypass optics are mounted at the centers of the dispersing elements and mirror, in the hole of the entrance pupil. The bypass optic in the center of the mirror is a mirror, but the optic in the center of the grating and the prism is a grating. Since the MODS prisms are back-coated, the bypass grating must be mounted, not directly to the optic as is the case for the mirror and grating, but to the prism cell, and this concession degrades the performance of the IMCS for prism spectroscopy.
The collimator must be realigned after each slew of the telescope and rotator and after every change of configuration. The “imcslock” command accomplishes this; it opens the shutter, measures the quad cell counts and adjusts the position of the collimator until there are equal numbers of counts in all the quadrants, at which time the shutter is closed. It is usually run implicitly after the “instconfig” command sent by the script, but it also may be run manually, either by clicking the “IMCS Lock” button on the MODS User Interface or by typing “imcslock”, “blue imcslock” or “red imcslock” into the Command Window of the User Interface.
Commissioning data showed that the specification, that the relative motion of the spectrum in the x- (dispersion) direction should be less than 0.6 pixels over a 15 degree elevation change under normal sidereal tracking, is met. A 0.6 pixel drive is just over 1/10th of the resolution element (~5 pixels) for the 0.6″ slit and assuming the 0.12″/pix scale of the blue channel.
The performance of the IMCS is measured in relative terms, minimizing the effect of flexure from the start to end of an exposure. In absolute terms, there may be a few pixel shift. Every time the “instconfig” command is run, it sets the positions of the three actuators on the collimator back to their zero points. These zero points are usually determined once per year, after the instrument is mounted, and with the telescope at 60 degrees and for PA=0, these being average positions that better represent actual observing conditions than the extremes. The implicit “imcslock” command which follows an “instconfig” issued by a script then realigns the collimator based on feedback from the quad cell.