The IRS mechanisms were originally designed for 300 micron pixels on a linear array. The current array has 24 microns pixels and thus pushes the IRS mechanisms to their limits. 1 stepper motor step of the grating drive is about 2 pixels, thus it is not possible to position wavelengths much more accurately that about 1 pixel. The grating mechanism does reset to an accuracy of about 0.7 pixels RMS, which is a fraction of a motor step. Thus, when centering an object in either zeroth order or in the mirror position (or anytime when sub-pixel accuracy is needed) it is best to image the slit being used for observing each time the grating drive is moved to record an accurate slit center positon. We have tested the stepper motor drives extensively and have found that they do not lose any steps even with hundreds of moves.
The flexure with the new f/14 mounting has been measured and is substantially improved. First, there is internal flexure which causes wavelengths to shift by about 0.3 pixels per hour of telescope tracking. Secondly, there is differential refraction and flexure between the tip-tilt camera and the spectrometer slit. This amounts to about 0.21 pixels (=.05 arcsec) per hour of tracking.