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The powerful tools of integral field spectroscopy and adaptive optics have made great contributions to the progress in astronomy in recent years. The combined use of these techniques now enables spectroscopy in the near IR close to the telescope diffraction limit. This will provide new and interesting insights into a variety of objects such as AGNs, QSOs, circumstellar disks around highly extincted YSOs, etc. Spectroscopy at or close to the telescope diffraction limit has some caveats which one has to be aware of when designing the instruments so as to maintain the maximum possible throughput and to optimize spectral resolution. Astronomical campaigns with our H- and K-band integral field array spectrograph 3D in combination with the laser guide star adaptive optics system ALFA at the 3.5-m telescope at Calar Alto require special observational techniques in order to make the most efficient use of the observing time available. Chopping by moving the telescope to do background subtraction makes it necessary to relock the A.O. system on the guide star after moving the telescope back to source. This procedure is usually rather time consuming. The aperture interchange module (AIM), which we present here, enables us to perform chopping between source and black sky while keeping the telescope fixed at a certain point in the sky. For this purpose AIM uses two different optical channels. The ON channel always points to the center of the 3'ALFA FOV, picking off a FOV of roughly 4 inch by 4 inch. With the OFF channel one can choose any off-center position within the ALFA FOV except a central obscuration of 36 inch diameter. The AIM optics are designed in such a way that the optical pathlengths for the on- and off-axis positions are kept equal. AIM also includes a scale changer which magnifies the scale from 0.25 inch/pix to 0.07 inch pix. The 3D spectrometer itself is equipped with two interchangeable grisms, so that one can choose between H- and K-bands and between spectral resolutions of 1100 and 2100. The commissioning run of AIM together with 3D and ALFA took place in July 1997 at the 3.5m Calar Alto telescope.
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