The spectrograph is just a rectangular box with grating, mirror and doublet lens mounted inside. The ALPY slit is bolted to the box and the ALPY guider and science camera are coupled to the box with T mounts. The slit should be positioned to correspond to the positon in the ALPY (The slit is in same plane as the front face of the guider I believe). The distance from slit to doublet via the mirror and doublet to camera = focal length of the doublet. make position of doublet and science camera adjustable. (Collimate by pointing a camera focused at infinity through the end of the box with the grating removed and adjusting the doublet to give a sharp slit image)
No guarantees, I will leave the detail to you 😉
Robin
EDIT corrected distances (to doublet, not grating) – suggest using LHIRES III dimensions and position of elements