The FSQ optical assembly is the wide field telescope tandem with the RC24, and the focusing optics of the small Littrow echelle used for stellar spectroscopy.
The FSQ-106ED uses two ED (extra-low dispersion) glass elements based on the classic two-doublet Petzval design. It provides very low levels of aberration and image sizes less than a single pixel of the CCD camera which is used with it:
Aperture: 4.2" or 106 mm
Focal ratio: f/5
Focal length: 530 mm
Focal plane scale: 0.108 degrees/mm or 389 "/mm