Hi William,
The size, that is span of wavelength, is just a consequence of what bit of the spectrum was in the field of view.
The actual dispersion is the same. The sporadic fireball was a bullseye. It went across the fov slightly diagonally but dead centre. So the whole range of the (silicon) detector was utilised and a full range spectrum was caught, from the UV cut off of the glass to nearly the edge of the silicon “band gap” at just over 1000nm. Actually that was a remarkably lucky catch!
You have a greater chance of catching the whole spectrum if one uses a lower dispersion grating. But then the resolution drops, it’s all a compromise.
cheers,
Bill.