Model 3 Has Two Alternate Form Factors (F.F.): Bookshelf F.F. and Slider F.F.

The "Bookshelf Form Factor" of Model 3 is a compact form factor has the laser and detectors permanently aligned and glued into place -- rather than on adjustable sliders (the other form factor). This form factor also directly connects the timing triggle cables, rather than letting you set them up with BNC cables.

The "Sliders Form Factor" of Model 3 is less compact, but lets you slide around the laser and detectors to misalign them. It also has BNC connections exposed for trigger cables to be connected and disconnected at will.

Is the Bookshelf Form-Factor what I want?

With this form factor, you lose some fidelity to real-world uncertainties in laser and detector alignment, as well as losing some flexibility in the trigger cable connections.

However, this "Bookshelf" form factor is much more compact and "no-fuss" than the other form factors for Model 3. It is convenient when you don't have any desire to make daily mechanical adjustments of laser and detector alignment. E.g. use the Bookshelf design only when you want a setup that (unlike a real laser-plasma experiment!) "just works" consistently, with no re-alignment.