Revive PE Paragon 1000 FTIR

Hello fellow experimenters,

recently I got my hands on a Perkin Elmer Paragon 1000 FTIR spectrometer in largely useable condition. Switching it on and putting in the reference polystyrene sample resulted in an immidiate spectrum that closely resembled literature, some minor deviations excluded.

Of course, luck comes to an end, and after it sat for some days (in a dry room), switching it on resulted in an error "erratic laser values" and no more spectra.

Well, idiotic me quickly googled, got something about slight laser misadjustment in the comparably built FTIR 1600, and of course, I got to work screwing around with the laser alignment, instead of researching more and using the auto gain feature which would probably have solved the issue right away.

Now, a few days later, I gained some insight into the device, scored a (mostly useless) manual which was surprisingly hard to come by, and having fucked up the alignment beyond self-repair.

Today, after inspecting the salt windows and beamsplitter, finding them pretty corroded, I polished them back to translucency using my limited means (optic wipes as "abrasive" and varying mixtures of water (to get rid of crusts), methanol and acetone. They won't win an award but at least now the laser dot is pretty much a dot again.

Alas, my question of adjustment. The HeNe-Laser is apparently reflected by an very small mirror (~3 mm) on the optical bench. It is then directed into the interferometer and onto the beam-splitter, which has a reflective (aluminium?) part with a clear smaller window.

It is my understanding that the coated part is for the IR, and the clear window is for the laser. Behind the window, there is a sensor for the laser, which gives a reading in the laser-gain adjusting menu. So far, all I do leads the sensor to do nothing, the intensity at maximum gain is somewhere at 0.35 (which is the lowest value, covering the sensor doesn't reduce this value).

And here is where I need help. By screwing around (immensely dumb, I know) I misaligned laser and mirror to a point where I cant fine tune it to maximum gain value, I first need to know where the laser even needs to go.

By aiming it into the window, I can get a beam into one fixed mirror, get a beam through the parabolic mirrors all the way to the IR detector, but dont get any increase in signal. Also, the reflection on the splitter aims the laser to the arm of the periscopic mirror.
If I want to get the laser into the periscopic mirror, it invariably goes onto the coated part which leads to almost no light for the fixed mirror and no more visible dot for the detector.

So, I'm clueless here. Do I really aim for the window? Is there some magic angle that will allow the laser to shine directly onto the sensor? Or is there some reflection I, so far, "skip" which will make all good again?

Alternatively, considering my repair of the salt materials isn't perfect (but I know it worked even pre-polish) there might be some issue, or the laser sensor in the interferometer is broken (it seems to react to daylight and a flashlight, but im not perfectly sure of the value range here), or the board is broken (but self test shows no issue).

Any help to get this godsent tool working again is greatly appreciated.