The order sorting filters prevent second and third order reflections from the gratings from interfering with the output. They are long pass filters, one around the 350nn range as well as 600nm range, some have filters higher and lower, this one has one down in the deeper UV as well as a spot for one in the NIR area. And thats why I was getting such a sharp cut off at 1115nm, there was no filter installed, just a blank black aluminum disc. The wheel uses standard 1" filters, I had a bunch of different ones in the NIR range so I checked them out on the spectrometer. Unfortunately they were all notch filters like this one here:
It is rated for 880nm and a 20nm pass which is pretty dead on.
Earlier this year I picked up a little Acton SpectraPro150 from the tektronics company store, it is a little unit maybe about 8"on a side and is controlled by an on-board serial or GPIB port. On a later visit I found the order sorting filter wheel that went with it and picked up that too. Someone added a 900nm long pass filter to that (and marked what they had added) at some point. That sounded like just what I needed so i popped it out and put it in the beam path to check it out before installing it.
Yep, exactly what it said it was. So it is now installed in the filter.
Now I need to figure out how to do baseline subtraction. Its harder than I thought it was going to be and have not have had any luck finding out what kind of math I need to handle that.