The mirror slit does have a couple of drawbacks too though. Bright stars are normally guided when on the slit using the overspill. (Typically the slit width is chosen to match the star FWHM so there is always some light which does not pass through the slit) The resulting image is “Hamburger” shaped though, split by the slit, which some guider programs struggle with. Also because unlike an air slit, the light has to pass through glass this can limit measurements at the UV end of the spectrum. (The beam spliiter has the same problem though.) The original not for profit LHIRES kit had jaws lovingly hand polished by volunteers but that would be expensive to do commercially. Christian Buil has been experimenting with reflective air slits (from the base ALPY module) for the UVEX, along with using all reflective telescope optics and a different glass for the camera window to reach a remarkable 3200 Angstroms. Beyond that atmospheric ozone absorption becomes an issue.
http://www.spectro-aras.com/forum/viewtopic.php?f=45&t=2277&start=10#p12459
Cheers
Robin