Dear List,
We've been using the older olympus XLUMPFL 20x 0.95 NA objective, and have 
been getting decent images at large imaging depths. But for two reasons we're 
looking into purchasing a new objective. First, when we do point spread 
functions measurements for two-photon fluorescence at 780nm excitation with 
this lens, we always get measurements that are significantly larger than the 
size predicted from the diffraction limit (~30% bigger laterally and ~50% bigger 
axially). We're careful about index matching, overfilling the back aperture, etc., 
and can get close to diffraction limited PSFs with other lenses, so I believe this 
is a limitation of the lens, probably at NIR wavelengths. Second, we'd like to 
get a lens that can be used with a coverslip (preferably one with a correction 
collar).

It seems there are at least two objective lenses I can find that would be good 
for our upgrade (we also want to have a working distance ~> 1mm): the 
olympus 25x 1.05 NA W, and the nikon CFI Apo LWD 25x 1.1 NA W.

Their specs are very similar, so I'm wondering if anyone has had any experience 
characterizing them, or can suggest other options that I haven't found. They 
both say they are "corrected" out to 950-1000nm, but based on our experience 
with the older olympus, I'm doubtful they perform at the diffraction limit in this 
range. The Leica HCX 20x 1.0 also seems nice, but doesn't have a correction 
collar. Has anyone done PSF measurements on any of these objectives? Or 
compared them in any way?
Best Wishes,
-Nick