I am not sure this is the correct thread for a long answer to scatter, and with no mention of Fluorite, but there are questions of scatter in eyepieces and I am very interested in this topic. I test scatter a lot especially while viewing brighter double stars and sometimes the planets. I have a lot of eyepieces and enjoy comparing them for differences.
I evaluate the area size of the scatter but also how bright the scatter is.
I believe it was Thomas Back who recommends using averted vision to judge the amount of scatter. You can see more scatter by doing that, of course.
I see scatter differences among my eyepieces but I haven't really evaluated how that effects the view of a planet, for example. How much smearing is done with the scatter? Supposedly more scatter will smear details but I haven't tested this yet. I imagine you would need very good seeing and conditions change constantly.
I am certainly not an expert on this and am still learning a lot so I don't want to appear that my results are "correct." These are my impressions with my Tec 180FL refractor. There are so many variables as well, of course, as in the humidity in the sky, and sometimes I wonder if I have slightly fogged the eyepiece so I have a Rocket Blower handy at times. Even when it isn't cold I might be fogging the eyepiece because my warm eye is so close to the lens. Sometimes I wear a hood and lately a mosquito net so that may cause fogging.
I have some favorite eyepieces that in my impression (again I don't want anyone to take this as correct) the scatter is obviously lower than other eyepieces. I do have some floaters and that can impact the view as well so I consider that.
For double stars, just last night, I took my entire line of Pentax .965 orthos and viewed some doubles with each of them from lower power to higher power. Really beautiful. I see some scatter but not a lot. Not last night, but I have compared to my Delos and Pentax XW and for me there is a noticeable increase in the scatter and I don't like the view of doubles nearly as much through these eyepieces. I wonder at times if the different field of view is contributing to seeing differences.
But what is really puzzling for me, is that my line of TMB Supermonocentrics which are only 3 lenses appear to scatter more than some of my other low lens eyepieces. I was very surprised by this initially and thought there would be less with only 3 well-coated lenses. These are some of my favorite eyepieces for smaller groups of stars and I feel I see more stars with them. I might not be seeing higher transmission because differences are small compared to other quality eyepieces but for me stars appear more pinpoint, very sharp or maybe the background is darker and so they stand out more or it could be the very small FOV so I concentrate on that area more. But I just love them on smaller clusters and globulars that don't have any bright stars in them.
I was curious about the higher scatter of the SMCs and so I searched CN and found that others have noticed this as well so perhaps it is not my imagination. I wondered and others wondered it there was more scatter because of the higher transmission which would show brighter stars and therefore more scatter. But any higher transmission is probably not enough to demonstrate this so I am not sure why I see more. It is not a lot of scatter, but more than some of my other favorites on brighter double stars.
I have some Zeiss monocentrics that could have older coatings, maybe single coated, I am not sure, but I also really love the double stars in these. There is scatter but it is a dull scatter, in others words the scatter is not bright so stars are really beautiful with a dark sky background. Very sharp too.
The Doctor 12.5mm is a wide angle eyepiece but it was really beautiful last night on many objects and double stars. Scatter seemed low but I didn't do too much comparing. I believe I did compare with my 12mm Pentax Ortho but there is a small difference in focal length.
Another wide angle eyepiece that seems better than I might expect is the Nikon HW 17mm. That is lower power though so that might contribute to lower scatter. I really like it though.
Lots of fun!!