Bought my FSQ-85 in 2010 and used it for a number of years with an SBIG8300. Kids came along so I put Astrophotography to one side but got back to it with the wave of interest in 2020. I upgrade to an ASI1600 that year and an ASI2600MM the year after. With both of these cameras and especially the APS-C I though my FSQ-85 had somehow lost collimation. I don’t remember it have such bad stars in the corners. Turns out it’s not a collimation issue but a small pixel issue. The optical design of the FSQ-85 is not fit for any of the popular CMOS cameras. This flattener is marketed as something to improve full frame sensors. However for both my 4/3rds and APS-C it’s completely fixed the issue and I now have nice round stars in the corners. The flattener does come packaged with the FSQ-85 now but from my experience you have to use it. Which kind of defeats the purpose of a quadruplet design that you need two flatteners to get round stars. As Takahashi glass goes this was a relatively cheap fix and it worked great so giving it 5 stars.