I know many people who do not want to ‘rent’ Adobe, and didn’t get CS6. On1 PhotoRaw 2017 doesn’t install to CS5, but I did manage to get it working with it.
I have a legit copy of CS5 E on my computer. There is a function in On1 Photo Raw 2017 stand along that allows you to ‘punt’ a processed image into photo editing program, but sometimes you don’t want to have to save the image, and reopen it in the Standalone.
This was my solution.
I downloaded and installed trial version of CC (I have no intention of renting it after the trial date). I installed On1 Photo Raw. It installed to the Photoshop CC but not (as expected) to CS5. I started the Stand Alone Photo Raw, signed in. All good!
Then went to Adobe Photoshop CC directory, navigated to the Plugins folder, copied the On1 2017 folders to the CS5 plugin folder.
I was able to use all the features of On1 Photo Raw with CS5.
Not sure if you will have to keep the CC trial, unused, on your system to update On1, but fairly sure you will. But nothing says you have to uninstall the trial if you choose to not have a membership, and as long as On1 ‘sees’ the newer version, you update On1 as the updates come along and just copy them over again.