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My modification was simple - no glass plate instead of removed filter.
Yes, I think you are right, it is IR.. I looked at couple of stars with halo and all of them have colour index >1.5, which means they are really red.
However, I am not sure the observed effect is because of removal of the glass, because uvir-cut filter is 2mm thick.. it is at the different location but still...
Most likely it is because this lens is Canon FD 400mm f/2.8L, from '80-ies last century, not corrected for IR.
And spurious far-IR bandpass of the uvir-cut filter (I will have a look with spectrograph tonight if it exists or not) allows too much IR.
BTW, all bright (saturated) stars have the ~same diameter, which is also indicative of something un-expected and counter-intuitive going on here.
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