No windscreen, no wiper ==> no problem
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I've been disappointed in the IQ of images I've taken through the big thermal-pane picture window in the study, even with the camera moved up close to the glass"
Same experience I have and you prove the significant better details visibility with the second picture. Good shot.
In my Sigma 500 is a simple glas element, which could be exchanged with a filter. But Sigma recommends to leave the glas in - it is part of the calculation of the lens. And it is true, if I remove the glas filter the images look somehow less sharp.