RF 50mm f1.2 users (and probably EF version as well), do you use a protective filter on it?

Franz Kerschbaum wrote:

And now the simple question: Why are such filters not part of the original lens?

Why even bother installing filter threads on a camera (with clearly marked filter sizes printed on the lens) at all?   I seem to recall that your observatory equipment uses (and/or has tested) special filters in order to maximize the light transmission of certain wavelengths.  The same can be said for conventional photography.

Obviousky because of disadvantages... No optical engineer would add an additional plan parallel element without critical need since it will degrade contrast and adds flare. 50% of our calls concerning UFOs or strange nocturnal light phenomena at my observatory origina from pics taken by people with front filters. Its always my first question. When I hear frpm a second moon or a unknown "comet".

Well - when it comes to UFOs (and I've seen some amazing UFOs whilst in the company of pilots in the past that defy logical explanation) that's because some people are incapable of simple reasoning deduction when assessing something unfamiliar to them.   I've attempted to photograph UFOs on three such occasions but was prevented from successfully doing so by circumstances at the time.  The only thing that interferes with my enjoyment of the night sky is Elon Musk's pesky StarLink satellites.
.

The image on the left was taken with what presumably cost millions of dollars to install on a mountain in Chile.  The image on the right was taken with a UV filter on the lens and is an unprocessed JPEG with an exposure time of 6 seconds. I was in a rural landscape at the time.
.

Someone should tell Canon to stop this.

.
It's also why Canon manufacture protective filters that perform no other purpose than to protect the lens.  It's also why Canon instructs the purchasers of certain "sealed" L-series lenses to "fit a filter to the front end of the lens to complete the seal".  It's not a verbal suggestion, it's actually printed on the instruction manuals supplied with the lenses.
.
When I purchased my RF 85mm f/1.2L USM lens earlier this year, I was immediately asked if I wished to purchase a filter to "protect it". Normally, this would be considered a normal "add-on" worthy of a sales promotion by the server.  But we chatted for a while about the practical benefits of using a filter to protect the lens itself.  It comes down to choice.  Some people won't use a filter.  Others might need to protect it from the elements and the harsh realities of travel. I needed TWO filters.  One for polarizing the light (which adds another advantage of allowing me to shoot with the aperture wide open in bright daylight... something the lens cannot do at f/1.2 by itself) and the other for protecting against splatter... which is something I'm not interested in hahving to risk my lens coatings over.  Salt air and food oils always ends up on my lens if I don't wear a filter.  And I'm sure Canon didn't anticipate the front end of an expensive EF lens getting etched by a stranger's diamond engagement ring.  But that's the risk we take.  I chose to lower/eliminate that risk by fitting a high quality optical filter that allows 99.7% of the light to pass through with the added benefit (and price) of anti-flare coatings and anti-reflection rims.  Your experience/requirements may differ.