M81

That's correct, it was a test outing after a long break caused by bad weather for astrophotography. I cleaned the optics before the season and slightly overtightened the mirror in its frame. That's why I hesitated for a long time to post the accumulated data.

Please share how you washed it. Did you soak it in a soapy solution? I understand that even gently rubbing the mirror is not allowed. Or is it not that scary?

You know, it's actually very simple. In advance, you need to take a bulb blower for cleaning optics, distilled water—about a liter—and Fairy dish soap; I use the clear one. First, I rinse the mirror with warm water, then apply Fairy on top and gently spread it over the entire mirror with a washed hand—the surfactant effectively neutralizes the grease from your hands. I try not to touch the area with the coating mark. Then, at an angle, I rinse off the Fairy with regular water. I finish with distilled water, which has no salts in it, place the mirror on absorbent wipes at a 90-degree angle, and blow the remaining droplets toward the edges. That's it. The devil isn't as scary as he's painted.

Thank you! I'll give it a try )

You can also carefully use tissues to collect water droplets without touching the mirror. Nowadays, factory-made mirrors and those crafted by individual professionals come with protective coatings, so there's no need to worry. My practice has been using this cleaning method for years, and I haven't noticed any scratches, peeling, or degradation of the reflective coating.