Exchanging the filters wasn't too bad with the Hyperstar filter drawer. But I can't do it easily with the dew shield on the scope - so, I had to remove it. I'm a little bit afraid if this will bring in some stray light from the side...
Taking flats turned out to be a major pain in the ... I tried with my flat panel, but it was too bright. And even with the dimmer I could not get them dark enough (I later found out about grey filters from Neumann that you can put over the flat panels to dim them). So, I took flat frames at dusk. It took me two evenings to do that as the hyperstar lens is so sensitive, that by the time I was done with 2 series of flats, it was already too dark and too many stars were coming through.
I then went into CCDStack and processed the images. During processing, I could already see that the flat frames weren't very good (they didn't neutralize the background evenly). And that's why the final picture looks like this:
Looking at the flat frames:
By far not perfect, but much better.