ASTRO: M81 LRGB

Hi everybody,
i was on holydays for a few days but unfortunately the weather was awful
onthe french riviera, quite unusual here. Anyway, i succeeded to image
between the clouds on friday night with a big moon high in the sky.
i have received my astrodon lrgb filters for my "I" chip, they are sold to
be parfocal, i noticed this was not really true as r,g and l are very near
for the focus the blue one needs some focus correction, did you notice the
same thing or is it a defect of my filter ?
Here is a classic M81 6X20mins for lum unbinned and 20 m each color unbinned
too, ST2K at F/D5 with lumicon focal reducer mounted on meade 8" SCT,
autoguiding with cam, mount is an old but great working vixen SPDX with
skysensor 2000PC.
The full process of the pic includes a ddp stretching.

Merry Christmas for you.

Patrice BOYER