Re: Seagull Nebula,
I have just three old observing notes with the 6" F/5 achro, OIII filters, and low magnification eyepieces from Swarovski and others. The full extent of the Seagull has been seen.
Good reason to make a revisit with the H-Beta around the next X-Mas, when I will be in the constellations of MON and CMA, anyway.
Re: H-Beta filter, and other nebular filters
There is lots of strange statements regarding the filters use, not supported by any in-depth observing experience. And as with anything, just a casual use of the filters may lead to questionable conclusions, sometimes reported.
Re: The blue shift on the nebular filters
This is easily checked with my small Zeiss spectroscope (Handspektroskop), when tilting the filter in front of the spectroscope slit.
The Baader OIII 10nm seems to be remarkably less affected due to the blue shift.
The Baader H-Beta 8.5nm shows through the spectroscope clearly the H-Beta absorption line in the Sun spectrum, and the line is within the passband during the tilt.
The Astronomik H-Beta 12nm filters stacked as a pair, keep again the line within their passband of around 9nm.
The tilt corresponds to +/- 30°, or about 60° AFOV.
Herewith, you can mount the filter behind the eye lens on the binoculars, why not.
The blue shift depends also on the design of the interference cavities in the filters.
Some filter designs from the other manufacturers add to the blue shift a more pronounced widening of the passband, and an enhanced leakage of light on the red and blue wavelengths.
And on the fast telescopes, and through the wide to ultrawide eyepieces, the leakage may show the bright stars in purple colors.
Thank you Cristopher for your very helpful reports, and experience shared,