Homofocal filters not genuinely homofocal [Deep Sky] Acquisition techniques · Markus Ohlig · ... · 4 · 211 · 0

I think this is quite normal. My Antlia filters are parfocal as well, but there is still some shift between them. Doesn't matter when I take flats, but it is noticeable during sub exposures if I don't refocus between each filter swap. It's been manageable on comets where I don't refocus after focusing with L, but I wouldn't advice it on regular DSO's. 

Most filters have some minor room for error which should be specified by the maker, for mine the error is about 0.05mm. Also temperature shifting will affect the optics and thus affecting your focus. As long as the filters are the same thickness that is what's most important as that will affect your backfocus distance.

When using NINA you could work around this by using filter offsets, with the Asiair you cannot though. I think generally filters are called parfocal if they are of equal thickness, but actual parfocality depends on your entire system as some optical systems focus the color wavelengths at a slightly different position. I think at least optical instruments using lenses, like refractors or regular lenses, are especially susceptible to this due to residual chromatic dispersion and are unable to focus all the wavelengths at the same position - although it can be pretty close.

Even if your filters are perfectly parfocal (and they probably aren’t) your telescope won’t be. You’ll need to focus after switching filters and/or use a filter offset.