[bojan]

I just repeated the (visual) measurement with Energy saving lamp as reference.
The line I was monitoring (650.5nm) is marked with ellipse on the attached annotated spectrum of the lamp (it is quite faint).

Summa sumarum: Without Rocolax filter it is visible, with filter it is not.
Conclusions;
- I have made a mistake with first measurement reported earlier (it is not easy to pinpoint the cutoff wavelength visually)
- Rocolax filter is NOT good for Halpha (contrary to manufacturer's claim and supplied transmission curve) (I contacted them and asked for advice)