Narrow Band Photometry and the Reddening of CYG A
The use of narrow band (FWHM 200 Å) filters in extragalactic research is not new. Our photometric system vz, bz, yz is very similar to the Stroemgren v, b, y passbands. It has many advantages in conection with a CCD camera compared to broad band UBV photometry. These narrow passbands are less affected by emission lines than other filters in use. As example the Syfert galaxy POX 4 (Kunth and Sargent 1983) has for the B filter a total emission line width of a factor four larger than in the v filter calculated for equal spectral width. The filters b and y are not affected by emission lines, except for the very faint He II 4686 Å line and the blend Ar IV and He I 4711 Å. The v bandpass is only affected by H delta.
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