MKN 421 (= MRK 421 = UGC6132 = CASG1364 = 3A1102+385 and many other
designations using its coordinates) was listed in a catalog
of blue objects off the Milky Way
by the Armenian astronomer Markarian about 30 years ago.
It is a BL Lac object; in some BL Lacs the nucleus is so bright that the fuzzy blob of the galaxy is not visible; for MKN 421 the galaxy does show as a fuzzy outer fringe. Still, the nucleus contributes a large fraction of the light in the visible, and in the UV.
In polarization, in the visible MKN 421 varies widely, from near zero, to about 10%, and rapidly, on time scales at least as short as days.
The object is so faint that the automatic tracking device could not keep it on the aperture for the whole observation, thus the observation is not as good as for bright stars. We found it necessary to calculate polarization as an average over the whole WUPPE wavelength range:
Wavelength range 1619-2837 Angstroms, polarization 3.7% at 169
This is similar to polarization reported in the visible, but we have no simultaneous ground based observation for comparison.