A cheap alternative could be the 20mm f/2.8 Arsat H.
290g & 62mm filters.
I haven't used mine much, but first impressions were seriously good (sharp corners, low distortion & vignetting), much better than the MIR 20mm lenses.
(My "seriously good" standard for ultrawides is Leica & Zeiss).
As with all Soviet & POS equipment, sample quality may vary...