It depends on the kind of astrophotography you're doing
The other comment covers the zoom aspect of it
Whereas for landscape astrophotography I use a mixture of 3 lenses mostly
This Panasonic 25mm f1.7 lens is pretty great for tighter shots with a lot of viewable stars: $150
This Laowa 7.5mm f2 lens is pretty great for really wide shots: $500
This Laowa 17mm f1.8 lens is pretty great for a middle ground: $150
EDIT: Prices and spleling.
Hi u/samzkri90 - for a 22mm FOV (35mm equivalent), I recommend the $499 Venus Laowa 7.5mm f/2 rectilinear lens ($599 for the 7.5mm T2.1 cinevized version).
Here are a few examples of the image quality this lens can produce when mounted on the OG:
Trešnjevka at night - BMPCC OG with Laowa 7.5mm f2 by SEKILE (1080p)
OG BMPCC + Laowa 7.5mm T2.1 | A different road trip across Canada by PL Arseneau (1080p)
Original BMPCC + Laowa 7.5mm F2 - Shooting Raw video by Jose Torralba (upscaled to 4K)
BMPCC (OG) in 2021with LAOWA 7.5mm T2.1 by NimbusPix (1080p)
This is a native micro four thirds lens, so no adapters or speed boosters would be required.
Hope this is helpful and good luck finding the right wide angle lens for your needs!
Hi u/sergiothequick - it depends on your budget and whether you want an auto or manual lens.
Auto
Manual
The widest manual lens is the $499 Venus Laowa 7.5mm f/2 - but if $499 is too much of a stretch for your budget, I recommend the $199 SLR Magic 8mm f/4 instead.
This lens can produce results like this, mounted on the OG BMPCC:
If you really need the extra 0.5mm of focal length and 2 stops of light - and can afford the $499 price tag - get the Laowa. Otherwise, the SLR Magic 8mm f/4 is a a very good value for your money.
Hope this is helpful and good luck finding the right wide angle lens for your needs!