B&H is discounting the Rokinon 8mm Fisheye by $40 in their 4th of July deals:

Rokinon 8mm Ultra Wide Angle f/3.5 Fisheye Lens FE8M-P B&H Photo

Final price: $239 - $40 = $199!

In-depth comparative review with other ultra-wide options for Pentax:
Ultra Wide Angle 4-Lens Showdown Review - Introduction | PentaxForums.com Reviews

[UncleVanya]

I'm a fan... that's my lens for this month's "single in" group. Sharper than I ever expected and just fun fun fun.

[robgski]

I bought one on PF a few months ago, it is a great lens, and the price makes it even more incredible

[ramseybuckeye]

I acquired one as a service award at work, I never really wanted a fisheye, but I love it. I'd highly recommend it, great price!

[Biro]

I've got the Pentax DA 10-17. I wonder if this 8mm would be worth the additional expense.

[aritra]

10-17 or 8mm? which one is better?
I have 16-85 as my widest

[richandfleur]

ThoUghts on this for Astro work?

---------- Post added 06-30-16 at 09:32 PM ----------

Details seem contradictory:

- Manual focus lens with no electronic connections to the camera for automatic focus nor metering.

- This lens features a mechanical aperture coupling that permits the use of aperture priority and automated exposure modes, as well as the ability to control the aperture setting with Pentax DSLRs

[UncleVanya]

ThoUghts on this for Astro work?

---------- Post added 06-30-16 at 09:32 PM ----------

Details seem contradictory:

- Manual focus lens with no electronic connections to the camera for automatic focus nor metering.

- This lens features a mechanical aperture coupling that permits the use of aperture priority and automated exposure modes, as well as the ability to control the aperture setting with Pentax DSLRs

This is entirely accurate. The way A series works uses passive pins that indicate maximum aperture and when you lock the aperture at the A point it engages a mechanical pin that shorts a contact on the body. At this point the camera can control the aperture by how far the aperture lever is moved. So fully mechanical automation and no electronics inside.