Question: I'm flying soon to Mongolia for a few weeks and I'm still somewhat of a beginner to photography, at least equipment-wise! So, I'm looking for equipment advice mostly. I currently have a Canon EOS 400D with an 18-55mm lens, and I recently purchased a 50mm F/1.8 prime (can't wait for it to arrive!), but I realize that to properly capture the open plains and skies of Mongolia, I'm going to need a wide angle. 

I don't have a massive budget to spend, and to be honest, I've been a bit rough on my camera in the past and don't want to bring a long an expensive lens while I'm backpacking. Is my 18-55 going to be good enough? I'm definitely willing to purchase a new lens to take with me, and I would prefer something that would stay useful to me after Mongolia when I move to a city for a bit (heading to Sydney, Australia!).

Also, what would be the right filter to bring along? I don't know much about those and only have a UV filter at the moment.

Answer: 
I spent a while in Mongolia and frankly I think that what you (will) have is fine, with some minor tweaks.

I'd add a good quality circular polarising filter for your kit lens as well (cheap ones have weird colour casts, good ones are colour-neutral at all angles).

For super-wide shots, I was quite happy with stitched 17mm shots. I don't think wide lens is a must. MS has a free stitcher that does a fair job if you make an effort to rotate your camera sensibly close to the lens' nodal point.

Night-time photography is also possible and with the crystal-clear skies I'd heartily recommend it. Watch out for the sharp temperature changes to night-time and the resulting condensation on your equipment. Also bear in mind that your batteries will have a significantly lower charge at low temperatures so it's worth keeping your spares close to body heat. A tripod isn't absolutely necessary if you find convenient resting places for your camera.

Unless you're in Ulaanbataar you might not find convenient charging points, so battery-saving measures make sense (disable review, lower lcd brightness, keep batteries out of camera unless shooting...) and I'd consider packing some spare memory cards too.

If you've got a point+shoot camera, bring it with you - you won't always have the time to whip out your dSLR ;-)

Lastly, I'd strongly recommend lots of practice of the various shots you'll be taking so you can spend more time enjoying the experience and less time setting up shots; having a list to hand of the shots you *know* you want is also quite helpful.

Oh yes, and expect your gear to get fairly dusty, no matter how much care you take to keep it clean. Dust spots can be removed after the fact fairly easily in any case.
what about fish-eye lens? I have Samyang 8 mm for few weeks and found it absolutely fantastic for stitched panoramas and also for those "planet" panoramas.

Ken Rockwell wrote a nice review about this lens here:
<http://www.kenrockwell.com/tech/8mm-f35.htm>

the lens is not so expensive, costs about 360 USD.