Maintaining your night vision is an important factor for night photography. If you want to fully take in the natural scene before you and be able to identify faint stars and the milky way, you will want to maintain your low light capability. The cones in your eyes quickly adapt for basic night vision, fully adapted in as little as 5 minutes. The rods on the other hand can take up to 45 minutes to fully adapt, and these are the key to seeing faint detail in the night sky.
Many people are aware that red light helps maintain your night vision, and is commonly used in the astronomy world. I've found my night vision to be fairly impaired using the red lights that are on my headlamps though, so I decided to do some digging to find the real answer.
One of the first articles I came across was Night Vision - The Red Myth, click through to really nerd out on the science behind it.
Short version: most red lights are entirely too bright, and affect your eyes just the same as a white light would. But, a very dim red light does preserve your night vision very well. So I went on a search for the perfect flashlight that is dim-able to very low levels. It took some digging but I found the holy grail.
This little beauty is adjustable down to extremely low levels, and when you need it can get fairly bright. Having the choice between red and white is a nice feature too.
It's fairly easy to use hanging it around your neck on the lanyard, but at times hands free use would be nice, so I will likely rig up a velcro solution to attach to my headlamp and have the best of both worlds.