When shooting hypersensitized film, especially in humid conditions it's essential to have some kind of system in place to keep the film dry during long exposure astrophotography. A good alternative to constructing a very expensive NO2 purge system is a Dry Air purge system. I constructed this system for about $70 using a cylinder of desiccant purchased from Drierite. I used fittings purchased from a local hardware store to step down the hose size from the cylinder to the size of model airplane fuel line. I then drilled and inserted a brass tube into the T-Ring that connects my camera to my Off Axis Guider. I use a battery powered air pump that's normally used for aerating live bait wells for fishing. It pulls in outside air, pushes it through the desiccant canister, and into the camera and keeps the film dry.

The main problem with shooting hypersensitized film is that it is very dry. This is a good thing for astrophotography because moisture causes resprocity failure and the film stops collecting light. This wouldn't be a problem for "normal" photography. But for astrophotography when your out all night in the cool night air it can cause ruined photographs due to film creep. As the evening temperature cools down near the dew point, your camera and equipment cool off. As dew collects, moisture gets inside the camera. Since (hopefully) your shooting hypersensitized film which is very dry, the film soaks up moisture like a sponge. Your film can creep inside the film carrier causing streaked photographs. Pumping dry air into the camera during exposures solves this problem.

I've used this system successfully with: hyped PJ400, LE400, PJM400, RG200 and Tech Pan 2415. It's a good alternative to what would otherwise be a very expensive system.