Closer studies showed that film tended to peak in performance in the -30 to -40C temperature range, which ruled out simply using ice to chill the film. Dry ice (frozen CO2 gas) weighs in at about -79C which is a bit too cold for film, but due to heat losses in most cameras the film usually sits somewhere near -40. One minor problem with cooling film this low is that it attracts moisture like a magnet! Within seconds frost forms on the film and inside the camera. Regular camera bodies would soon rust if adapted for dry-ice photography, or become unreliable when running so cold. So, a simplified camera body was needed, as was a way to keep air from hitting the film.

Early model cold cameras used one of two methods to keep the frost problem under control. The camera was a simple box, with an optical window at one end and film pinned against a cooling plate on the other. If the camera body was packed with dessicant, in theory there wouldn't be any moisture left to condense on the film. Another alternative was to pull a vacuum on the camera, again removing moisture from inside the camera body. This approach was best suited to the permanent observatory. A common problem with both of these approaches was that the film could droop away from the cooling plate, creating 'warm' spots on the film, and often dramatic changes to image density. Eventually someone hit upon the idea of pushing an optical plug in contact with the film, preventing airflow around the film and stopping condensation from forming. Early models (such as the Celestron/Williams model) used glass optical plugs. Glass has relatively good thermal conduction, which meant that within a matter of minutes condensation would form on the telescope side of the optical plug, which is exposed to the night air. For years, people used electrical heaters to warm the plug and keep it clear. Finally, someone used plexiglass for the optical plug which has much lower thermal transfer characteristics, and it now took an hour or more for the optical plug to fog over. With this, the modern cold camera was born.

A Comparison of Ambient vs. Chilled

Ambient at approx -5C	Chilled to approx -40C
6" f4.5 with 120 format Ektachrome 400, 30 minutes	6" f4.5 with 120 format Ektachrome 400, 10 minutes

• The shutter assembly - a 1/4" thick manual shutter, which can be closed for clouds, headlights and then re-opened when safe. The aluminum part on the left is threaded for filters.
• The optical plug - 1.75" diameter, machined from 2" plexiglass sheet.
• The camera body - a simple lightproof plexiglass box, which uses a re-useable plastic film cannister to roll the film into. Film is pressed against a brass cooling plate in the lid of the camera by the optical plug. The other side of the cooling plate is the dry ice chamber, with a threaded plunger as seen in the photo and cut-away drawing.
• Lower left, the knife-edge focusing tool. This plexiglass disc sits on top of the optical plug for focusing, focusing is done with the beveled edge running down the middle of the disc. (Sorry! I won't be getting into the details of knife edge focusing here!)
• Eyepice adapter. This is handy if you want to see where the camera is pointing! I find a 36mm plossl makes a great finder eyepiece.
• Odds'n'ends - the scotch tape is used to secure the film to the re-useable film cannister. (your take-up reel) The camel-hair brush and compressed air are used to keep the optical plug surface clean.

To start with, you will need a polar aligned, equatorially mounted scope with your choice of aperature, focal ratio, etc. If you plan on using a filter, attach it to the shutter assembly now. Attach the shutter assembly to the focuser, or to the off-axis guider if you use one. Put your favourite 1.25" widefield eyepiece in to the eyepiece adapter, put the adapter in to the shutter assembly and open the shutter. Find your target, and set up your guidestar as usual. Next, swing your scope to the nearest naked-eye star for focusing. Remove the eyepiece holder, and insert the optical plug that you plan to use. Optical plugs can vary in thickness by several thousandths of an inch, it is very important that you focus with the plug you will be shooting with. Gently put the focusing tool over the optical plug, and focus the scope. Put the eyepiece back in, and re-acquire your target and guide star. Remove the eyepiece, insert the optical plug and close the shutter! (voice of experience here!)

The next step is to ensure that the top surface of the optical plug is clean. I use a soft camel hair brush and a can of compressed air to clean the surface. Don't shake the can when spraying, you can shoot liquid propellant on to the plug if you are not careful. Next I shake out some dry ice into the Thermos 'cup' and crush it up a bit. Fill the dry-ice chamber with crushed ice, and quickly screw down the spring loaded plunger assembly. Remove the bottom cap from the camera (in total darkness) and loosly put the camera over the shutter assembly. Advance the film a couple of frames and then tighten up the camera to the shutter. Over the next minute or so, creaks and groans come from the camera and ice as everything settles down to working temperatures. Tighten up the dry-ice plunger, reposition the guide star and when everthing is ready, open the shutter.

After your exposure, there will a white ball of frost sitting on your scope with a plunger sticking out of the top! If you were to try to advance the film now it would shatter, and water would condense on the pieces. Now, the camera must be warmed up. Start by removing the shutter assembly from the focuser, and remove any filter you may be using. Unscrew the plunger assembly, and tip any unused dry-ice back in to the Thermos. Grab the nearest hair dryer, (12v or 110v) and point it into the dry-ice chamber. After a few minutes, the frost will have melted from the camera body, and the frost on the cold plate will turn to water. At this time lift the camera body straight up from the shutter about 1/2 inch, and advance the exposed film into the film cannister. Lift the camera off the shutter, and replace the cap. If you have only one plug, heat it up so that the ends don't fog up. Put the shutter back on the scope, and you are ready to go again.

Believe it or not, this whole process is easier to do than it is to write down! It doesn't take long to develop your own style.

I always carry four optical plugs with me, it speeds up the process by not having to heat the plugs up after heating up the camera. Eventually, no matter how careful you are with the focusing tool, the plugs will scratch. Small surface scratches don't cause much trouble, as they are in contact with the film and can't scatter light very far. Deeper scratches will cast shadows on the film. Jack Newton put me on to 'Novus Plastic Polish - #2' to remove fine scratches. You will need two soft cloths, one to polish the surface, the other to buff it. Jack also advised that toothpaste will also remove scratches, but I have never tried it.

I haven't shot the cold camera since 1996, films have changed during this time. I will publish newer data if and when I get it. If anyone has any data, I would love to publish it here. I am just one guy, there is only so much testing I can do! Plus, as I now use CCD cameras, I don't have much urge to test new films.

Further Reading

• The Guide To Amateur Astronomy by Jack Newton and Philip Teece, 1988, Cambridge University Press.
  ISBN 0-521-34028-4
• The Cambridge Deep-Sky Album by Jack Newton and Philip Teece, 1983, Cambridge University Press. (more of a picture book than camera info.)
  ISBN 0-521-25668-2
• The Messier Album by John Mallas and Evered Kreimer, 1978, Sky Publishing Corporation. (Another photo album, of cold camera shots)
  ISBN 0-933346-04-2
• Astronomy Magazine February 1977 - cold cameras part 1 - Richard Berry
• Astronomy Magazine March 1977 - cold cameras part 2 - Richard Berry
• Astronomy Magazine February 1981
• Astronomy Magazine November 1991 - The Newtonian advantage, by me! - Some cold camera shots and exposure data. (picture quality on pg 71 is poor)