Recently, I have decided to buckle under the pressure from my 'friends' for colour Schmidt shots. As conventional colour film wasn't up to the task, I considered my options. Stacking colour negatives seemed appealing at first, but my trial runs still didn't have enough contrast or colour saturation. By accident, I stumbled upon Bill and Sally Fletcher's web site, featuring tri-colour photography. After I recovered from being blown away, I knew this is what I was looking for. Some examples of this form of photography are here on my astrophoto page.

So, how does all this work? It has been known for over a century that any colour can be produced using the three primary colours - red, blue and green. If I was to shoot an object three times, each through a different coloured filter, I would be able to reconstruct a colour image. If I used a fine grained, high contrast film, such as Kodak tech-pan - the final result should be a high contrast colour image with good colour saturation. In practice, this technique works extremely well, but it does involve a lot of work! Exposure times - even at f1.5 are rather long. Red goes for 17 minutes, green for 60 and blue runs for 65 minutes. (To do this at f6, multiply the exposure times by 16!!) After the film is processed, an 8 x 10 print is made from each negative. All prints are made to the same background density using the same contrast grade of paper. Each image is then scanned, to produce a high resolution TIFF file.

The next part is what scares most sane people away - rotating and shifting the images to ensure good registration of the colours. With the tiny star images formed by the Schmidt camera (sometimes up to 50,000 stars in a single negative), registration was almost impossible to achieve. I couldn't get all the stars to line up. I then heard about a program from Auriga Imaging called RegiStar whose sole purpose in life was to register images. With this software, perfect alignment can be done in just a few minutes. I was finally over the last hurdle with tri-color, the results are the images you see on this web site.

Addendum November 20 1999 - I have discovered over the last few months that tri-color is extremely sensitive to variations in background brightness, especially with the wide field of the Schmidt. Variations in brightness cause violent color shifts in the final image. Unfortunately, I live far enough north to be affected by the northern lights. Until this current solar cycle finishes hammering us with aurora, I won't be able to shoot much tri- colour. This is due to changing background brightness levels over the 2.5 hours it takes to shoot a picture. Stay tuned, I will add tri-colour as often as possible.

Update - July 2003 - Still having background density problems! It looks like humidity might be causing random density changes in the film. Still working on it, but there doesn't appear to be an easy answer. However, I do have two examples that aren't totally hideous. They are NGC7000 and the WitchHead nebula.