I started to research Tri-color astrophotography because of some of the great pictures I have seen that were created using this technique. One of the best places to see tri-color astrophotography is at Chuck Vaughn's Astrophotography web page.
The process involves taking multiple pictures of the same subject (using black and white film) through three different filters. Each filter is designed to let only certain wavelengths of light to reach the film. These wavelengths correspond to the visible spectrum colors of red, green, and blue (RGB).
Once you have three black and white shots you can use a graphics program like Adobe Photoshop to combine each channel (red, green, and blue) into a single, color, graphic. The results of this technique are astrophoto's that have great detail and stunning color range. An example of tri-color combination is shown below.
Black and White through Red filter
Tri-Color combination of the three black and white pictures shown to the left.
(Special thanks to Chuck Vaughn for letting me use his Tri-color Horsehead picture for this article.)
Black and White through Green filter
Black and White through Blue filter
Note that the black and white picture taken through the blue filter has more sensitivity to the brighter stars. This is because the blue filter allows wavelengths down to 350 nanometers to reach the film (Hypered Kodak Tech Pan 2415) which is sensitive to these ultra-violet wavelengths. This sensitivity is evident in the Tri-color combination and is represented as a blue halo around the brighter stars. Some would say that an ultra-violet filter should be used to remove this halo but I personally like the effect.
That leads me to a discussion on filters. If you could design perfect filters their transmittance/density curves would look something like the graph below.
These curves are ideal because at any wavelength, the transmittance percentage adds up to 100%. All peak transmittance percentages are the same (100%) so that exposure times are as short as possible and are equal. This means that exposure times through each filter will be the same. Also the OIII emission wavelength (500.7 Nm) is equally distributed through the blue and green filters. The problem is, of course, that these curves are ideal and look nothing like real life transmittance curves.
The graph below was generated using Kodak transmittance data for their #25 (red), #47B (blue), and # 58 (green) gel filters.
As you can see, only the red transmittance curve even approaches 100% transmittance, and there is only a small amount of overlap in the different filters . This means that the wavelengths of light in the valleys will not be equally represented in the final RGB image (and note that one of the valleys lie directly on the OIII emission line). Granted, the final product may look very good but one of the main objectives of tri-color astrophotography is to try and represent the true colors of the subject being shot. The Kodak Gel filters do have a couple of things going for them. The filters are very thin (1mm) and can be placed in the light path without changing the focus. They are also readily available and inexpensive.
The graph below was generated using Schott Glass transmittance data for filters created using combinations of their different glass filters. The blue filter was created using a combination of Schott's BG12 (1mm) and BG40 (1.25mm). The green filter was created using a combination of Schott's BG39 (1mm) and VG9 (1.5mm). The red filter is a single 1.75mm OG590 filter.
The transmittance curves for the filters created out of Schott glass are much closer to ideal. Transmittance percentage is much higher and overlap is much better than with the Kodak filters. The exposure times through each filter will be slightly different due to the higher transmittance percentage with the red filter. The down side to this approach is expense. These would need to be created by a skilled craftsman.
The graph below was generated using data that was sent to me from IDAS Filters of Japan (via Hutech). They are being sold by Hutech Astronomical Products in the United States.
When I first went looking for Tri-Color filters it seemed that there were some acceptable filters made for CCD use (1.5" filters) but the larger sizes needed for film astrophotography were not available. I am happy to say "I finally found some". These transmittance curves are the closest to ideal of any that I have ever come across. Note the higher transmittance percentage (even compared to the Schott glass filters) and even overlap between the filters. These filters are available in various sizes including the all important 48mm size. IDAS also has filters in the Ultraviolet and Infrared wavelengths that may interest some astrophotographers. To see the full filter transmittance/density curve and data for the IDAS filters Click Here. Another thing to note is that IDAS also makes a great Light Pollution filter for color astrophotography. This filter is rapidly replacing the Lumicon Deep Sky filter in many astrophotographer's tool belts. I really recommend a visit to the Hutech site. It has more great information and products that are worth exploring. Let Ted Ishikawa (Hutech's owner) know if there is something you would like to see at his site. He is looking to expand his product list with those hard to find items.
I came across this information while looking into doing tri-color astrophotography myself. I will be purchasing the IDAS filters as funds become available. I will add to this article when I fully test the filters in person. For now the IDAS data is very impressive and is the best I have come across in all my searching.
I would like to credit Chuck Vaughn with explaining most of this information to me through e-mails and his web page. If you wish to explore tri-color astrophotography in greater detail, I recommend visiting his web page.
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