Achieving the correct color balance when doing tri-color CCD imaging has long been more of an art than a science. Due to the differing transmission characteristics of normal red, green, and blue CCD filters, exposures times vary for each of the colors. To complicate matters still further, different types of CCD detectors (full frame and back-thinned detectors versus interline detectors) have differing response characteristics to the incoming flux of light.
To get the right color balance with your particular camera, in the past you have had to take a separate G2V star measurement through your red, green, and blue filters individually (sometimes more than once) to arrive at the approximate individual filter exposure times needed to reach a white-point balance with your particular CCD/filter combination. The eventual resulting weighting of the exposure times (for example, a 1.2 to 1.0 to 1.7 ratio) produced reasonable star colors for galaxies and globular clusters.
However, these exposure weightings were not likely to produce the correct “teal” color of OIII emissions within planetary nebula. We have all seen images of the same planetary nebula (the Dumbbell, for example) where the central OIII color ranges from very green to very blue, depending on the astrophotographer. These colors are unrealistic, although often pretty. However, if you maintain the different exposure times required to achieve the correct G2V white-point balance, there is little you can do to achieve a true “teal” nebula color. Wouldn't it be simpler if you could achieve correct star and nebula colors without having to worry about weighting the exposure times? Tru-Balance filters do that for you.
Astrodon Generation 2 Tru-Balance filters simplify tri-color CCD imaging by equalizing the photon flux through each filter so exposure times are identical for each individual red, blue, green, and dark frame exposure. Rather than a perhaps a 1.2 to 1.0 to 1.7 exposure time ratio found by experimentation or trial and error, your RGB color combine weights will be approximately 1:1:1, within perhaps 10%. This can never be perfect, but it does allow you to take equal time exposures through each filter, simplifying your imaging in the cold and the dark. You can then combine the 1:1:1 color data to achieve virtually automatically a G2V white point balance for correct star colors and balanced OIII teal color in emission objects, with the least amount of color balancing needed in the computer later during image processing. Trial and error exposures, guesswork, and extensive color-balancing work at the computer become things of the past.
There are specific Astrodon Generation 2 Tru-Balance filter sets with passbands designed for either full frame and back-thinned detectors (E-Series) or interline (I-Series) detectors, depending on your camera type. Each set provides ease-of-use, high optical throughput, and great resulting color for galaxies, star clusters and nebulae.
Compared to previous filter designs, the Astrodon Generation 2 filters provide better color separation (spectra) and better color rendition for galaxies based upon color theory. They yield significant reflection and star halo reduction and enhanced contrast for HII regions in galaxies. A spectral “gap” between the green and red passbands minimizes the effect of mercury and high and low pressure sodium city light pollution, as well as airglow emission lines due to ionized nitrogen in our atmosphere. The width of the gap depends on the type of camera for which the filter set has been optimized (whether it is an E-Series or I-Series filter set).
The Astrodon Generation 2 filters are made with striae-free 1/4-wave fused silica substrates, polished to 30 arc second parallelism. They use ultra-hard and durable sputtered coatings for long life. Each is parfocal with Astrodon's high-performance narrowband (H-alpha, Oxygen III, etc.) and near-IR filters. They have the highest efficiency blue filter available, with less UV transmittance.
Previous Astrodon filters have been known for being parfocal on most telescope systems. With a thickness tolerance of 3mm +/-0.025mm (25 microns), the new Generation 2 filters are a factor of 2 better than the original Astrodon filters, and should be parfocal for optical systems as fast as f/3.5, if your refractor or camera lens optical system is well color-corrected.
Astrodon has reduced the UV contribution in the luminance and blue filters to reduce star bloat in systems that are not particularly well corrected. The clear (no near-IR blocking) filter may produce bloated stars if your optical system has poor near-IR focus. You will need to use the near-IR blocked Astrodon luminance filter in this case. The clear (no near-IR blocking) filter is likely a better choice to use with reflectors, such as Ritchey-Chrétiens, rather than for use with refractors or camera lenses.