Color Filter Wheels

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The filters are parfocal within the ability of most motorized focusers to make adjustments. Tests with a Finger Lakes Instruments DF-2 focuser showed the RGB filters to be parfocal within 5 units (only 0.0005"). This is within the error imposed by seeing and the mathematical algorithm used to determine the full-width (FWHM) pass band.

All of the filters have UV- and NIR-blocking. For best results, the luminance filter passband should match the combined passband of the RGB filters. The luminance image in LRGB imaging provides the detail, while the lower resolution color images (often binned 2x2) colorize the detailed luminance image. The photons in the Tru-Balance L (luminance) image cover the same stretch of the visible spectrum as do the RBG images. This can be seen in the response curve accessed by clicking on the “Images of Some Features" link in the “More Information" box to the right.

However, some astophotographers prefer to use a clear filter that is not NIR-blocked, to obtain as much luminance signal as possible in the shortest amount of time. However, the NIR photons collected by an unblocked wider passband clear filter do not relate to those collected by the RGB filters. This can create an artifact in which strong NIR radiating stars can appear brighter on your images than they do to the eye. Tru-Balance filters are supplied as a matched NIR-blocked LRGB set to eliminate this possibility. The Astrodon Tru-Balance L filter matches the passband of the RGB filters precisely, as shown in the spectral graph mentioned above. However, for those who are comfortable using a non-NIR-blocked clear filter, Astrodon makes a parfocal clear filter (#TBFC1) available as an option. This filter is UV-blocked only, and has >95% transmittance out to the extent of the CCD detector range, including the NIR region.

Why a 1 : 1 : 1 exposure ratio is important . . .

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 the red, green, and blue filters, exposures times vary for each of the colors. Typically, you must take a G2V star measurement through your red, green, and blue filters to arrive at the approximate exposure times through each filter to reach a white-point balance with your particular CCD/filter combination. The resulting weighting of the exposure times (for example, a 1.2 to 1.0 to 1.7 ratio) will produce reasonable star colors for galaxies and globular clusters.

However, these exposure weightings are 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.

The 1.2 : 1.0 : 1.7 exposure time weighting example means that the combination of your CCD’s quantum efficiency (QE) and the transmission characteristics of your filters is not very responsive at blue wavelengths. The blue image needs longer exposures to bring its signal-to-noise (S/N) ratio up to the level of the other colors. But, if you zoom into the image background you will see blotchy blue pixels – the result of amplifying the noisy blue data. Wouldn't it be simpler if your blue filter was as efficient at transmitting light as the red and green filters? The Tru-Balance blue filter is.

Furthermore, if you take longer blue exposures to improve the S/N, you will have to take dark frames at least as long as your longest exposure. You will then have to scale your dark frames, since the green and red images are typically shorter than the blue. You would also have to take different length bias frames in order to properly scale your different dark frame exposure times. Wouldn't it be simpler if you could take your color frames and dark frames using the same exposure times? Tru-Balance filters let you do that.

Also, many tri-color imagers using SBIG self-guided CCDs have had the experience of losing a faint guide star when the lower efficiency blue filter rotates into place, since the RGB filters cover both the internal imaging and guiding CCDs. A common response is to increase the length of the guiding exposures, but that may affect the mount's ability to guide accurately if a periodic error jump occurs midway through the longer exposure. Wouldn't it be simpler if you didn't have to worry about losing the guide star? Tru-Balance filters are designed to minimize this problem.

Lastly, CCD astrophotographers are used to reducing their color images (applying dark, bias and flat field frames), registering them to align the stars, and then applying the color combine weights, using programs such as Software Bisque's CCDSoft and Diffraction Limited's MaxImDL. Once they have created a RGB image in these programs, they save it as a TIF file with 16-bit resolution and bring it into Adobe Photoshop for final processing. They generally avoid taking the reduced red, green and blue images directly into Photoshop because there is no simple way to achieve the 1.2 : 1.0 : 1.7 G2V white-point balance in Photoshop with levels and curves. Wouldn't it be simpler if your filters were designed to be equally combined so that after reduction and registration, you could bring them directly into Photoshop and simply use the “Merge Channel" tool for RGB images? Tru-Balance filters are designed to do just that. And, with the new Photoshop CS, you can do much of your color processing (e.g. layers, masks) while staying in the higher-resolution 16-bit mode.

An examination of the spectra of the LRGB Tru-Balance filters shown in the “Images of Some Features" link will prove of interest. Notice the high transmission of the photographically important H-alpha and OIII emission lines near 500 and 656 nm labeled on the graph. Notice also the wide passband of the blue filter. This compensates for the lower blue sensitivity of the Kodak full-frame CCDs compared to green and red.

This high-efficiency blue filter is 50% more efficient than the Custom Scientific blue filter used in SBIG color filter wheels. It is also 30% more efficient than the Astronomik blue filter, which for years has been the most efficient dichroic RGB filter available to the amateur imaging market. This means that you will immediately see tremendous S/N and detail in your individual blue images as they download.

Notice also the “gap" between the green and red filter passbands. Its purpose is to minimize the effects of light pollution, primarily from the ubiquitous sodium street lamps.

Getting the correct “teal" nebula color requires that the OIII emission lines end up equally in the green and blue color channels. Notice in the graph the high and equal transmittance of the blue and green filters at the OIII emission lines with the Tru-Balance filters. This not only accomplishes the goal of correct “teal" OIII color, but does so with high efficiency. Astrodon Tru-Balance filters will produce OIII nebula colors within 8% of being balanced. No other dichroic filters come close to being balanced for Kodak -E CCD detectors, including Custom Scientific and Astronomik 2c filters (~25% too green), IDAS Type 3 and Optec (~40% too blue), etc. Schuler RGcBc colored glass filters come closest at ~9% too blue. However, Tru-Balance filters have ~90% throughput at the important emission wavelengths for OIII and H-a. The Schuler filters are only half as efficient for these emissions due to their being colored glass, rather than dichroic. Therefore, unlike the Schuler filters, HII regions stand out with Tru-Balance filters as briskly as they do with the Custom Scientific filters supplied with SBIG color filter wheels, but the Tru-Balance filters are up to 50% more efficient in the blue than Custom Scientific and other dichroic filters.

To sum up, the Tru-Balance filters have the excellent color balance of Schuler RGcBc filters, but with higher efficiency and better visualization of HII regions. Emission nebulas glow teal through the Tru-Balance filters; galaxy cores are yellow-orange, while their spiral arms are blue. The Tru-Balance filters have better color balance than dichroic filters, and higher efficiency, as well. Tru-Balance filters are tru-winners for accurate nebula colors with balanced star colors.

The Astrodon Tru-Balance filters won’t make tri-color CCD imaging as easy as using a “point and shoot" camera, but they will make imaging easier for you than it is now.