Astrodon 36mm Unmounted Gen2 E-Series LRGB Filters (set of 4)
Astrodon Tru-Balance RGB filters have revolutionized CCD imaging. Their popularity is due to ease-of-use, high optical throughput and great resulting color for galaxies, star clusters and nebulae. There are two varieties of Tru-Balance filters - E-Series and I-Series.
E-Series filters are designed to approximately equalize the flux of Kodak's full-frame (E-Series) CCD detectors, including compensation for the solar photon flux. This means that 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 for your RGB data and also just one corresponding dark exposure time.
Generation 2 Tru-Balance filters (LRGB2) were introduced in April, 2008 in all sizes. This was the first change in Astrodon filter design and fabrication since their inception. We listened to your requests.
- 1.25" mounted, 31, 36, 49.7 mm dia., 49.7 mm square
- ~1:1:1 color combine weights for G2V white-point for Kodak Full-Frame (E) detectors
- Best choice for backthinned and Sony detectors
- Equal RGB exposures and one dark time
- Better color separation (spectra)
- Better color rendition for galaxies based upon color theory
- Significant reflection and star halo reduction (see Alnitak image)
- Enhanced contrast for HII regions in galaxies
- Spectral "gap" to minimize effect of light pollution
- Highest efficiency blue filter with less UV
- Correct "teal" OIII color for planetary nebula (OIII signal >97% in blue and green)
- More parfocal - thickness 3+/-0.025 mm*
- Parfocal with Astrodon's high-performance narrowband and near-IR filters
- Striae-free 1/4 wave fused silica substrates
- 30 arcsec parallelism
- Proudly made in the U.S.A.
Do not force-fit filters designed for interline CCDs to these full-frame CCDs. Here are some G2V color weights taken with Baader filters on full-frame CCDs:
R:G:B 0.84:1.00:1.25 SBIG STX16803 Mr. P.
R:G:B 0.7:1.0:1.4 SBIG ST10XME Mr. A
and compare those to a recent measurement with Astrodon E-Series filters
R:G:B 1.0:1.0:1.0 Apogee U16M Mr. F.
This has siginficance for imaging planetary nebula, where, after applying the G2V color weights, the resulting signal near 500 nm in the blue and green color channels should be equal or nearly equal to produce the correct blue-green or teal color. This is the case for the Astrodon E-Series on the 16803. However, the %T at the OIII wavelength is equal for the Baader filters. When a blue weight of 1.25 or 1.4 is applied, this will result in planetary nebular core being 25 to 40% too blue, unless this problem is corrected in localized post-processing. This is the reason why Astrodon offers E- and I-Series filters. They simplify your imaging.
The E-Series filters were designed for cameras containing the full-frame KAF8300, KAF3200, KAF6303, KAF16803 sensors. The E-Series red filter is a deep red filter about 60 nm wide, which excludes much of the light pollution from high pressure sodium street lamps. Other filter suppliers try to "force fit" their orange filter (designed for the interline KAI11000, KAI2000 sensors) for use with these full-frame sensors. Their orange filter is about twice as wide spectrally as our deep red filter. As a result, their orange filter tends to "wash out" HII regions in galaxies and detail in nebula. Their blue and green color weights are no longer balanced and as a result, the OIII color in planetary nebula is skewed strongly toward blue, rather than the correct teal color. These are the reasons why Astrodon designed I- and E-Series filters for these very different classes of cameras for optimum results.
Our filters are made from the highest quality, striae-free, <1/4-wave fused silica substrates with the specifications listed above. They are coated and cut from larger plates. This has an important advantage over filters that are coated individually in that our filters are coated to the edge. Individually coated filters, as shown with the orange filter above, leaves an uncoated "rim" near the edge that needs to be blocked in the optical path. This becomes critical for narrowband filters. Since our coatings utilize hard-oxide material, edge sealing is not required. There are some claims that cutting filters from larger coated plates somehow degrades the optical quality. They present no evidence, because this is simply not true. Most optical coating companies have to size their filters to meet a variety of shapes and sizes. It is common practice.
Nevertheless, we would like to present data on one of our 50 x 50 mm square E-Series red filters. First, after coating, we measured the transmitted wavefront, TWF (a measure of flatness using the number of fringes at 632 nm) to be at most 0.176 over the entire part. That is less than our 1/4-wave (0.25) specification. So, coating and cutting from larger plates do not alter the flatness. Second, power (a measure of curvature that would create a lens effect) is measured at only 0.09 fringes. So, the coatings are very uniform. Third, the spectral variations of the red bandpass are at most 0.02% or 1 nm over the 50 mm part, meaning that your red data will be the same over your entire image. In summary, you can be assured of the highest optical quality available when you purchase Astrodon filters.
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