Astrodon Tru-Balance Generation 2 E-Series - LRGB 31mm Round Unmounted Filters
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.
The E-Series is designed for the red-sensitive Kodak full-frame detectors with the goal of equalizing the fluxes, and hence, the signal-to-noise, for the red, green and blue filters. This simplifies your imaging, because it allows you to take the same length exposure for each color and only one dark frame exposure time. You will appreciate this since each 16803 FITS file is ~32MB in size. Within ~10% your color combine weights based upon a G2V (sun-like) star white point should be 1:1:1, making processing much easier.
Generation 2 filters are well known for minimizing or eliminating halos around bright stars due to their construction. The bandpass and blocking layers are all placed on one side of 1/4-wave fused silica glass substrates. The other side has the highest quality anti-reflective coating. As a result, there is virtually no "cross-talk" between the two sides, thereby eliminating halos. We do not need absorptive glass substrates that lower the transmission in order to minimize halos. We keep performance at a maximum. All coatings are made with durable hard oxides using the latest sputtering deposition methods.
The E-Series filters are also designed to minimize the leading source of local light pollution from high-pressure sodium street lamps. Their yellow color occurs in the 580 to 600 nm region. This is the region in the E-Series BETWEEN the green and red filters that is not passed to the STX detector.
The high-performance luminance filter blocks both UV and Near-Infrared (NIR) light, and is supplied with the RGB filters as an LRGB set. Astrodon also offers an optional Clear filter that is similarly UV-blocked, but allows NIR light to pass.
Lastly, Astrodon pioneered filter sets with two different formulations for red-sensitive Kodak full-frame detectors (Astrodon E-Series) and less red-sensitive interline detectors (I-Series). The E-Series has a much deeper red filter (left) with a bandpass of about 60 nm, whereas the I-Series red is an orange filter (right) with a bandpass of about 110 nm.
Other filter suppliers only offer LRGB filter sets that contain an orange filter. They were most likely designed for the popular SBIG STL11000 (Kodak Interline KAI 11000 detector). As a result, HII emitting regions in galaxies tend to get "washed out" with the orange filter. Conversely, most imagers who use the Astrodon E-Series filters with the deep red filter report HII regions in galaxies that "pop out". They do not need to augment their galaxy images with H-alpha.
Finally all Astrodon filters are coated on 3 +/-0.025 mm substrates and will likely be parfocal on reflectors (f/4 or slower). Systems with refractive optics, such as refractors and SCTs, all have some degree of focus difference between red, green and blue. This tight thickness tolerance in Astrodon filters will therefore not make the existing focal differences in the optical system any worse.
Astrodon Tru-Balance Generation 2 E-Series Filters Features
- RGB filters designed to produce true color for galaxies and stars based upon a G2V (sun-like star) white point.
- Equal exposure time for each RGB filter leads to equal fluxes and equal color weights (combine ~ 1:1:1).
- Correct blue-green color for OIII in plane-tary nebula with 1:1:1 RGB color weights for correct star color.
- Two LRGB designs; E-Series for full-frame, and I-Series for Interline CCDs.
- Deep red filter in E-Series brings out HII regions in galaxies and better definition of H-alpha in emission nebula.
- All filters transmit an amazing 98% in their passbands for the highest perform-ance available.
- All filters are UV– and NIR-blocked. No extra filters are needed.
- Breakthrough design eliminates halos around bright stars.
- Hardest optical coatings on 1/4-wave fused silica substrates for durability.
- Coatings extend to the edge (no uncoated rims that need to be blocked) and no edge-sealing is required.
- Light pollution suppression included in the design.
Astrodon Product Number: LRGB2-E31R