Bring out the best in your nebulae astroimages with the H-alpha H-beta OIII (Hα-Hβ-OIII) Imaging Filter for the 8” RASA. This specialized filter allows high transmission of light from emission nebulae while blocking light pollution, helping to make your images pop with detail off the black sky background with superior contrast.

Fortunately, many sources of light pollution have relatively discreet emission spectra; they emit primarily at specific wavelengths of light. By strategically blocking these “bad” wavelengths, the filter essentially eliminates the effect of light pollution while letting the “good” wavelengths of light associated with the emission nebula pass through to the camera. You’ll notice much higher contrast in your images immediately, even if imaging from your light polluted backyard! And since the transmission at the key wavelengths is so high, the nebula will not be dimmed—only the background will be blacker.

Nebula Hunting

The Hα-Hβ-OIII Imaging Filter for the 8” RASA is perfect for imaging emission nebulae like the North America Nebula (NGC 7000), Lagoon Nebula (M8), Orion Nebula (M42), and the Carina Nebula (NGC 3372). The imaging filter transmits key Hα-Hβ-OIII wavelengths that produce high contrast for emission nebulae.

The transmission spectrum for the Hα-Hβ-OIII Imaging Filter is 85% or greater at key wavelengths (486nm, 496nm/501nm, 656nm) and less than 0.5% transmission elsewhere. The Hα-Hβ-OIII Imaging Filter for the 8” RASA is a narrowband filter that blocks much of the visible spectrum and only lets specific wavelengths through. Because of this, it will not work well on broadband emission objects, such as galaxies and star clusters.

Other similar imaging filters on the market have much narrower bandpasses at the critical Hα-Hβ-OIII wavelengths, but these filters will actually block much of the incoming Hα-Hβ-OIII light for an F/2 optical system. This is because the light is coming into the filter at a steeper angle, which causes a spectral shift. Using filters not designed specifically for use at F/2 will cause some of the light from the nebula to be blocked, resulting in a dimmer image. The Celestron Hα-Hβ-OIII Imaging Filter for the RASA 8 was custom engineered to work at F/2, so none of the “good” light from the nebula is inadvertently blocked by the filter.

Body and Storage

The filter is machined aluminum black anodized cell with knurled rim for easy grip during installation.  An anti-reflection coating has been added to the filter to help suppress reflections on the glass. The filter fits directly into the internal filter mount in the RASA 8’s front lens cell, where the optical window normally mounts. By removing the clear window and replacing it with the filter, optical performance for the entire system is maintained. No other adapter is needed.

Each filter comes with a foam-lined plastic storage case and retaining ring to safely store the filter or the placeholder optical window glass that came with your RASA when not in use. You will also receive a spectral transmission graph unique to your specific unit, verifying its performance.