This 50mm round unmounted Astrodon Generation 2 Oxygen III imaging filter has a 5nm FWHM (F
idth at H
aximum transmission) passband centered on the ionized oxygen emission line at 5007 Ångstroms, the stronger of the pair of oxygen emission lines that radiate at 4959Å and 5007Å. Transmission at 5007Å is guaranteed to be greater than 90% (typically 94-97%).
For maximum contrast, the contribution of the 4959Å oxygen emission line, which is only one-third as strong as the 5007Å line, is placed outside the filter’s 5nm passband. To include both emission lines, and account for the blue spectral shift in faster focal ratio scopes (see below), would require a passband in the 12-15nm range, two and a half times as wide as this Astrodon filter. This would result in a marked and very visible loss in contrast, particularly when the Moon is out (which affects the Oxygen III portion of the spectrum more strongly than it does the H-alpha and Sodium II regions).
The filter rejects light outside the desired Oxygen III passband to a mere 0.0004% transmittance from 300nm to 1150 nm. This yields a very high signal to noise ratio and high contrast, even with a bright Moon nearby. This is particularly important as the Oxygen III emission line is very susceptible to being washed out by Moonlight. In addition, the filter edges are blackened for high contrast.
The filter uses a 3mm (+/-0.05) thick striae-free fused silica substrate, rather than ordinary glass. The filter faces are polished flat to an accuracy of 1/4 wave (or better) and 30 arc second parallelism. For durability, the filter uses the hardest multiple layer dichroic sputtered coatings available. The filter comes in a hard plastic snap-lid storage case.
Unlike some other narrowband filters that use a sandwich design comprised of several pieces of glasses epoxied together, the single substrate design of the Astrodon H-alpha filter minimizes or eliminates internal reflection halos. Astrodon filters are renowned for not producing halos around stars and not leaking near-infrared light that can bloat star images.
This 50mm unmounted filter drops into the filter receptacle of any color filter wheel that accepts standard round unmounted 50mm filters, or into the color filter wheel integrated into SBIG Research Series cameras. It is parfocal with Astrodon's other high-performance narrowband (Sulfur II, H-alpha), near-IR, LRGB, and CRGB filters. This eliminates the need to waste valuable observing time finding the correct focus when switching between filters in a filter wheel.
Narrowband filters shift into the blue with very fast optical systems. This Astrodon filter is designed to have lower blue spectral shift with faster optics than a conventional filter. By placing the emission line at the left edge of the flat-topped passband when used with f/9 to f/10 and higher systems, any shift into the blue with a faster focal ratio simply moves the emission line towards the center of the passband. This keeps the desired transmission at a constant 90% or more, all the way down to an f/3 system.
In addition to monochrome images of Oxygen III emission features using this filter, many people use a combination of OIII, SII, and H-alpha filters for tri-color CCD imaging, rather than the traditional RGB filters. This allows them to emulate the amazing false-color Hubble Space Telescope images, such as the famous “Pillars of Creation." One of the feature images below (courtesy Astrodon Filters) shows the “Gulf of Mexico" area of the large North American emission nebula (NGC7000) near Cygnus. This was taken with an SBIG ST10XME on a Takahashi FSQ106N refractor using OIII, SII, and H-a filters. Blue represents ionized oxygen from the OIII filter. Green represents ionized hydrogen from the H-alpha filter. Red represents ionized sulfur from the SII filter. It is clear that the “Great Ridge" is strong in sulfur. Thus, narrowband filters can provide additional detail about the structure of emission nebula – not to mention striking images.