This 1.25” Astrodon Generation 2 Sulfur II imaging filter has a very
narrow 3nm FWHM (F
idth at H
aximum transmission) passband centered on the ionized sulfur emission line at 6724 Ångstroms. Transmission of this ionized sulfur line is guaranteed to be greater than 90% (typically 94-97%).
The filter rejects light outside of the desired Sulfur II 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. 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 Sulfur II 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.
The Astrodon 3nm Sulfur II filter is supplied in a 1.25” cell. It threads into any color filter wheel that accepts standard 1.25” filters. Unlike conventional filters that are assembled using a metal retaining ring in direct contact with the glass filter, this Astrodon filter uses a rubber O-ring between the filter holder’s retaining ring and the glass filter element. This gently cushions the filter in place, reducing the possibility of scratching or chipping the filter element from direct glass to metal contact.
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 Sulfur II emission line at the left edge of the flat-topped 3nm 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/4 or f/4.5 system. By way of contrast, conventional filters with a passband centered on the emission line at f/10 shift the emission line off the passband with a resulting drop in transmission as the focal ratio gets faster and blue spectral shift comes into play.
Transmission does drop off to about 80% when this very narrow/high contrast 3nm filter is used with an f/3 system, but this is offset by the shorter exposure times inherent in such a system. It is instructive to compare the Astrodon transmission with that of higher-priced special order 3nm filters from other manufacturers, who generally claim only a >70% transmission, with no specification of the telescope’s focal ratio.
Astrodon keeps this 3nm filter in stock, unlike other manufacturers who generally only make their 3nm filters on a special order basis, usually with a 2-3 month delivery time.
The filter is parfocal with telescopes as fast as f/4 and is parfocal with all Astrodon LRGB, CRGB, OIII, and H-a filters. This eliminates the need to waste valuable observing time finding the correct focus when switching between filters in a filter wheel.
In addition to monochrome images of Sulfur II emission features using this filter, many people use a combination of SII, OIII, 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." The feature image 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 SII, OIII, and H-a filters. Red represents ionized sulfur from the SII filter. Blue represents ionized oxygen from the OIII filter. Green represents ionized hydrogen from the H-alpha 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.