Why is this Chroma SII filter "optimized for f/3"?
Fast telescopes (like an f/3 system) produce a steep cone of light that shifts a standard filter's passband towards shorter (blue) wavelengths, causing it to miss the target emission line. This Chroma filter is designed with a slightly reddish center wavelength. When placed in an f/3 light cone, this intentional offset causes the passband to shift perfectly onto the 672.4 nm SII line, with a measured 98.6% peak transmission at f/3.
How will this 3nm f/3-optimized SII filter perform on the Soul Nebula (IC 1848) with my f/2.8 astrograph?
It is a good match. The Soul Nebula has SII emission, and f/2.8 sits at the fast end of the filter's f/2.8 to f/3.6 design range. The 3 nm bandpass gives high contrast, separating the faint, wispy sulfur structures from the background skyglow and dense starfield. Measured data puts the passband centre at 672.0 nm at f/2.5 and 672.4 nm at f/3, so it stays close to the SII line at your focal ratio.
Can I use this f/3-optimized Chroma filter on my slower f/5 refractor?
Yes, you can, and it will still perform very well. In a slower f/5 system, the blue shift is minimal. This filter will have its passband centered slightly red of the SII line at 673.0 nm, but with a peak transmission of 99.9%. Given the filter's 3.6 nm measured FWHM at this f-ratio, it will still capture the vast majority of the light from the SII doublet, delivering excellent contrast.
I'm building a Hubble Palette (SHO) set for my f/3 system. Is this the right SII filter?
Yes, this is the SII filter Chroma designs for f/3 beams. For Hubble Palette images you need strong signal in all three channels. Chroma's f/3-optimized H-alpha and OIII 3nm filters are made for the same f/2.8 to f/3.6 range, and the three are sold together as set 27108.
What does "parfocal" mean for this Chroma filter?
Parfocal means that you will not need to significantly refocus your telescope when switching between different filters in the same set. This Chroma filter is manufactured on a 3.0 mm thick substrate, a standard shared by Chroma's other astronomy filters except the LoGlow. This uniform thickness keeps the focal point consistent, saving you critical time during an imaging sequence.
Which size Chroma SII filter do I need for my full-frame sensor?
To avoid vignetting (darkening of the corners) on a full-frame sensor (approx. 43mm diagonal), you will typically need the 50 mm round unmounted or 50 x 50 mm square unmounted filter. For smaller APS-C sensors, a 36 mm filter is typically sufficient, while smaller sensors can typically use 1.25" or 31 mm filters. Vignetting also depends on filter-to-sensor spacing and focal ratio.
