What is the purpose of the Chroma Clear Filter if it doesn't block any light?
The Chroma Clear Filter's job is mechanical, not spectral. It is a 3.0 mm thick piece of high-quality optical glass designed to be parfocal with other Chroma astronomy filters (except LoGlow). It holds a slot in your filter wheel and adds the same optical path as your other Chroma filters, so an unfiltered position does not shift focus the way an empty slot would.
Is the Chroma Clear Filter the same as a Luminance (L) filter?
No. A Luminance filter is an actual filter with specific coatings designed to pass a very broad spectrum of light while typically blocking UV and IR wavelengths. The Chroma Clear Filter is essentially a high-grade optical window with anti-reflection coatings; it does not have a defined passband and is not designed to block UV or IR light. Its purpose is to maintain focus, not to capture luminance data.
Should I focus with the Chroma Clear Filter instead of my L-filter?
Not for tighter stars. The Clear filter has no passband, so it passes a broader range of wavelengths than a luminance filter, and any residual color error in your telescope shows at least as much through it. Because it is parfocal with other Chroma astronomy filters, focus found through it carries over to them only as far as your telescope's color correction allows.
Is the 3.0mm Chroma Clear Filter parfocal with all other Chroma filters?
Yes, the 3.0 mm Chroma Clear Filter is parfocal with all other Chroma astronomy filters that also use a 3.0 mm substrate. This includes their LRGB, H-alpha, OIII, and SII filters. The only exception noted by Chroma is the LoGlow Light Pollution Filter (part #27030), which is manufactured on a thinner substrate.
Will the Chroma Clear Filter work on a fast f/2 system?
Chroma officially designs their standard astronomy filters for systems of f/4.0 or slower. While the Clear filter does not have a passband that can shift, using any unoptimized filter in a very fast optical system can potentially introduce minor aberrations. For critical applications at f/2, it's best to test performance, but its primary role as a focusing tool is generally less sensitive to focal ratio effects than a narrowband filter.
What size Chroma Clear Filter do I need for an APS-C camera?
For an APS-C class sensor (about 28 mm diagonal), the 36 mm unmounted size is typically used. These filters would be installed in a filter wheel designed for 36 mm unmounted filters.
Does the Chroma Clear Filter act as a UV/IR cut filter?
No. The Chroma Clear Filter is not a UV/IR cut filter. It is an optical flat with anti-reflection coatings and will pass ultraviolet and infrared light. If your optical system requires UV/IR blocking, you would need to use a separate Luminance filter that includes this feature.
Why is the transmitted wavefront of the Chroma Clear Filter important?
The transmitted wavefront of better than 0.25 waves/inch is a measure of the filter's optical flatness. It means less than a quarter wave of wavefront error per inch of filter, which helps the filter keep the sharpness your telescope delivers.
