Chroma UBVRI Filter Set

The Johnson-Cousins UBVRI System for Scientific Imaging

This is not an LRGB set for creating pretty pictures; it is a scientific instrument for measuring the brightness and color of celestial objects. The UBVRI system is a widely used standard system for photometry, allowing your data to be compared with professional observations worldwide. Each filter isolates a specific passband for quantifying stellar magnitudes.

• U (Ultraviolet): Captures the near-UV, crucial for characterizing hot, young O- and B-type stars and active galactic nuclei.
• B (Blue): Isolates the blue portion of the spectrum, forming the blue-end baseline for color index calculations.
• V (Visual): Centered in the green, this band closely matches the peak sensitivity of the human eye and serves as the standard for visual magnitude.
• R (Red): Covers the red part of the visible spectrum, essential for studying cooler stars like K and M-types.
• I (Infrared): Samples the near-infrared, providing critical data on the coolest red dwarf stars and heavily reddened objects.

3.0mm Parfocal Substrates and 0.25 Waves/Inch Wavefront

Each filter in the UBVRI set is manufactured on a single 3.0 mm thick substrate, ensuring they are parfocal. The filters themselves do not shift focus between bands, though your telescope's color correction can leave small focus differences between the U and I ends. Transmitted wavefront is better than 0.25 waves/inch.

The coatings are applied by reactive magnetron sputtering on a single substrate with no laminations; Chroma states its coatings withstand humidity changes and extreme temperature fluxes, while anti-reflection coatings on the rear surface prevent ghosting and internal reflections from bright stars.

Focal Ratio and Photometric Accuracy

Chroma gives no focal-ratio rating for its Bessell photometric filters. All interference filters experience a "blue shift" in their passband when used in the converging light cone of a fast telescope; broadband photometric filters are far less sensitive to this than narrowband filters, but the shift grows as the focal ratio gets faster.

For precise photometry, calibrate with standard stars through your own optical system, which accounts for your telescope, filters and camera together.

UBVRI vs. LRGB: When to Choose a Photometric Filter Set

The choice between a UBVRI set and a standard LRGB (Luminance, Red, Green, Blue) set comes down to your primary goal: data or aesthetics. LRGB filters are designed for color rendering, and their passbands are not standardized for measurement.

The Chroma UBVRI set, in contrast, is matched to the Bessell realization of the Johnson-Cousins system, whose broad passbands are defined for measurement. This allows you to produce light curves of a variable star or determine the color index of an asteroid with data that, after standard calibration, is comparable with professional research. While you can combine UBVRI data to create a color image, its purpose is analysis, not just presentation.