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Chroma B-Bessell Filter
The Chroma B-Bessell Filter is a scientific-grade photometric filter built on a 3.0 mm thick substrate for parfocal operation within a UBVRI filter set. Its transmitted wavefront of better than 0.25 waves/inch preserves the resolution of your optical system for precise stellar measurements. Manufactured in Vermont, USA, this filter uses durable sputtered hard coatings on a single substrate to deliver the standardized blue bandpass required for Johnson-Cousins photometry.
The Bessell B-Band: A Standard for Johnson-Cousins Photometry
This is not a general-purpose "blue" filter for RGB imaging; it is a precision instrument for photometry. The Chroma B-Bessell filter isolates the specific blue portion of the spectrum defined by the standard Johnson-Cousins UBVRI system. This allows you to accurately measure the apparent magnitude of celestial objects and calculate their color index (e.g., B-V), a fundamental technique for classifying stars and studying variable star behavior.
3.0mm Parfocal Design for UBVRI Filter Sets
All Chroma astronomy filters except the LoGlow are built on a 3.0 mm thick single-glass substrate. This uniform thickness is critical for photometry, where you rapidly cycle through different filters in a wheel. The parfocal design means filter thickness does not change focus when you switch between this B-Bessell filter and Chroma's U, V, R, and I filters, saving time in your data acquisition sequence.
Better Than 0.25 Waves/Inch Wavefront and Sputtered Hard Coatings
Chroma specifies a transmitted wavefront of better than 0.25 waves/inch, so the filter adds very little wavefront error. This maintains sharp, point-like star images, which is essential for accurate magnitude measurements. The coatings are applied via a durable sputtering process on a single substrate, with an anti-reflection coating on the rear surface to prevent ghosting and internal reflections that could corrupt photometric data.
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What is the primary use for the Chroma B-Bessell Filter?
The Chroma B-Bessell Filter is designed specifically for scientific photometry. Its purpose is to isolate the standardized "B" (blue) bandpass of the Johnson-Cousins UBVRI system, allowing astronomers to accurately measure the brightness and color of stars for research on topics like stellar classification and variability.
Is the Chroma B-Bessell Filter parfocal with other filters?
Yes. It is manufactured on a 3.0 mm thick substrate, making it parfocal with all other Chroma astronomy filters (with the exception of the LoGlow filter). This means filter thickness does not change focus when you switch between filters in a UBVRI set, which helps efficient data collection.
Can I use the Chroma B-Bessell Filter for imaging nebulae like the Orion Nebula (M42)?
While you can, it is not the ideal tool for that task. The B-Bessell is a wideband filter designed to capture starlight for measurement. For deep-sky objects like the Orion Nebula (M42), you would achieve much higher contrast and reveal more detail by using narrowband filters, such as Hydrogen-alpha or Oxygen-III, which isolate the specific wavelengths emitted by the nebula's gases.
What kind of targets would I observe with the Chroma B-Bessell Filter?
This filter is best used for photometric studies of stars. Excellent targets include:
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Variable Stars: Measuring the light curve of stars like Algol (Beta Persei) or Delta Cephei as their brightness changes over time.
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Star Clusters: Creating a color-magnitude diagram (Hertzsprung-Russell diagram) for a cluster like the Pleiades (M45) to determine its age and distance.
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Exoplanet Transits: Detecting the slight dimming of a star as a planet passes in front of it.
Which size Chroma B-Bessell filter do I need for my camera?
The right size avoids vignetting (darkening of the corners); as a typical guide:
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1.25" Mounted: Typically for cameras with smaller sensors.
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31mm Unmounted: Typically for smaller sensors in a filter wheel. 36mm Unmounted: Typically for APS-C class sensors.
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50mm Round/Square Unmounted: Typically needed for full-frame (35mm) sensors and larger. The 2" mounted size typically suits APS-C class sensors; vignetting also depends on filter-to-sensor spacing and focal ratio.
What does "sputtered hard coating" mean for the Chroma B-Bessell Filter?
It refers to the manufacturing process where layers of optical material are deposited atom-by-atom onto the 3.0 mm glass substrate. Chroma states its sputtered coatings withstand humidity changes and temperature extremes and remain spectrally accurate, and its sputtered catalog filters carry a lifetime warranty.
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| Filter Type |
B-Bessell Photometric |
| Manufacturer |
Chroma Technology Corp (Vermont, USA) |
| Chroma Part Number |
27052 |
| Glass Thickness |
3.0 mm substrate |
| Parfocal |
Yes, with other Chroma astronomy filters (except LoGlow 27030) |
| Transmitted Wavefront |
Better than 0.25 waves/inch |
| Coatings |
Durable sputtered hard coatings on a single substrate |
| Anti-Reflection |
Coating on the rear surface |
| Available Sizes |
1.25" mounted, 2" mounted, 31 mm, 36 mm, 50 mm round unmounted, 50 x 50 mm square unmounted |
| Cell Thread (Mounted) |
1.25": M28.5x0.6 male; 2": M48x0.75 male |
| Made In |
Vermont, USA |
| Warranty |
Chroma lifetime warranty for sputtered catalog filters |
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Chroma B-Bessell Filter
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Chroma Lifetime Warranty
