The CXB ExoPlanet-BB Filter (formerly Astrodon) is a specialized blue-blocking filter designed for amateur astronomers measuring exoplanet light curves with backyard telescopes and sensitive CCD/CMOS cameras.

It blocks ultraviolet and blue light shorter than approximately 500 nm (corresponding to the start of the conventional V-band) while transmitting light through the visible spectrum into the near-infrared. This provides most of the high signal-to-noise ratio advantages of unfiltered (clear) observing, combined with reduced systematic errors typical of V-band or R-band photometry.

According to Bruce Gary in Exoplanet Observing for Amateurs (Chapter 7), a “clear with blue-blocking” filter is the recommended default choice for most amateur exoplanet transit observations (BTEs brighter than 10th magnitude with telescopes under 16" aperture). Benefits include:

• Minimizes air-mass curvature caused by color differences between target and reference stars
• Reduces moonlight interference for improved signal-to-noise ratio (SNR)
• Lowers atmospheric extinction compared to broader filters
• Only ~0.11 magnitude penalty versus a clear filter for an average star

Key Specifications

• Blocks wavelengths below ~500 nm
• Transmits from ~500 nm into the near-infrared
• 3 mm thick – matches other CXB/Astrodon LRGB, narrowband, NIR, and photometric filters for minimal refocusing
• Anti-reflective (AR) coatings on both sides
• Available in standard sizes (1.25", 31mm, 50mm round/square, etc.)

Recommended Use

• Primary choice for transiting exoplanet light curves
• Photometry of stars brighter than ~10th magnitude with telescopes <16" aperture
• Situations with moonlight or moderate airmass

For observations at very low elevations or with a nearly full moon present, Bruce Gary recommends switching to a near-infrared filter (such as the CXB NIR Luminance filter starting at 700 nm).

Note on Availability We aim to keep these filters in stock, but larger or less common sizes may have a lead time of up to 30 days.