Astrodon introduced the ExoPlanet-BB filter for use in amateur-scale telescopes to measure light curves of exoplanets. Although it may not seem possible at first, astronomers with typical backyard equipment and sensitive CCD cameras have the capability to quantitatively measure small changes in light intensity as large exoplanets pass in front of their suns.
Based upon the extensive work of Bruce Gary in his book, “Exoplanet Observing for Amateurs”, Chapter 7 concludes that a “clear with blue-blocking filter should be the amateur’s default choice for exoplanets”. The Astrodon ExoPlanet-BB filter is that filter, where BB stands for “blue blocking”. He concluded that this filter “has most of the high signal-to-noise advantages associated with unfiltered observing, yet it has most of the reduced systematics advantages associated with V-band and R-band observing.He recommended this filter for transiting exoplanets, or BTEs (those brighter than 10th magnitude assuming a telescope <16 inches in diameter). The rewards for using the CBB filter are that reference stars that are a different color from the target star won’t produce annoying air mass curvatures in the light curve. Also, most moonlight is blocked by the CBB filter, leading to greater SNR. Finally, observations with the CBB filter exhibit lower atmospheric extinction. For an average star, the CCB filter will have a magnitude penalty of about 0.11 compared to a colorless (clear) filter.
The spectrum of the Astrodon ExoPlanet-BB filter is shown at left. It blocks UV and blue light. It starts transmitting light near 500 nm, corresponding to the shape of the conventional V-band filter and continues to transmit light into the near-infrared. Bruce Gary also states that a “near-infrared filter should be used when observations at low elevations are necessary or when a nearly full moon is above the horizon”. He specifically mentioned the Astrodon NIR Luminance filter which starts transmitting at 700 nm into the near-infrared. The filter is 3 mm thick and is thus the same thickness as all Astrodon LRGB, narrowband, NIR and photometric filters for minimal refocusing. It has anti-reflective coatings on both sides.