Optical filter for objective lens
Apogee Imaging Systems

Astrodon® tru-balance filters are the first filters designed to match the sensitivity of modern CCD cameras. Using filters matched with the detectors greatly simplifies all aspects of deep-sky tri-color imaging. Benefits include shorter imaging times, G2V white point balance, balanced OIII nebula color, and pre-balanced RGB channels when using Kodak Blue Plus CCDs.
They are an outgrowth of over a year and a half of research, published in Sky and Telescope and presented at AstroImage and Imaging the Sky conferences. The narrowband filters, H a, SII and OIII, allow you to image when the moon is up, thereby extending the amount of imaging time you have each month. Since they are parfocal with the LRGB filters, you can easily focus on the L or C filter and then switch to the narrowband filter WITHOUT refocusing.