Violet Light Rejection - Blocks the UV and violet light shorter than 420nm. Most refractor optics including apochromatic telescopes are not well corrected in the short wavelengths. Add to this, the enhanced quantum efficiency of the newer CCDs in this part of the spectrum. Without this filter, the stellar profiles of hot blue stars become bloated and off-axis stars show a small comet like tail due to laterial chromatic aberration. The NextGEN 0.33X comes standard with this filter.