Astrodon 1.25'' (27mm) Clear CCD Filter NIR Unblocked

Clear (C) filter with no NIR-blocking has >97% transmittance throughout the spectral region, decreasing to only 95% at 400 nm in the blue. The C filter will increase the luminance signal by 45% for Kodak Interline CCDs.

If your goal is to bring out magnitude 22 galaxies by combining many long "luminance" exposures, then it may be best to use this C filter (or no filter at all).

However, if your goal is to bring out vibrant color in galaxies, these tests suggest that the L filter, matched spectrally to the RGB filters (and included in the LRBG filter set), is a better choice to eliminate the NIR "galaxy glow" contributed by a C filter. This becomes even more important in systems with refractive optics (SCTs, refractors).

If you have a refractor or telephoto lens, remember that a C filter also lets in UV. This is why less color-corrected refractors often need "minus violet" filters to eliminate the optical dispersion in the short wavelength region (blue halos around bright stars). An L filter does cut out some of this. For these systems, an L filter is likely to be a better choice.