[BruceG (Bruce)]

Removing the rear filter only leaves the front filter to block IR wavelengths, This is still needed to avoid stars becoming bloated due to chromatic aberration. The second reason for leaving the front filter in place is this is the one that has the sensor autoclean and also is a protective layer for the sensor. The rear filter removal will dramatically increase the sensitivity to Ha wavelengths even though the front filter is intact. The only reason some people install a clear filter in the rear position is to keep the autofocus accuracy (not considered as necessary for my use).