Lumicon used to give a print-out of the throughput curve, back in the old days, but I haven't bought from them since sometime around 1990. When you purchase a filter mail-order, you have to take what the order puller grabs off the shelf, so it boils down to the luck of the draw. It should be remembered that interference filters are dependent on the f-ratio of the telescope for maximum efficiency at the stated bandpass. They generally work better on longer f-ratio scopes. A larger angle of convergence (shorter f-ratio) means the light path is longer between the top and bottom layers of the filter coating. This affects the effectiveness of the filtration because of the nature of interference filters. They do not work in the same manner as colored glass filters, which work well regardless of the telescope's f-ratio. Something to consider. On Wed, Jun 4, 2008 at 4:42 PM, Richard Tenney <retenney@yahoo.com> wrote:
. I do know that Lumicon actually puts the filter bandpass rating on each one they sell, so you should have some idea up front how "good" it is.