Is the DG Spectrum Filters grating blazed?
Blazed gratings have five inherent short-comings when it comes to using them for CCD astrophotography. They are: 1. They allow more intensity to pass in the center of the visual region at the expense of having decreased efficiency over the rest of the visual region. 2. They generally only have a useable band width from 420 nm to 670 nm, when blazed for 550 nm (or 500 nm either). By their nature the UV and the IR regions are cut off in order for a slight increase in efficiency where our eyes are most sensitive. 3. Blazed gratings are designed for a parallel light beam passing directly perpendicular to the normal grating surface. If the light reaches the grating at angles other than 90 degrees then the blaze angle changes and its band width warps. This is generally not a problem for telescopes with a F/D > 20 (a F/D 10 SCT with a 2X Barlow). However, for short scopes or scopes with focal reducers there is a problem.
