1.5mRC Gratings

The 1.5-meter Cassegrain spectrograph has a fixed 55^o angle between the optical axes of the collimator and camera. For sufficiently large grating tilts e.g., when observing at high dispersion, the beam coming from the collimator can overfill the grating resulting in the loss of a small amount of light. For the 1.5-m Cassegrain spectrograph, the collimated beam begins to overfill the grating for tilts greater than 20.^o6 (grating tilt readout > 29.^o 85 ).

The amount of light which might be lost due to overfilling the grating is almost always negligible, typically a few percent. In practice, this small loss would be more than offset by the ability to observe using a wider slit while preserving spectral resolution that results from the anamorphic demagnification at large grating tilts.

Table 2 lists the twelve 127mmx152mm Bausch & Lomb plane reflection gratings available with the spectrograph. Gratings tilts for the 1.5-m Cassegrain spectrograph are listed in Table 3.

Table 2: 1.5-Meter Cassegrain Spectrograph Gratings, 160mm Camera, Loral 1K (1200x800) CCD

¹ The value given for the blaze wavelength is the first order Littrow blaze. Effective blaze wavelength when used in the 1.5-m Cassegrain spectrograph is 0.89 of the Littrow value.
² Gratings 11 & 22 are silver coated and thus does not reflect very much light below ~4000A.
³ Grating cannot be tilted far enough to be used in 2nd order.

Table 3: 1.5-Meter Cassegrain Spectrograph Gratings Tiles Center Wavelength