127x152 mm Gratings for Use with the 1.5-m R-C Spectrograph

The 1.5-m R-C spectrograph has a fixed 55° angle between the optical axes of the collimator & camera. For sufficiently large grating tilts e.g., 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 R-C spectrograph, the collimated beam begins to overfill the grating for tilts > ~20.6°.

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

At present, there are twelve 127×152 mm gratings available. Their nominal specifications are listed below, along with resolution and coverage in 1st order available with the L1K.

Notes:

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