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  6. Goodman Spectrograph Gratings
  7. Goodman Spectrograph Gratings

Goodman Spectrograph Gratings

Updated Apr 2024.

Up to three (3) gratings can be installed in the spectrograph at a time, in a linear stage which allows the rapid interchange of gratings. Installing different gratings is a day time operation. No grating installations are done during the night.

The available complement is: 400, 600, 930, 1200, 1800, 2100 and 2400 l/mm transmission VPH gratings.

NEW (Apr 2024):

A new 1200 l/mm grating is now available. This VPH grating has been designated 1200CaNIR, and is optimized for best throughput in the Ca II (8498, 8542, 8662A) triplet region. Its measured wavelength range is 7670 - 8810 A.

Note that for some resolutions we have several gratings, optimized for either the Blue or Red, or in a particular wavelength range.  Use the following table as a guideline to which specific grating best fits your science case.
 
Grating designation Match with this Goodman Camera Recommendation  
  400 BLUE or RED UV to far red. Use BLUE camera if observing below ~4500A
  600OLD BLUE or RED UV to ~6000A. Use BLUE camera if observing below ~4500A
  600RED RED Use for observations redward of ~6000A
  930 BLUE or RED UV to far red. Use BLUE camera if observing below ~4500A
  1200BLUE BLUE Use for observations below ~4500A.
  1200BLUE BLUE or RED Observations between ~4500 - 5500A
  1200RED RED Use if observing beyond ~5500A.
  1200CaNIR RED Optimized for the Ca II (8498, 8542, 8662) triplet.
  1800 BLUE or RED UV to red. Use BLUE camera if observing below ~4500A
  2100 BLUE or RED UV to red. Use BLUE camera if observing below ~4500A
  2400 BLUE or RED UV to red. Use BLUE camera if observing below ~4500A

Long Wavelength Limit for High Resolution Gratings
Because of limits in the camera rotation stage, it is not possible to use the 1800, 2100 and 2400 l/mm gratings beyond the central wavelengths indicated below:

  • 1800 l/mm grating: Littrow mode centered at 760nm, spans 7200 A - 8000 A
  • 2100 l/mm grating: Littrow mode centered at 650.8nm, spans 6130 A - 6710 A
  • 2400 l/mm grating: Littrow mode centered at 565nm, spans 5395 - 5905 A

The table below shows the dispersion and wavelength coverage for observations in our preset spectroscopic modes.  Please note that the 1800, 2100, and 2400 l/mm gratings are operated in Custom mode (Littrow Configuration), in which the observer selects the central wavelength for their observations.

 

 

The VPH gratings operate via Bragg scattering and their efficient operation requires Littrow or near-Littrow operation of the spectrograph. A grating rotation stage sets the incident angle to the desired value, which depends upon the line density of the grating and the central wavelength of interest. A concentric camera rotation stage must then be set to nearly twice this angle to intercept the diffracted beam.  A set of fixed observing modes for each grating are given below, where applicable.  All gratings can be used in the Custom mode.

Updated on April 25, 2024, 7:04 pm