The dispersive elements include grisms available on the Blue side and gratings that are available on the Red side. The grisms on the blue side are always installed and available to the observers. The gratings on the red side are available to observers, but need to be specified on the instrument configuration form to ensure that the desired gratings are installed during an observing session.

Grisms: Blue Side

The table below lists the grisms available for use with LRIS on the Blue Side only The values presented were empirically measured. Chuck Steidel (Caltech) supplied the wavelength ranges and efficiency data. The FWHM of lines using a 1 arcsec slit width were obtained using the internal lamps, and ranges in FWHM are from blue to red in wavelength. The efficiencies on the blue side are the efficiency of the instrument with that grism.

Grism	Dispersion	Efficiency	Wavelenth Range (Å)		Flat Field	FWHM resolution for 1" slit (A)
			MOS	Longslit
300/5000	1.43	data	2240-8090	1600-7450		8.4-9.2
400/3400	1.09	data	1790-6250	1300-5770		6.5-7.1
600/4000	0.63	data	3330-5910	3040-5630		3.8-4.1
1200/3400	0.24	data	3040-4030	2940-3920		1.56

Note: The efficiency data was acquired by Chuck Steide (4 June 2002). The efficiency plots reflect the efficiency of LRISb with each grism. For the 300, 400, & 600 grisms, the mirror was used at the position of the dichoric. The UV response improves with the use of dichroics instead of the mirror. The 1200 grism efficiency is in combination with the 460 dichroic. All other optics were the same for all grism observations. Below is a comparison plot of the instrument throughputs for the four grisms as measured by Chuck Steide (4 June 2002)

Gratings: Red Side

The following gratings are available for use with LRIS red side. The grating information is up to date for the red side upgrade. Dispersion and wavelength coverage are calculated and are not measured values. The efficiency plots are not up to date and were determined using the old detector.

Grating	Dispersion	Efficiency1	Δλ (Å)	Flat Field	FWHM resolution for 1" slit (Å)
150/7500	3.0		12288	n/a
300/5000	1.59		6525	n/a	9.18
400/8500	1.16		4762	n/a	6.9
600/5000	0.80		3275		4.7
600/7500	0.80		3275		4.7
600/10000 Gold coated	0.80		3275		4.7
831/8200 Gold coated	0.58		2375	n/a
900/5500	0.53		2175
1200/7500	0.40		1638
1200/9000 Gold coated	0.40		1638	TBD
Mirror	n/a		n/a	n/a

1. The grating efficiency was measured by acquiring slitless spectra of spectrophotometric standard stars. The grating, standard star, date of observation, and filter(s) used are indicated on the plots. The spectra were reduced as follows:

• Bias subtraction of stellar spectrum and calibration images.
• Flat-fielding of stellar and arc-lamp spectrum images; the flat-field images were taken with internal illumination, generally at the same sky-PA as the stellar spectrum.
• Extraction of stellar spectra with background subtraction; no variance weighting is applied but the spectra have S/N typically greater than 50. The atmospheric A and B-O2 bands, and the H-alpha absorption line when appropriate, are removed.
• The extracted spectra are extinction corrected and integrated over bandpasses from the STScI "medium"-resolution tables (2-10 Å spacing). Then, the spectra are fluxed against the tabulated brightness values.
• The resulting efficiencies are smoothed over 50 Å (typical) centers, and plotted as postscript files.
• The plots for the 600/10000 and 1200/9000 were derived in 2022.

Additional plots for the red side

Low-res gratings
PDF file
Medium-res gratings
PDF file
Med-res gratings relative efficiencies
PDF file
High-res gratings
PDF file

Notes:

• Several spectra that have overlapping free spectral ranges at the same resolution display an efficiency reduction at the blue end of each progressively red spectrum; see 1200g/mm for example.This effect must be due to a reduced transmission of the CCD over some range of columns on the left side of the CCD (see also Grating flat-field plots).
• Several of the curves (150/7500, 600/7500, and 831/8200) display "enhanced" efficiencies near 6500 Å; in the case of 600/7500 it is seen also in the flat-field plot and is apparently due to a grating ghost. In addition 1200g/mm has a ghost at 4500 Å of similar nature.
• More data exists beyond 9200 A but these tables end there. The spectrum 400g/mm-8500 Å blaze with GG495 was not flat-fielded.