Gratings and their Tilts

Two gratings can be mounted back to back inside of the IRS. Additionally, there is a flat imaging mirror mounted at the end of the gratings which is useful for imaging the field in undispersed light. CTIO has several gratings available. Installation and removal of the gratings is a process that requires warming up and opening the dewar. The total turn around time for this process is about 2 days so users must specify the gratings they wish to use at the time they request telescope time. Changes in grating configurations once the telescopes have been scheduled will probably not be possible since we try to block together observing runs using similar gratings. The available gratings are given in the following table. The two factors which affect your choice of grating are resolution versus the wavelength coverage. If you need to work beyond 3$\mu$m then you must use either the 75 l/mm grating with the 4.5$\mu$m blaze or the 210 l/mm grating. If you are working shortward of 3 microns, the gratings blazed at shorter wavelengths are preferable if the wavelength coverage and resolution are acceptable. since the efficiency is higher and they don't have order overlap problems.

IRS Gratings: 2 Pixel Resolutions ($\lambda$/ $\Delta\lambda$) and Orders

Grating	Blaze	J	H	K	L	M
l/mm	$\mu$m	1.2$\mu$m	1.6$\mu$m	2.2$\mu$m	3.5 $\mu$m	4.7 $\mu$m
632	2.4	9800	5370	8370	NAa	NAa
		2nd	1st	1st
210	4.2	3860	5360	4830	3760	5240
		3rd	3rd	2nd	1st	1st
75	4.5	1800	1800	1650	1320	1760
		4rd	3rd	2nd	1st	1st
75	1.9	900/450	600	825	1320b	NAc
		2nd/1st	1st	1st	1st

^a Grating ruling is too fine to be used at this wavelength

^b Grating is being used well off blaze, not recommended

^c Background count rates are too high to permit use at this wavelength

IRS Gratings: $\Delta\lambda$ per Pixel and 250 Pixel Coverage

Grating	Blaze	J	H	K	L	M
l/mm	$\mu$m	1.2$\mu$m	1.6$\mu$m	2.2$\mu$m	3.5 $\mu$m	4.7 $\mu$m
632	2.4	0.000061$\mu$m	0.00013$\mu$m	0.00013$\mu$m	NAa	NAa
		0.015$\mu$m	0.033$\mu$m	0.033$\mu$m
210	4.2	0.00015$\mu$m	0.00015$\mu$m	0.00023$\mu$m	0.00046$\mu$m	0.00046$\mu$m
		0.037$\mu$m	0.037$\mu$m	0.057$\mu$m	0.11$\mu$m	0.11$\mu$m
75	4.5	0.00033$\mu$m	0.00044$\mu$m	0.00067$\mu$m	0.0013$\mu$m	0.0013$\mu$m
		0.083$\mu$m	0.11$\mu$m	0.17$\mu$m	0.33$\mu$m	0.33$\mu$m
75	1.9	0.00067/0.0013$\mu$m	0.0013$\mu$m	0.0013$\mu$m	0.0013$\mu$mb	NAc
		0.17/0.33$\mu$m	0.33$\mu$m	0.33$\mu$m	0.33$\mu$m

^a Grating ruling is too fine to be used at this wavelength

^b Grating is being used well off blaze, not recommended

^c Background count rates are too high to permit use at this wavelength

When the gratings have been changed you must run the initmtrs command once to tell the motor control microprocessor which gratings are in place. To select grating A or B issue the command sidea or sideb. These commands also place the grating in zeroth order. The mirror command will place the flat mirror into the beam to allow the IRS to be used as an imager. When in zero order the durant counters should read 320 and 5320 respectively for side A and side B. To set the grating tilt zero point (so that the lambda command will work properly) it is necessary to image the zeroth order image of the slit and use the gjog [n] command to move the slit image to pixel 128. A single motor step will move the slit image about 2 pixels. A positive motor step moves the slit image up on the detector in the IRAF display. Since the two back-to-back gratings will have slightly different tilts in their grating cells it may be necessary to jog the grating drive when you change from one grating to another.

To select a wavelength to observe you issue the command lambda [n] [l] where [n] is the order and [l] is the wavelength in microns. For example to observe in 3^rd order at 2.3$\mu$m (at pixel 128) you enter the command lambda 3 2.30 in the command window. A single motor step for the grating drive is 2 pixels and this is the limitation to the precision with which wavelengths can be selected. The repeatability of the grating drive appears to be about 0.7 pixels.