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Observing Strategies

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Re-centering Target in the slit will be done every ~1.0 hour, as flexure between NIRI and the guider causes the slit to slowly move off of the target as the telescope tracks. PIs do not need to provide reacquisition sequences, but do need to factor in five extra minutes per hour into their time estimates. This time is not accounted for in the OT, so PIs should underfill their time allocations accordingly.

Slit Orientation should be EW whenever possible, as the differential flexure discussed above tends to be in that direction and thus the target will remain in the slit for a longer time than if the slit is, say, NS.

Spectroscopic Offsetting along the slit is recommended for sky and detector defect removal. Smaller nods more accurately retain the target in the slit than do large nods. Although the slit is 50-90 arcsec long it is not necessary to use all of it on small targets. For a point source nods of 3 arcsec at f/6 are ample.

JHK Spectroscopy of faint sources should use the "Low Background" read mode with exposure times longer than 44 sec and shorter than a few minutes. Plan to discard the first spectral image of a sequence, because of dark current instability when the background changes (between acquisition imaging and science spectroscopy). Bright star observations are much shorter and should use the "Medium Background" read mode.

L-band Spectroscopy can be done with exposures of 1-3 seconds - or longer, depending on the slit width and the wavelengths of interest. Note that the background rises steeply with wavelength at 3-4μm. At 4.1μm using the widest slit, a 2 second exposure half fills the detector well, putting that detector close to the non-linear regime. Although the "Medium Background" mode (for which the minimum recommended exposure time is 2.7 sec) can be used, for higher efficiency or for bright standards the "High Background" mode (minimum recommended exposure time is 0.9 sec) is recommended. For the "High Background" mode, the widest slit and a 2 second exposure (or the narrowest slit and a 6 second exposure) the background fluctuations exceed the read noise except near 3.0-3.2μm, where the two are comparable. Except for bright sources, many exposures should be coadded between nods (so that the total integration time between nods is 30-60 seconds).

M band Spectroscopy must be done in the "High Background" read mode. The 768x768 subarray should be used rather than the full array; it will result in greater efficiency (because of the shorter read time), and no spectral information is lost, because the excluded portions of the array are outside the bandpass of the blocking filter. Use of the 512x1024 subarray does not gain any improvement in readout time and is equivalent to using the full array. As at L multiple coadds should be used for faint targets. At f/6 flats can be taken with the 768x768 subarray but they saturate the full array.

Acquisition modes include "Bright Object" for sources which can be easily seen in short exposures (<15s) without sky subtraction (J<17, H<16, K<16). "Faint Object" is for sources requiring sky subtraction and exposure times between 15 - 60 seconds (17<J<21.5, 16<H<20.5, 16<K<20.7). "Blind Offsetting" is used for sources which cannot be seen in a 60s sky subtracted image (21.5<J, 20.5<H, 20.7<K).

Acquisitions involve placing a near-IR image of the target at the position of the slit before introducing the appropriate slit, blocking filter and grism. Fainter objects can be acquired via accurate user-supplied blind offsets from a nearby bright object. In this mode the bright reference (User1) star will be centered in the slit, and then the blind offsets will be applied to shift the science target into the slit. In the thermal IR (3-5um), because of the high background it is much easier (except for the most extreme red objects) to center a near-IR image of the target on the slit than an image in the thermal IR. Examples of both types of acquisitions are given in the NIRI OT library.

Slits and Grisms are specific to each camera. The f/6 slits and grisms are designed for use at f/6, and generally speaking, higher spectral resolution cannot be achieved using the f/14 camera and the present set of grisms, and the wavelength coverage at f/14 will be severely reduced. At f/32 (using adaptive optics) three grisms are available to cover the JHK bands although these provide somewhat narrower wavelength coverage than the f/32 grisms. In general, if NIFS is available it rather than NIRI should be used for JHK spectroscopy with AO.

Slitless spectroscopy may be useful for some near-IR programs. The background will be much higher, of course. Some observations may use narrow-band filters in conjunction with slitless grism spectroscopy to good advantage. This mode has not been tested yet.

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