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Target Acquisition

GNIRS contains a "flip-in" mirror that bypasses the dispersive optics (i.e., gratings and prisms) and provides a long and somewhat narrow acquisition field. This allows precise positioning of an object in a slit without moving the grating, prism, or camera. The choice of acquisition filter is independent of the wavelength of the science observations. 

Target acquisition is currently done in the following manner:

  • Using the acquisition mirror an image of the requested slit and decker position is obtained with the telescope offset from the location of the target. The image thus shows the slit illuminated by the night sky.
  • The slit and decker are removed and the acquisition field is imaged.
  • The object is identified and the telescope is moved to center the object in the slit at the desired position. Nominally, this requires 2 images (to measure initial position and to confirm offset) for objects visible above the background; 3 images if a sky exposure is needed for background subtraction.
  • The slit and decker are reinserted to image the object through the slit and make final pointing adjustments if needed.
  • The acquisition mirror is removed and spectroscopy is started.
  • This technique, involving moving the slit out and back in, is being used because it is not easy to determine whether sources are properly centered in GNIRS' narrow slits. The positional repeatability of the slit has been measured to be considerably better than 1 pixel.

    Acquisition observations must be defined by the PI in the Phase II definition.  PIs should specify target brightness (and finding charts if needed) in the filter specified for acquisition, if possible. It is also useful to add notes with acquisition instructions if any special conditions apply (e.g., "center on fainter source W of bright nucleus","acquisition in K filter to match finding chart", etc). See the OT details for more information on defining GNIRS acquisition observations.

    Observers must also specify the exposure times for acquisition, using one of the four tables below. The first two tables, which are used in almost all instances, are for science involving spectroscopy in the 1-2.5µm range with one of the blue cameras and for science involving spectroscopy in the 3-5µm range and use of one of the red cameras. For example, for 1-2.5µm spectroscopy with the short blue camera the normal choices is the H2 filter for targets brighter than K~12, and the J, H, or K filter for fainter targets.

    The second two tables are used only when the target is so bright that its spectrum will saturate the array. In such a case a neutral density filter is employed both for acquisition imaging and for science spectroscopy.

    Recommended exposure times for "comfortable" (i.e. high S/N) acquisitions are given in the tables. Note that when using one of the long cameras (0.05 arcsec/pixel) without AO there is a 2.5 magnitude offset from the values in the table, which apply to the short cameras (0.15 arc sec/pixel). For example for an H~K~7.5 mag star to be acquired and observed with a long camera and no AO, use the "coadds x exp time" listed for H~K~10 mag.

    In all situations (even very bright stars) we recommend total exposure time (no. of coadds x exposure time) of at least 5 seconds to average out seeing and pointing fluctuations.

    For faint targets (see tables for details) acquisition should include obtaining a "sky" frame with the telescope offset at least a few arcsec from the nominal coordinates, for the purpose of sky subtraction.

    Objects fainter than J~20 or H~K~19 require "blind" acquisition. The PI must provide IR-visible reference star(s) that can be acquired with the same guide star as the science target, and with the same coordinate system as the science target.  See "Blind or Offset Acquisitions."

    Individual exposure times greater than 10 seconds at J, 3 seconds at H, and 2 seconds at K with the short camera (0.15"/pix) can lead to persistence in the form of a faint image of the acquisition field on the array in the initial few spectral frames. For that reason the recommended individual exposure times in the table below are short, and for faint targets numerous coadds are employed instead of longer exposures and fewer coadds. For the long camera (with AO) fewer, longer exposures should be used for faint targets, as indicated in the table.


    Recommended GNIRS Acquisition Exposure Times - Science at 1.0-2.5 Microns
    (applies to non-AO+short blue camera and to AO+long blue camera;
    for long blue camera without AO subtract 2.5 mag from values in column 1,
    reduce no. coadds by ~5X, and increase individual exp time by ~5X)


    mag Filter coadds x exp. timea total time (sec) a Read Mode
    K<7.0b H2 25 x 0.2 sec 5 Very Bright
    K=7.0b H2 25 x 0.2 sec 5 Very Bright
    K=8.0 H2 10 x 0.5 sec 5 Very Bright
    K=9.0 H2 5 x 1 sec 5 Bright
    K=10.0 H2 3 x 2 sec 6 Bright
    K=11.0 H2 2 x 4 sec 8 Bright
    K=12.0 H2 1 x 10 sec 10 Bright
    H=11.5 H-MK 25 x 0.2 sec 5 Very Bright
    H=12.0 H-MK 16 x 0.3 sec 5 Very Bright
    H=13.0 H-MK 8 x 0.6 sec 5 Bright
    H=14.0 H-MK 5 x 1 sec 5 Bright
    H=15.0 H-MK 4 x 2 sec 8 Bright
    H=16.0c H-MK 4 x 3 (AO 1 x 12) sec 12 Bright
    H=17.0c H-MK 8 x 3 (AO 1 x 24) sec 24 Bright
    H=18.0c H-MK 20 x 3 (AO 2 x 30) sec 60 Bright
    J=11.0 J (order-blocking) 25 x 0.2 sec 5 Very Bright
    J=12.0 J (order-blocking) 10 x 0.5 sec 5 Very Bright
    J=13.0 J (order-blocking) 5 x 1 sec 5 Bright
    J=14.0 J (order-blocking) 3 x 2 sec 6 Bright
    J=15.0 J (order-blocking) 1 x 6 sec 6 Bright
    J=16.0 J (order-blocking) 1 x 8 sec 8 Bright
    J=17.0c J (order-blocking) 1 x 10 sec 10 Bright
    J=18.0c J (order-blocking) 2 x 10 sec 20 Bright
    J=19.0c J (order-blocking) 6 x 10 (AO 1 x 60) sec 80 Bright
    K=10.0 K (order-blocking) 25 x 0.2 sec 5 Very Bright
    K=11.0 K (order-blocking) 10 x 0.5 sec 5 Very Bright
    K=12.0 K (order-blocking) 5 x 1 sec 5 Bright
    K=13.0 K (order-blocking) 3 x 2 sec 6 Bright
    K=14.0 K (order-blocking) 4 x 2 sec 8 Bright
    K=15.0c K (order-blocking) 6 x 2 sec 12 Bright
    K=16.0c K (order-blocking) 8 x 2 (AO 1 x 16) sec 16 Bright
    K=17.0c K (order-blocking) 15 x 2 (AO 2 x 15sec 30 Bright
    K=18.0c K (order-blocking) 45 x 2 (AO 5 x 18) sec 90 Bright

    a Values in table are for good conditions (IQ70 CC50). For worse conditions increase number of exposures. E.g., for poor seeing or for thin clouds increase number of coadds by about 50%; for bad seeing or for thick clouds roughly double the number of coadds.
    b Point sources brighter than K~7 with the short blue camera (K~4.5 when using long blue camera without AO) will saturate the array in imaging mode. Saturation results in image persistence in subsequent frames for several minutes. In the OT library the nod sequences are set up so that spectra avoid the array rows where saturation might occur during acquisition. Alternatively PIs may set up their own nod sequences to avoid saturated areas.
    c In order to acquire objects fainter than J~17, H~16 or K~15 (J~14.5, H~13.5 or K~12.5 with long camera without AO) sky subtraction is usually needed (see above text above the table). In that case the total time (for a pair of frames) is double the value in column 4 above.



    Recommended GNIRS Acquisition Exposure Times - Science at 3-5 Microns
    (applies to non-AO+short red camera;
    for long red camera subtract 2.5 mag from values in column 1,
    reduce no. coadds by ~5X, and increase individual exp time by ~5X)

    mag Filter coadds x exp. timea total time (sec)a Read Mode
    K<5.0b H2 25 x 0.2 sec 8 Very Bright
    K=5.0b H2 25 x 0.2 sec 5 Very Bright
    K=6.0 H2 10 x 0.5 sec 5 Very Bright
    K=7.0 H2 5 x 1 sec 5 Bright
    K=8.0 H2 3 x 2 sec 6 Bright
    K=9.0 H2 2 x 4 sec 8 Bright
    K=10.0 H2 1 x 10 sec 10 Bright
    H=11 H-MK 10 x 0.5 sec 5 Very Bright
    H=12.0 H-MK 5 x 1 sec 5 Bright
    H=13.0 H-MK 5 x 2 sec 10 Bright
    H=14.0c H-MK 5 x 3 sec 15 Bright
    H=15.0c H-MK 7 x 3 sec 21 Bright
    H=16.0c H-MK 10 x 3 sec 30 Bright
    H=17.0c H-MK 15 x 3sec 45 Bright
    L=6d PAH 25 x 0.2 sec 5 Very Bright
    L=7 PAH 10 x 0.5 sec 5 Very Bright
    L=8e PAH 10 x 0.8 sec 8 Bright
    L=9e PAH 15 x 0.8 sec 12 Bright
    L=10e PAH 20 x 0.8 (2 x 7) secg 16 Bright
    L=11e PAH 25 x 0.8 (3 x 7) secg 20 Bright
    L=12e PAH 50 x 0.8 (7 x 7) secg 40 Bright

    a Values in table are for good conditions (IQ70 CC50). For worse conditions increase number of exposures. E.g., for poor seeing or for thin clouds increase number of coadds by about 50%; for bad seeing or for thick clouds roughly double the number of coadds.
    b Point sources brighter than K~5 with the short blue camera (K~2.5 when using long blue camera without AO) will saturate the array in imaging mode. Limits are about 2 magnitudes brighter for the red cameras. Saturation will result in image persistence in subsequent frames for several minutes. In the OT library the nod sequences are set up so that spectra avoid the array rows where saturation might occur. Alternatively PIs may set up their own nod sequences to avoid saturated areas.
    c Acquisition should include obtaining a "sky" frame with the telescope offset at least a few arcsec from the nominal coordinates, for the purpose of sky subtraction.
    d Point sources brighter than L=6 will saturate the array in imaging mode.
    e In order to acquire objects fainter than L=7 (with either pixel scale) sky subtraction is needed. See text above the tables. In that case the total time (for a pair of frames) is double the value in column 4.
    f Do not use individual exposures longer than 0.8 seconds (7 seconds with the long red camera) or the sky+telescope background will saturate the array.



    Recommended GNIRS Acquisition Exposure Times - Science at 1.0-2.5 Microns
    (when ND100X filter is used for spectroscopy; applies to non-AO+short blue camera and to AO+long blue camera;
    for long blue camera without AO subtract 2.5 mag from values in column 1)


    mag Filter coadds x exp. timea total time (sec) a Read Mode
    K=2.0b H2+ND100X 25 x 0.2 sec 5 Very Bright
    K=3.0b H2+ND100X 10 x 0.5 sec 5 Very Bright
    K=4.0b H2+ND100X 5 x 1 sec 5 Bright
    K=4.0b H2+ND100X 5 x 1 sec 5 Bright
    K=5.0b H2+ND100X 3 x 2 sec 6 Bright
    K=6.0b H2+ND100X 2 x 4 sec 8 Bright

    a Values in table are for good conditions (IQ70 CC50). For worse conditions increase number of exposures. E.g., for poor seeing or for thin clouds increase number of coadds by about 50%; for bad seeing or for thick clouds roughly double the number of coadds.
    b Point sources brighter than K~2 with the short blue camera (K~-0.5 when using long blue camera without AO) will saturate the array in imaging mode. Saturation results in image persistence in subsequent frames for several minutes. In the OT library the nod sequences are set up so that spectra avoid the array rows where saturation might occur during acquisition. Alternatively PIs may set up their own nod sequences to avoid saturated areas.



    Recommended GNIRS Acquisition Exposure Times - Science at 3-5 Microns
    (when ND100X filter is used for spectroscopy; applies to non-AO+short red camera;
    for long red camera subtract 2.5 mag from values in column 1)


    mag Filter coadds x exp. timea total time (sec)a Read Mode
    K=0.0b H2+ND100X 25 x 0.2 sec 5 Very Bright
    K=1.0b H2+ND100X 10 x 0.5 sec 5 Very Bright
    K=2.0b H2+ND100X 5 x 1 sec 5 Bright
    K=3.0b H2+ND100X 3 x 2 sec 6 Bright
    K=4.0b H2+ND100X 2 x 4 sec 8 Bright

    a Values in table are for good conditions (IQ70 CC50). For worse conditions increase number of exposures. E.g., for poor seeing or for thin clouds increase number of coadds by about 50%; for bad seeing or for thick clouds roughly double the number of coadds.
    b Point sources brighter than K~0 with the short red camera (K~-2.5 when using long red camera without AO) will saturate the array in imaging mode. Saturation results in image persistence in subsequent frames for several minutes. In the OT library the nod sequences are set up so that spectra avoid the array rows where saturation might occur during acquisition. Alternatively PIs may set up their own nod sequences to avoid saturated areas.