iSHELL: 1.1-5.3 micron high-resolution spectrograph and imager
NASA InfraRed Telescope Facility(IRTF), Institute for Astronomy, University of Hawaii

@ Observer Documentation


IShell Manuals and Tools

  • iSHELL manual 20210827.pdf
  • The iShell Observing Manual (27 August 2021).
  • Parallactic angle calculator
  • Used to estimate the parallactic angle and differential atmospheric refraction for observations with SpeX.
  • ishell_spectrograph_log.pdf
  • iShell Log Template
  • ishell_guider_log.pdf
  • iShell Guider Log Template
  • A0V Stars and G2V Stars
  • Used to locate A0V and/or G2V stars near a target object
  • Spectrograph Exposure Time Calculator
  • iSHELL's exposure time calculator form L
  • iSHELL Echelle Format Simulator
  • Employs a set of ab-initio grating equations and simplified parameters that are accurate enough to describe the overall behavior of the cross-dispersors and the gratings

    iSHELL Data

    With the 2048x2048 H2RG array in the spectrograph the individual file size is 16.8MB. However, we now store three files per image: pedestal minus signal, pedestal, and signal, for a total image size of 50MB. The reason for the extra fiiles, which are stored as extensions to each image, is to accurately compute corrections for non-linearity. Consequently,

    Observers can compress data for transfer and uncompress on arrival by using the -C flag: 'sftp -C yourname@yourhost'. This should only be used for slow links since it requires extra CPU time.

    iSHELL Darks

    Dark current frames are taken at daytime by the support astronomers, for the itimes that were used in the preceding night. For each itime, the frames are automatically averaged (using the Spextool combine option Robust Weighted Mean at the default threshold of 8). The observer can download the averaged darks using scp and their observer account (2020AXXX):

    If the observer prefers to download the raw (un-averaged) dark frames, this can be done using the above instructions, replacing the directory with /scrs1/cartman/2020A901/200205/ (for 5 Feb 2020 in the 2020A semester, in this example).

    Spextool - Data Reduction Tool

    Spextool5 is an IDL-based spectral reduction program to reduce iSHELL cross-dispersed and prism data. It is written and maintained by Mike Cushing and Bill Vacca.

    Spextool version v5. Spextool v5 reduces data obtained with iSHELL only, and is currently limited to reducing the discrete cross disperser settings. Future versions of SpeXtool will reduce both SpeX and iSHELL data. All users should read Sections 1 and 2 of the manual (found in Spextool/manuals/) before attempting to run the software, if only because some new IDL libraries are required. Raw K-band and L-band data can be downloaded to test drive the downloaded Spextool5 package. Comments, suggestions for improvements, and bug reports should be directed to Mike Cushing at


    Observers publishing results obtained with iSHELL are requested to reference the following paper:

    iSHELL: a 1–5 micron R=80,000 Immersion Grating Spectrograph for the NASA Infrared Telescope Facility
    John Rayner et al. 2022 Publications of the Astronomical Society of the Pacific, 134:015002 (26pp), 2022 January

    Observers who use Spextool are also kindly asked to reference the following papers:

    Spextool: A Spectral Extraction Package for SpeX, a 0.8-5.5 micron Cross-Dispersed Spectrograph
    Michael C. Cushing, William D. Vacca and John T. Rayner
    (2004, PASP 116, 362).

    A Method of Correcting Near-Infrared Spectra for Telluric Absorption
    William D. Vacca, Michael C. Cushing and John T. Rayner
    (2003, PASP 115, 389).

    Note: The general version of the telluric correction routines, xtellcor_general, previously available as a stand alone package, has now been incorporated into the Spextool package. Users must download the full Spextool package in order to use xtellcor_general.

    Another paper may be of interest to iSHELL users:

    iSHELL: construction, assembly, and testing
    J. T. Rayner, A. Tokunaga, D. Jaffe, M. Bonnet, G. Ching, M. Connelley, D. Kokubun, C. Lockhart, E. Warmbier (2016, SPIE, 9908E, 84R)

    Nonlinearity Corrections and Statistical Uncertainties Asociated with Near-Infrared Arrays
    William D. Vacca, Michael C. Cushing and John T. Rayner
    (2004, PASP 116, 352).

    Contact John Rayner for further information or assistance