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

@ Observer Documentation

ALWAYS CONSULT WITH A SUPPORT ASTRONOMER BEFORE USING OR PROPOSING TO USE iSHELL.

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):

Find out which darks are available:

ssh 2020AXXX@stefan.ifa.hawaii.edu
cd /home/quicklook/Data_ishell/Cal_library_ishell/
find the dark with the desired itime and date closest to the observing date (hint: use 'ls -ltr dark*' for a time-sorted list). For example, dark-29pt65:200205_191039ut.fits is for data obtained on 5 February 2020 (UT) and an itime of 29.65 seconds (note that this is the actual itime used when entering 30 seconds in the XUI).

Copy the dark to your own machine: 'scp 2020AXXX@stefan.ifa.hawaii.edu:/home/quicklook/Data_ishell/Cal_library_ishell/dark-29pt65:200205_191039ut.fits .'

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 michael.cushing@gmail.com.

Acknowledgements

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