User's Guide to IRCAL

Table of Contents

Summary Table
Quick start
Instrument Characteristics
Software Overview
Data File Info
Filters, Grisms, Polarimeter
Nodding and Dithering Scripts
Observing Procedures and Tips
Technical Details

Mt. Hamilton Homepage

IRCAL Data and FITS Files

Data File Names and Directory

IRCAL data are written as FITS files to ircalbox2 directory /data/today/ (which is a link to /data/ircal/YYYY.MM.DD, e.g. /data/ircal/2007.02.16/). This directory is accessible from gouda and karnak by the directory path /data/ircal/. Each day at noon, a new directory is created for the coming night's data.

IRCAL file names are ircalxxxx.fits where xxxx is 0000 through 9999. For example, exposure number 51 will be in a file named ircal0051.fits. If you take data in imstream mode, the filename is imstreamX where X is the first number of the 100 exposure block. For example, if you start a 200 frame exposure sequence in imstream with exposure file number 349, the output filenames will be imstream349 and imstream449. imstream files are specially formatted FITS files designed for fast readout and writing to disk. For more information about imstream files, contact a support astronomer.

The data are automatically copied to the on-line data archive,, shortly after being acquired. If you need to set or reset your data archive password, contact a Support Astronomer.

Data Format

Data are written to FITS files. The recorded value of each pixel is the product of the counts per coadd exposure time, number of reads, and number of coadds.

total counts per pixel = counts/coadd * coadds * nreads

Reference Biases

Reference biases are stored dark frames for the most often used exposure times and number of reads. If you take an exposure using the same exposure time and number of reads as a reference bias, that reference bias frame is subtracted from the data before writing the FITS file and being displayed in the ircaldisplay window.

Reference bias files are archived in case you wish to add them back into your data. The reference bias file name is listed in the FITS header under the BIASFILE keyword.

FITS Header Keywords

IRCAL FITS files have some unique header keywords to describe the data, state of the IRCAL instrument, and the state of the Adaptive Optics system.
Exposure Parameters
EXPTIMEExposure time per coadd (seconds)
COADDTIMExposure time per coadd (milliseconds)
NREADSNumber of reads
NRESETSNumber of resets
NCOADDSNumber of coadds
Motor Positions
FILTER1Filter wheel 1 contents
FILTER2Filter wheel 2 contents
APERTUREAperture wheel contents
WHEELSWheels by index position
Readout Mode Details
BIASFILEReference bias file subtracted
MODEMode summary string from ircalui: COADDTIM:NREADS
READMODEDetector Readout Mode (1=3us,2=1us,3=Fast,4=Hybrid)
READMD_SDetector Readout Mode Description
SUBARRXSubarray X size (per quadrant)
SUBARRYSubarray Y size (per quadrant)
CCDSECSubarray location in IRAF CCDSEC notation
IMSTREAMImstream mode? (0=no, 1=yes)
AO Parameters
AO_MODEAO Guide Star Mode: NGS or LGS
TTMODEtipTilt Mode, ngs or lgs
AOLOOPAO loop status
TTLOOPTip/Tilt loop status
AOGAINAO loop gain
AOINTAO integrator gain
AORATEAO frame rate
TTGAINTip/Tilt loop gain (NGS TT loop, or LGS laser uplink)
LTTRATETip/tilt sensor frame rate
LTTFOVTip/tilt sensor field of view
LTTGAINTip/tilt sensor gain
TTSDNTip/Tilt sensor ND wheel
NOD_NSCumulative nod north-south, arcsec
NOD_EWCumulative nod east-west, arcsec
FIBERCassegrain focus fiber position
WPLATWaveplate angle
WPLINWaveplate insertion

Elinor Gates
Last modified: Thu Jan 19 11:53:12 PST 2012