MMIRS Observing

Note: New HAWAII-2RG detector has been installed for 2014B. Information below is tentative.
Field-of-view: 6.9 x 6.9 arcmin
Pixel scale: 0.202 arcsec/pix (optical layout)
Detector: 2048 x 2048 pix, HgCdTe Hawaii-2RG
Electronics gain: 1 e-/DN
Read-noise (per read): 11 e-
Full well: TBD
Linearity: Linearity correction TBD
Dark current: < 0.01 e-/s/pix expected at 78K set point.
Readout: 32 channels at 100 kHz; 1.4 sec for a single read.
Data Acquisition: The HAWAII2-RG is a CMOS device that reads non-destructively. There is no shutter. Images are obtained by differencing a pair of reads N seconds apart ("double-correlated sampling"), or, preferably, by taking multiple reads and then fitting a linear slope to each pixel ("up-the-ramp sampling"). Up-the-ramp sampling can measure count rates for pixels that saturate or have cosmic rays hits during an exposure (using mmfixen).

Bad Pixel Mask Dark Frame K-band Flat Field

Filters: Transmission plot

Transmission Data
Filter Cuton Cutoff Comments
Y 0.97 1.07
J 1.17 1.33
H 1.50 1.78
K 2.00 2.32K-short filter, primarily for imaging
HK 1.25 2.49HK spectroscopic filter available 2010-. The red extent of this filter causes some loss of sensitivity in the H-band due to scattered thermal emission. Also order overlap in the wings of the bandpass causes contamination of the reddest end of the spectrum.
HK3 1.25 2.34New HK filter available 2014-. Has shorter red end cutoff to minimize scattered light. This filter also has very high throughput, so if you don't need to go beyond 2.34 microns, it is a better choice than HK.
zJ 0.95 1.50Spectroscopic filter
Kspec 1.93 2.45New K spectroscopy filter available 2014B-.
HKold 1.26 2.25Old HK filter used prior to August 2010. No longer available

Imaging: Exposure Time Calculator

Exposure times: Must be integer seconds >= 1 sec.
Focus: Instrument focus is stable and is not changed. Telescope focus is provided by the wavefront sensor.
Sky background: Dominated by narrow OH emission lines and broad H2O absorption bands (additional discussion).
Overhead: 14 sec total overhead per exposure, including 30 arcsec telescope dither.
Guiding: Images are unguided.
Strategy: Long integrations are made from a series of short exposures, otherwise the night sky quickly saturates the detector (see Imaging Senstivity table). Images are typically obtained in a dither sequence. Sky background is measured, and removed, by median-combining a set of dithered images. This works well for sparse fields and point sources, however observations of extended objects require dithering on an OFF field to measure the sky background. Worked example.

Imaging Sensitivity
Magnitude of Point Source
(Vega mag) with S/N=10.
Time [sec] Y J H K
60 20.3 20.1 19.5 18.8
300 21.2 21.0 20.4 19.7
3600 22.5 22.4 21.7 21.0
Exposure time for sky
background to reach half
of saturation [sec]
500 300 40 55
Total System Throughput 0.23 0.24 0.51 0.40

Spectroscopy: Signal-to-Noise Calculator

Slit mask field: 4 x 6.9 arcmin
Slitlets: Recommend 0.4" wide slits at least 7" long (slit lengths can differ, but widths should be the same if possible)
Long Slits: 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 2.4 x 420 arcsec
Wavelength coverage:Provided by filters; grisms cannot be tilted.
Cal lamps: Continuum, Argon
Telluric calibration:Bright A0 stars
Observing Strategy: Similar strategy as imaging, except that the target(s) must be dithered along the slit(lets). Exposure times can be much longer because the sky background is dispersed. In addition, argon lamp exposures are required at each pointing to perform wavelegth calibration, and a bright telluric standard should be observed at identical airmass.

Grism Modes
Grism Order Filter Wavelength
range (um)
over (pix)
(0.4'' slit)
Field for
complete spectra
over band
S/N plots
for an
A0 star
HK 1 HK 1.25-2.45 1800 1400 -0.5 to +0.5 (plot) 70% plots now
HK 1 K 1.98-2.32 550 1700 -2.0 to +1.2 (plot) 70% plots now
HK 1 H 1.50-1.79 450 1300 -1.7 to +2.0 (plot) 80% plots now
HK 2 Y 0.96-1.07 360 1600 -2.0 to +2.0 (plot) 60% plots now
HK 1+2 zJ 0.95-1.5 var 800,1600 n/a (plot1, plot2) var plots now
J 1 J 1.17-1.33 720 2800 -2.0 to +2.0 (plot) 65% plots now
J 1 Y 0.96-1.07 520 2400 -0.5 to +2.0 (plot) 33% - now
J 1 zJ 0.94-1.51 2600 2400 n/a (plot) 50% plots now
H 1 H 1.50-1.79 800 2400 -1.0 to +2.0 (plot) 80% plots now
H 2 Y 0.96-1.07 670 3000 -2.0 to -0.3 40% plots now
K3000 (VPH) 1 Kspec 1.95-2.45 - 3000 - - - now
H3000 (VPH) 1 H 1.50-1.79 - 3000 - - - now
* Range of slitlet positions East of field center at Position Angle = 0, or South of field center at PA=90, in arcminutes.