IRIS2 Overview

This page gives a quick summary of the current capabilities of IRIS2 which potential users will need to know before preparing an observing proposal. This page will be updated regularly as we gain more experience with the instrument, and users are recommended to consult it before submitting (or re-submitting) a proposal. Please contact the Instrument Scientists, Chris Tinney or Stuart Ryder, if you require any further information not listed here.
 

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Contents:

• Optical design
• Cryogenics and Dewars
• Wheels within IRIS2
• Filters available
• Spectroscopic Formats
• Detector Performance
• Image scale and Orientation
• Slit Orientation
• Imaging Sensitivity
• Imaging Exposure Time Calculator
• Spectroscopic Sensitivity
• Spectroscopic Exposure Time Calculator
• Count rates, sky brightness and filter profiles for Imaging

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Optics

The figure shows the general IRIS2 layout.  It is available in several forms.  Yellow on Black GIF (20K). Black on White GIF (20K). Black on White Postscript (55K).

 

Top end	Pixel scale	Field of view
f/8	0.4486+-0.0002"/pixel (measured)	7.7'x7.7'
f/15	0.24"/pixel (estimated)	4.1'x4.1'
f/36	0.10"/pixel (estimated)	1.7'x1.7'

Cryogenics & Dewars

The instrument is kept cold by two Cryodyne closed-cycle refrigerators. In order to ensure minimal thermal cycling of the fragile (and valuable) detector, the instrument has two dewars. The main dewar contains all the collimator-camera optics, the detector and the pupil, filter and grism wheels. This dewar has a long cycle time, and it is planned that once commissioned, this dewar would be kept cold and untouched permanently. It also has an auxiliary liquid nitrogen cooling system which is used to accelerate cool down.

The fore-dewar contains the aperture wheel, and room for future expansions to include a multi-slit juke-box, IFU elements and polarimetry analysers. It will be able to be cycled on a short (1-2 day) time-scale, so user provided filters, masks etc., will go here.
 

Wheels

The IRIS2 wheels are self explanatory. The aperture wheel or slit wheel has: fully clear and fully opaque apertures; two 1" wide slits for spectroscopy at f/8 (one of these is offset relative to the other in order to provide better centering of the J- and H-band spectra on the detector); a matrix mask for focus checks and optical distortion mapping; and up to two multi-slit masks.

The 12 position coldstop wheel sits at the re-imaged telescope pupil, and is loaded with cold stops for use in applications where thermal background is critical. This wheel is also used for narrow band filters at short wavelengths, where the instrument can be used without a cold stop.

The 12 position filter wheel  contains filters for use with the cold stop.

The 8 position grism wheel  contains up to five locations for mounting spectroscopic grisms, as well as an opaque position for obtaining detector darks.
 
 

Filters

The current filter complement is listed below - see the IRIS2 Filter page table for more details. All filters are 60mm in diameter, which is required for unvignetted operation at f/8. These filters were purchased as part of the GEMINI IR Filter Consortium.

The J,H,K,Ks filters were manuactured by OCLI. These are the filters discussed at Alan Tokunaga's 2001 and 2002 papers. These papers contain links to other work using NSFcam on the IRTF and IRCAM3 on UKIRT to define colour terms for these filters. These can be used as indicative for the AAT, but detailed colour terms will de different due to those instruments using InSb (not HgCdTe) detectors, and having different optical trains.

The narrowband filters and methane filters were manufactured by NDC Infrared Engineering. The spectroscopic blocking filters were manufactured by Custom Scientific.

 

Filter Name	Cut-on  (µm)	Cut-off  (µm)
Broadband
J	1.164	1.325
H	1.485	1.781
Ks	1.982	2.306
K	2.028	2.364
Z	0.996	1.069
Jshort spectroscopic	0.95	1.25
Jlong spectroscopic	1.05	1.35
H spectroscopic	1.44	1.82
Narrowband
He I *	1.075	1.091
J continuum *	1.198	1.216
Pa Beta	1.272	1.292
H continuum	1.558	1.582
[Fe II]	1.632	1.656
Methane Offband (CH4_s)	1.53	1.63
Methane Onband (CH4_l)	1.64	1.74
H2 nu=1-0 S(1)	2.106	2.138
Br Gamma	2.150	2.182
H2 nu=2-1 S(1)	2.231	2.265
K continuum	2.253	2.287
CO(2-0) band head	2.278	2.312

* The He I and J continuum filters have a red leak
in the K-band, so must be used in combination with
the J broadband filter.

Information on the sky count rates, throughput and filter bandpasses can be found at the IRIS2 Filter Page.

Spectroscopic Formats

With its combination of two different long slits (SLIT_150 and OFF_150), two useful grisms (SAPPHIRE_240 or S-K, and SAPPHIRE_316 or S-H, plus several less useful grisms) and a range of order sorting filters (K,Ks,H,Hspect,J,Jshort,Jlong) IRIS2 offers the possibility of a wide range of wavelength formats. In practise, most observers will be using the grisms in a limited number of formats, which are listed in the following table along with the standard name we use for that format
 

 

Format	Grism		Order	Slit	Filter	R	Start (um)	End (um)	Mean Dispersion (nm/pixel)	Comments
K	SAPPHIRE_240	S-K	m=1	SLIT_150	K	2250	2.02	2.37	0.442	Cutoff wavelengths determined by K filter
Ks	SAPPHIRE_240	S-K	m=1	SLIT_150	Ks	2200	2.02	2.31	0.442	Blue cutoff set by array; red cutoff set by filter.
H	SAPPHIRE_316	S-H	m=1	OFF_150	H	2270	1.47	1.79	0.341	Cutoff wavelengths  determined by H filter
Hspect	SAPPHIRE_316	S-H	m=1	OFF_150	Hspect	2270	1.46	1.81	0.341	Blue cutoff set by array, red cutoff set by filter+array.
J	SAPPHIRE_240	S-K	m=2	OFF_150	J	2490	1.17	1.33	0.225	Cutoff wavelengths determined by J filter
Jshort	SAPPHIRE_240	S-K	m=2	SLIT_150	Jshort	2460	1.04	1.26	0.232	Blue cutoff set by array; red cutoff set by filter.
Jlong	SAPPHIRE_240	S-K	m=2	OFF_150	Jlong	2490	1.10	1.33	0.225	Blue cutoff set by array; red cutoff set by filter+array.

For more details on wavelength formats and wavelength claibration see the IRIS2 Wavelength Calibration page.

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Detector performance

The IRIS2 HAWAII1 detector has been configured for use in two read modes.

Double Read Mode (DRM): The array is reset, a read is done, followed by a second read at the end of the exposure. The image delivered is the difference between the two reads. There is an overhead of one array read time per exposure. This is the mode used for all imaging applications. (Also known as Double Correlated Sampling)

Multiple Read Mode (MRM): The array is reset and then non-destructively read repeatedly throughout the specified exposure time. The image delivered is a least-squares fit through the non-destructive reads. There is an overhead of one array read per exposure (usually a tiny fraction of the total exposure length) and several seconds to complete the least-squares fit. This is the mode used for all spectrscopic applications. (Also known as 'Up-the-Ramp' sampling).

For each read mode we have configured two read speeds. In almost all situations the NORMAL speed in each mode will be the optimal one (the IRIS2 software offers three speed options,  but NORMAL and FAST are the same speeds).
 

 

Mode	Speed	Read Time (s)	Gain (e/adu)	Read Noise/NDR (e)	Typical Read Noise (e)	Full Well (ke-)	Linearity (at Full Well)	Comment
DRM	NORMAL (=FAST)	0.5982	5.3	10.0	14.1 (for DRM)	180	~1%
	SLOW	0.9925	4.4	8.7	12.3 (for DRM)	70	~0.25%	Slightly better cosmetics & linearity then NORMAL
MRM	NORMAL (=FAST)	0.7866	4.3	8.6	4.8 (for 61reads)	75	~0.3%
	SLOW	1.3109	5.2	8.6	~4.8 (for 61 reads)	130	~1.2%	No reason to prefer to NORMAL.

Dark current

Measured dark current performance is typically <1 e- per second. There is some structure to the dark frames, most noticeably a "hot spot"  of about 50 pixels near the centre of the lower-right quadrant, an excess of charge in the lowest rows of each quadrant, and a diagonal stripe across the top-right quadrant. All of these artifacts subtract out quite well, so we recommend that matching dark frames be obtained for each combination of exposure/cycles to be used in imaging mode.

Quantum Efficiency

QE has not been measured directly for our science device. The image below shows the 'standard' Rockwell curve. Rockwell have provided a histogram of QE values accross the detector, which peaks at 69%. However, as they don't bother to tell us what wavelength they did that measurement at, we have no idea how to normalise the curve below.

Full Well / Linearity

Tests show that non-linearity varies with read speed. In most applications, the non-linearity can be kept below 0.5% by keeping fluxes to less than ~15,000 adu. The non-linearity can be strightforwardly calibrated out (see the Linearity Technical page for more information) which raises the usable well-depth considerably.

Residual Images / Mode switching dark current

Tests made with the slit in place illuminated by a flat field lamp, and then followed by darks, seem to indicate residual images are present at the 0.01% level. That is if you illuminate the slit with light at a rate of about 6667adu/s (ie 10000 adu in a 1.5s exposure), and then follow this with a 30s dark, you will get a residual image at a level of about 15adu (or 0.5adu/s). Clearing these residual images seems to be a matter of both taking lots of resets and reads of the detector, and just physically waiting a while.

Switching between read modes (ie DRM and MRM) also seems to induce additional dark currents. So all acquisition exposures for spectroscopy should be acquired in MRM mode.

Residual images / switching images appears as an additional dark current. Even though the 0.01% of the HAWAII array is quite good, short exposures on bright sources followed by long exposures on faint sources may show residual images. We reccomend

• Acquire your object in a passband with the lowest sky counts possible. This will usually be J.
• Acquire your object in MRM mode.
• Be aware that the first spectrum you take after acquisition may still have slightly higher noise, though if you follow these guidelines, the additional dark current is not usually significant.
  

Cross Talk between Quadrants

HAWAII arrays are know to suffer from cross-talk between their quadrants. If you have a bright object at pixel (660,700), the inter-quadrant cross talk appears as additional flux in all pixels of row 700 and row 700-512=188. Correction of this effect is straight forward. If you collapse your raw data down into a 'spectrum' from adding up all the columns, fold this spectrum at row 512, multiply it by 2.e-5, and then subtract it from all columns of the detector, the inter-quadrant cross talk is almost completely eliminated.

Windowing

Is not implemented for IRIS2.

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Image Scale and Orientation

Image scale with the science-grade array measured using 2MASS astrometry and a linear astrometric fit to tangent-plane projected and radial distortion corrected images = 0.4486 +/- 0.0002"/pixel. For non-astrometrically corrected images, this plate scale will be correct at the field centre, but up to 1.4% incorrect in the array corners. (See IRIS2 Distortion Appendix for more information on the form of the IRIS2 astrometric distortion.) The detector orientation as it appears on the SKYCAT display (and in FITS files) is North to the bottom, W to the left, when IRIS2 is used in its default Cassegrain rotator=90 orientation. Instrument rotation is currently limited to between 90^o (N-S slit) and 180^o (E-W slit), and requires an AAO staff member be in the Cass cage while the instrument is rotated to watch out for cable and hose fouling.

With the Cassegrain instrument rotator set to 90^o, the array is aligned such that true N is rotated by 0.1 deg clockwise relative to the array columns. This information is recorded in the FITS header WCS, and updated whenever the rotator is set to a different angle.

For J and K spectroscopy, wavelength decreases with pixel number (i.e. redder wavelengths are to the left as seen on the Skycat display).

For H spectroscopy, wavelength increases with pixel number (i.e. redder wavelengths are to the right as seen on the Skycat display). The H grism is reversed relative to the J/K grism so that it can be used with the ofset slit to obtain the whole H bandpass in one go.
 
 

Slit Orientation and Re-positioning Accuracy

The slit is rotated relative to the array columns by 17 arcminutes, in a clockwise sense.

All spectroscopic acquisition with IRIS2 must be carried out using IRIS2 in imaging mode.

Tests of slit re-positioning show that the slit seems to re-position with no backlash to <0.1" when the instrument is nearly upright. When the telescope is slewed off the meridian East-West, the slit re-positions to < 0.1" as long as the wheel is always moved in the same direction (which it always does during spectrscopic acquisition).

While the slit repositions very precisely, however, the place it re-porisions to does depend on the telescope's East-West orientation, with the slit moving by ~0.5 pixels when the telescope is slewed 3 hours to the East or west from the meridian.

For significant motions East-West from the last spectroscopic acquisition, the acquisition of a new slit image (to re-determine the slit location) is recommended.

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Sensitivity figures

Imaging sensitivities & S/N calculator

 
 

Broadband Imaging Figures of Merit.
Filter	JHK Zeropoint 1 ADU/s at 1 airmass	Flux for JHK=0 (ADU/s)	Sky Brightness (mag/arcsec2)	5-sigma limiting magnitude  in 1 hr (on sky) 1.2" seeing, 2" aperture.
J	22.78	1.3e9	16.4	22.4
H	22.87	1.4e9	14.4	21.6
Ks	(22.46)	(9.6e8)	13.8	21.6
K	22.46	9.6e8	13.5	21.4
Notes: - For the purposes of proposal preparation and S/N calculations we temporarily adopt the catalogued K magnitude as the Ks magnitude  for the standards to get these numbers (and do the reverse to calibrate the available deep imaging which is usually only done in Ks). - The sky brightness has so far only been measured in winter (July 2002), and can be expected to be up to 0.5 mag brighter in summer time at K, or when observing close to the Moon at J or H. - The median optical seeing of 1.5" at the AAT corresponds to 1.2" at H-band (FWHM ~ lambda-1/5).

Imaging overheads


The major overheads for broad band imaging is the detector read-time and telescope movement. For the purposes of preparing an observing proposal, you should factor in overheads of ~25%.

Readout

There is an overhead of one detector read-time on every DCS exposure. So for DRM FAST with an exposure of 10s, this is 0.6s/10s = 6%. Windowing is not possible. There is also a "dead-time" between new exposures of up to 7 seconds, even if no telescope offsets are commanded, as the acquisition system finishes writing the previous image to disk, and sets itself up for the next exposure.


Telescope

You should assume an overhead of roughly 12s for a typical telescope offset of 10" (note: this includes the 7s detector overhead mentioned above). If dithering every 60s, this corresponds to a 20% overhead. If nodding the entire detector to sky (i.e. nodding by 9') then overheads will be more like 20s per offset for N-S nods, and twice as large for similar E-W nods (the telescope offsets more rapidly N-S).

Spectroscopic sensitivities & S/N calculator


Providing a single S/N estimate for a given magnitude object in a given passband is only crudely possible. Sky brightnesses and atmospheric transmission within an atmospheric window can vary by factors of up to ten. We have prepared an Exposure Time Calculator for IRIS2 which is based on the median sky brightness per pixel, and median object brightness per pixel as measured in practise with IRIS2. This means that in a given spectrum half the pixels will have S/N better than that listed below and half will be worse. However, these estimates should be adequate for most proposals.

Notes

• Note that two 1" long slits are provided in IRIS2. The SLIT_150 and OFF_150 slits.
  • SLIT_150 is used with the S-K (SAPPHIRE_240) grism, and K/Ks filters for K-band spectrscopy, or Jshort filter for J-band spectroscopy.
  • OFF_150 with the S-K (SAPPHIRE_240)  grism and J/Jlong filters for J-band spectroscopy, and the S-H (SAPPHIRE_316) grism and H filters for H-band spectroscopy.
• The wavelength format delivered by the various filter+grism+slit combinations are discussed above. The different filters and slits determine the start and end wavelengths within the J, H and K atmospheric windows, and the grisms determine the dispersion, throughput and so S/N.
• Exposure time estimates should assume 1.2" seeing for IRIS2 proposal purposes.
• Only S/N estimates for the R~2400 saphire grisms are provided.
• Nodding along the slit is assumed. If the target object is nodded to blank sky, then your S/N will be a factor of sqrt(2) worse in a given exposure time than that provided below.
• These estimates are based on observations acquired in October 2001. Sky brightnesses in J and H will vary from night to night. In winter the sky brightness at K may be better by up to a factor of two. The Calculator should be used for proposals, but please be aware that actual performance on your run may be different.

Absolute Spectroscopic Efficiency

Absolute throughputs have been measured for the SAPPHIRE_240 grism, when used with the K and J filters.
• Sap240+J transmits 36.4% of the total J-band photons incident on it. This means throughput peaks at 40% and is greater than 30% accross the entire J bandpass.
• Sap240+K transmits 38% of the total K-band phtons incident on it. This means its throughput peaks at 52% and is greater than 35% accross almost the entire K bandpass.
• Detailed measurements have not been done for the Sap316+H grism, but counts on flat fields would indicate it has  throughput intermediate to the two above - peaking at about 45%, and greater than 35% accross the H band-pass.

Spectroscopic overheads.


The major overhead for spectroscopy is acquisition

Readout
Readout overheads for spectroscopy exposuresacquired in MRM are small. There is a slight delay (1-2s) at the end of an exposure as the least-squares fit is re-done a final time and a 0.9s readout overhead over the length of the whole exposure.

So, for example, for K-band spectroscopy where you might be exposing for 300s with 61 non-destructive reads (before nodding the telescope to sky), the overheads are (0.9s+2s)/300s or 1% of the exposure time.
 

Telescope
You should assume an overhead of roughly 5s for a typical telescope offset when dithering or nodding along the slit. Again, these are usually a small fraction (<1.2%) of typical exposire times of >300s.


Acquisition

Acquisition is the major overhead for spectroscopy. You will acquire your target using short imaging exposures. You should be able to get away with acquisition using 4-5 such images to put your object down the slit. If your target is faint (or hard to identify) this can easily take up to 15  minutes. The night assistant will also need to acquire a guide star. This is usually done in parallel to getting the target close to the slit, but can add an extra overhead of a few minutes.

You should assume acquisition will take 15min at the start of your run, improving to 5min as you gain experience.

Acquisition should always be done in MRM mode itself (eg. 5s exposures with 2 MRM reads). This ensures you get a 'clean' detector with no switching dark current in the first of your deep MRM images.

These pages contain information on the functionality of the IRIS2 Infrared Imager and Spectrograph. Local project information can be found at the Local IRIS2 Project Page (AAO Local access only). Pages maintained by Chris Tinney mailto:cgt@aaoepp.aao.gov.au and Stuart Ryder (sdr@aaoepp.aao.gov.au).

Last modified July 19, 2004 by Stuart Ryder.