Instrument Performance

This page describes the performance of the upgraded CRIRES, currently installed at UT3. An overview for the original (pre-upgrade, pre-August 2014) CRIRES can be in the following link.

Sensitivities

Table 1 lists preliminary values for the maximum S/N values in the stellar continuum and the maximum flux on the detector, both derived from commissioning data. They were determined for the spectro-photometric standard star Pi.02 Ori (H=4.2; spectral type A1V) using a 0.4 arcsec slit, adaptive optics under a Turbulence category of 30% (seeing 0.8 arcsec and coherence time 4.1 ms) and an airmass of 1.3. The DIT and NDIT were 15 s and 3, respectively.

 

Table 1. Sensitivities for Pi.02 Ori  
Band
max S/N
max flux on detector [ADU]
Y (950-1120 nm)
570 11000
J (1116-1362 nm)
530
10000
H (1423-1769 nm)
480
8300
K (1972-2624 nm)
350
5200
L (2869-4188 nm)
- 9600 1)
M (3583-5300* nm)
(* detector cutoff)
- DIT=15s: saturation;
DIT=5s: ~20000s 1)

Please note that IR observations are sensitive to the precipitable water vapor (PWV) in the Earth's atmosphere. Observations in the L and M bands should only be requested for PWV values lower than 4 mm.

1) Given that in the L and M bands the thermal background is the dominating flux contributor, the maximum integration times (DITs) should be limited. Particularly, in the M-band these limits are 15s and 30s, respectively for the 0.4" and 0.2" wide slit. Longer DITs will result in both heavy saturation and reminiscence effects in the detectors that severely could affect the data quality of subsequent observations.

 

Slit Viewer Characteristics

For guiding on target (Target=SVGS), the slit viewer guide star needs to be relatively bright as only a small fraction of the light is reflected by the slit jaws to the slit viewer detector. During the on-sky commissioning run, under excellent conditions (0.5” seeing) and a slit of 0.2”, reasonable guiding was possible with stars of H=14.5 in NoAO. In AO mode (NGS), however, less light is available for guiding; the magnitude limits in AO mode are H=13 and 13.5, respectively for the 0.4" and 0.2" wide slit.
 

Adaptive Optics Correction

The Adaptive Optics (MACAO - Multi-Applications Curvature Adaptive optics) is used to concentrate the light of the target into the slit and thereby significantly enhance the signal-to-noise ratio of the observations. MACAO is equipped with a wavefront sensor (Avalanche Photo Diodes) sensitive in the R band. Since the flux on these diodes is limited to 1 million counts to not damage the devices, stars brighter than R=0.2 mag cannot be used as AO guide stars. Stars fainter than R =15 mag will not result in any improvement. Good correction under good atmospheric conditions can be still obtained with stars as faint as R = 14 mag. Any star fainter than this will require excellent atmospheric conditions (Turbulence category = 10%) to provide any image quality improvement. Please note that the quality of the AO correction will degrade with increasing airmass; we recommend to limit the use of AO to airmass values smaller than 1.4. Please note that the AO mode cannot be used with THICK clouds.

The field selector allows the selection of the AO star within a 30 arcsec field centered on the nominal position of the science target. However, if the AO star lies at more than 10 arcsecs away from the science target, then the AO correction degrades and very good atmospheric conditions are required. Under excellent conditions, R < 11 mag stars as far as 20 - 30 arcsec can still lead to a modest improvement of the image quality. Please note that in P108 off-axis guiding will not be offered.

Science detector characteristics

Typical values for the main characteristics of the science detectors are listed in Table 2.

Table 2. Typical detector characteristics
Read-out noise in e- (rms) for Det1, Det2, Det3
11, 12, 12  
Read-out mode
Sample up the Ramp
Dark current  (e-/second) for all detectors
< 0.03 
Gain (e-/ADU) for Det1, Det2, Det3
 2.28, 2.19, 2.00
Saturation level (e-) for all detectors
37,000 ADU
Non linearity <1% below 6000 ADU/px
>5% above 18,000 ADU/px
>10% above 29,000 ADU/px

Users can select predefined detector integration times (DITs) of 1.427, 2, 3, 5, 7, 10, 15, 20, 30, 45, 60, 90, 120, 180, 300, 450, 600 and 900 seconds.  

 

Radial velocity precision

The short gas cell (SGC) provides a stable long-term wavelength reference in the H and K bands. For a S/N of 150 per spectral pixel in the spectral continuum, an RV precision of 4 m/s has been attained in the lab by employing the short gas cell (SGC) as a simultaneous wavelength calibrator in the K-band with the 0.2” slit. However, this value has not been confirmed on sky so far.

For this reason, users should not expect an RV precision better than 20 m/s for exposure times up to 120s. For longer exposure times, the RV precision might be even worse due to grating drifts. In any case, we note that ESO does not provide any support for the analysis of RV data taken with the gas cell; this is left to the experienced user.

Users can also make use of the telluric absorption lines of Earth’s atmosphere as a simultaneous wavelength reference. In most CRIRES wavelength settings, these lines will anyways be imprinted on the science data. Figueira et al. (2010; A&A, 515, 106) demonstrated that telluric lines are intrinsically stable down to 10 m/s (rms). For information on how to model the telluric lines we refer to the paper by Seifahrt et al. (2010; A&A, 524, 11).

Observations without any simultaneous wavelength calibrator can result in an RV precision on the order of a few times 100 m/s due to grating drifts.

 

Schematics of the lightpath in CRIRES