IRCS camera filters

The following table lists the filters available in the camera section of IRCS, together with their sensitivities. The transmission curves of each filter are available as plots or ASCII lists (lists kindly provided by Alan Tokunaga of the IRTF) if indicated in the respective columns.

All sensitivity figures are for 5 sigma detections in 1 hour of on-source integration with the infrared secondary mirror. They assume background-limited performance (see the guide for exposure time estimation). Point source sensitivities assume 0.5'' seeing and a 1.0'' diameter extraction aperture; extended source sensitivities are per pixel (the first column is for the 20 mas pixel scale, the second for the 52 mas pixel scale); binning over N pixels will improve the sensitivity by 1.25 log N mag.

Broad-band filters


Observers should be aware that variations in the strength of the OH airglow can cause changes of up to 1 mag in the sky background, with significant changes in sensitivity.
Filter Wavelength (μm) Sensitivity(1) mag for 
1 e-/s
(mag) (mag/arcsec2)
w/ AO w/o AO w/ AO w/o AO w/ AO  w/o AO
Name Plot List Center Width 20 & 52 mas 20 & 52 mas 20 mas 52 mas 20 mas 52 mas 20 & 52 mas 20 & 52 mas
Y 1.02
0.103 TBD
TBD
- - TBD
TBD
25.8
TBD
J 1.25 0.16 23.5 23.6 - - 19.6 20.6 26.1 26.3
H 1.63 0.30 22.7 22.8 - - 18.6 19.6 26.3 26.5
K' 2.12 0.35 - 22.6 - - 18.4 19.4 - 25.8
K 2.20 0.34 22.2 22.3 - - 18.2 19.2 25.5 25.7
L'(2) 3.77 0.70 - 17.1 - - 13.3 14.3 24.7 24.7
M'(3) 4.68 0.24 - 14.6 - - 10.8 - 22.5 22.6

(1) The sensitivities for w/ AO will be improved about 1-2 magnitude with full AO correction. Please check the improvement with Imaging ETC with seeing ~ 0.1", aperture ~ 0.2" - 0.3" parameters.
(2) L' observation with the 52 mas pixel scale is limited to a maximum 512x512 (27"x27") subarray with AO and 896x896 (47"x47") subarray without AO.
(3) M' observation is limited to 20 mas pixel scale mode.

Narrow-band filters


The following amounts are estimated while a filter was cold and tilted by 5 degrees in IRCS (except Br gamma filter).
Filter Wavelength (μm) Sensitivity flux for 
1 e-/s 
(μJy)
Name Plot List Center Width (μJy) (μJy/arcsec2)
CH4 short 1.570 0.100 1.7 60 23 0.06
H cont (NB1550)


1.549 0.018 -
-
-
0.25
CH4 long 1.690 0.100 1.7 60 23 0.06
[Fe II] 1.644 0.026 3.3 120 47 0.4
NB2070

2.070 0.030 -
-
-
-
NB2090


2.091
0.035
-
-
-
0.33
H2 1-0 2.122 0.032 3.0  110 44 0.4
Br gamma (1) (1) 2.166 0.032 3.0 110 44 0.4
K cont (NB2315)

2.314 0.030 -
-
-
0.63
H2O Ice 3.050 0.152 41 2193 832 0.45
PAH 3.295 0.050 55 2900 1100 0.6
H3+


3.413 0.022 -
- - -
Br alpha 4.052 0.060 140 7500 3000 0.6

(1)Transmittance at room temperature.

The Transmission Curves of the ND Filter installed in the Filter Wheel 3


There are two ND filters installed for IRCS, one each in the filter wheel 1 and 3. Although we do not recommend to use this filter in combinations with any other filter for scientific photometry, it is often used for bright objects along with other filters. For its transmission curves throughout the IRCS wavelength coverage, please refer to this plot.

The Shift of Transmission Curve within the FOV


The actual wavelength coverages for these filters will change as a function of the angle of incidence (AOI) to the filter. This mean that the filter transmission will have the significant dependence with the position on the image, especially for narrow-band fileters. You can refer to these plots for the transmission shifts of several NB filters. Regarding the informations for the other filters, please contact the support astronomer.




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