NIRC2 Sensitivity Measurements

The NIRC2 sensitivity values listed in the table below are a compilation of all throughput/sky measurements made during the first years of NIRC2 operations. Differerences in the zero point may represent real changes in throughput, poor tracking and correction of atmospheric extinction, or merely measurement noise. Differences in the sky surface brightness can be due to different amounts of moonlight, differences in atmospheric conditions, etc. Differences in "sky" for the thermal IR (e.g. Lp and Ms) could be due to changes in the cleanliness of the AO bench, differences in atmospheric conditions such as water vapor content, etc.

The next table shows sensitivity values obtained during the First Light commissioning on July and August 2001.

Filter¹ Camera² Pupil³ Zero Point⁴
^(magnitudes) Sky⁵
^{(mag/arcsec^2)} PSdet⁶
^(magnitudes) SB Lim⁷
^(magnitudes) BL Time⁸
^(seconds)

J Wide Circ 25.3 15.5 26.7 25.6 52

J Wide Insc 24.8 15.1 26.3 25.1 57

J Medium Circ 25.1 15.2 27.2 25.3 190

J Medium Insc

J Narrow Circ 25.1 14.9 27.8 25.2 57

J Narrow Insc

H Wide Circ 25.3 14.1 26.0 24.9 14

H Wide Insc 24.9 13.3 25.4 24.3 10

H Medium Circ 25.2 13.7 26.5 24.6 43

H Medium Insc

H Narrow Circ 25.2 13.6 27.2 24.6 158

H Narrow Insc

K Wide Circ 24.6 11.4 24.3 23.2 2

K Wide Insc 24.2 22.9 24.4 23.2 5

K Medium Circ 24.5 11.3 25.0 23.1 9

K Medium Insc 24.1 11.8 25.0 23.1 21

K Narrow Circ 24.5 11.3 25.7 23.1 36

K Narrow Insc 24.0 11.7 25.7 23.0 83

Ks Wide Circ 24.5 11.5 24.3 23.2 3

Ks Wide Insc

Ks Narrow Circ

Ks Narrow Insc

Kp Wide Circ 24.7 11.7 24.5 23.4 3

Kp Wide Insc

Kp Narrow Circ

Kp Narrow Insc

Lp ⁹ Narrow Circ 23.2 1.6 20.0 17.6 0.016

Lp Narrow Insc 23.0 2.2 20.2 17.8 0.033

Ms ⁹ Narrow Circ 21.2 -1.1 17.5 15.2 0.008

Ms Narrow Insc 20.8 -0.5 17.6 15.3 0.021

Next table shows the sensitivity values obtained on March 31, 2004 after realignment of the AO rotator. The telescope pupil may match the NIRC2 pupil mask better than before.

Filter¹ Camera² Pupil³ Zero Point⁴
^(magnitudes) Sky⁵
^{(mag/arcsec^2)} PSdet⁶
^(magnitudes) SB Lim⁷
^(magnitudes) BL Time⁸
^(seconds)

J Narrow LHex 25.47 ± 0.07 13.60 ± 0.25

H Narrow LHex 25.51 ± 0.07 13.24 ± 0.13

K Narrow LHex 24.73 ± 0.06 12.34 ± 0.10

Kp Narrow LHex 24.84 ± 0.07 12.66 ± 0.12

Lp ⁹ Narrow LHex 23.60 ± 0.09 3.01 ± 0.09

Lp Narrow Insc 23.34 ± 0.06 3.03 ± 0.07

Ms ⁹ Narrow LHex 21.42 ± 0.29 0.23 ± 0.30

Ms Narrow Insc 21.11 ± 0.22 0.19 ± 0.22

General notes:

Inconsistencies in the above table are due primarily to data being taken on different nights.
Mean detector gain = 4.0 e-/DN, Readnoise = 37e-/pixel/read

Footnotes:

1. All broadband filters are correctly blocked to 5 microns and are used with an open position on the second filter wheel.

2.Camera:

Wide = 0.04"/pixel
Medium = 0.02"/pixel
Narrow = 0.01"/pixel

3. Pupils: NIRC2 contains 6 pupils, 4 of which rotate in order to track the rotating pupil image. Pupil rotation was not available during the first commissioning run, so only the circumscribed (circ) and inscribed (insc) circular pupils were used (note that the inscribed pupil is designed to rotate).

The circumscribed pupil is a completely open aperture, blocking none of the thermal radiation from the secondary, spider vanes, or even the telescope/building seen around the edges of the mirror. This pupil has no mask for the central obstruction or spider vanes.

The inscribed pupil is the largest circular aperture which lies completely within the zig-zag hexagonal edge of the pupil image. It does have a central obstruction and spider vanes and normally would be used in a rotating mode. It was used without rotation for the measurements below. The thermal contribution to the background from the spider vanes should be minimal. The thermal emission is dominated by the mirrors in the AO system. It does block the outer points of the primary, effectively reducing the telescope to a ~9m size.

Ideally, the sensitivities and backgrounds through these two pupils bracket the expected performance of NIRC2 when full pupil rotation is implemented.

4. Zero Point. Defined as:

magnitude = ZPt - 2.5log[counts/second]

There is currently no explicit solution for the airmass correction. These numbers may improve a bit as the AO system mirrors are cleaned.

5. Sky surface brightness in magnitudes per square arcsecond.

6. Point source detection limit defined as a 5-sigma detection in 1 hour on source. An aperture diameter of 1.22×λ/D with a minimum 2 pixels was used.

Note that this assumes perfect AO correction. If your Strehl ratio is 40% then your point source must be one magnitude brighter than the quoted limit. If your Strehl is 10%, your point source must be 2.5 magnitudes brighter.

7. Surface brightness limit (noise produced by the background) defined as 1-sigma in 1 hour integration within an area of 1 square arcsec

The surface brightness limits assume the observed sky magnitudes.

These numbers may improve, especially in the K-band and longer wavelengths as the AO optics are cleaned and cooled relative to these initial measurements.

8. Time to reach background limit, in seconds. Defined as the time when the CDS readnoise (52.3e-) is equal to the square root of the number of photoelectrons. Note that it is better to be safely beyond the point of equality, so you should actually go 2-3 times as long as this. The readnoise on long integrations can effectively be reduced by using multiple end point reads (Fowler sampling or Multiple Correlated Double Sampling - MCDS), allowing one to reach the background limit in a shorter time.

9. Thermal Imaging with the AO system. For the Lp band (and presumably Ms as well) there is a mottling of the background at a level of one part in 500-1000 when the AO system is on.

Grism efficiency: Extensive testing of the spectroscopic mode in NIRC2 has not been done at the time of this writing. To first order, you can assume a mean grism efficiency of ~50% over any photometric band filter relative to the filter without the grism. Slit losses are not included.