III. 2MASS Facilities and Operations

1. Facilities

b. Camera and Detectors

A detailed description of the 2MASS camera optical design appears in Milligan et al. (1996, SPIE Proceedings Volume 2863, p2).

i. Cryostat and Optical Configuration

Each 2MASS camera consists of a liquid nitrogen cryostat (Figure 1, below) which contains three NICMOS3arrays. A raytrace (Figure 2, below) of the system shows that each array views the same region of the sky via beamsplitting dichroics. The light passes through both dichroics to the Ks-band array. The first dichroic reflection feeds the light to the J-band array. All three optical paths share the same first element which lies behind a cryogenic field stop. Each optical path has six other lenses. This set of six lenses is identical for each band. The lenses are composed of water-free fused silica (Infrasil) or calcium fluoride. All lens surfaces are spherical. All optical elements are anti-reflection coated and the integrated optical assembly transmits ~80% of the incident light. A band-limiting interference filter located near a pupil image establishes the system bandpass. The 2MASS bandpasses are:

J-band 1.11 - 1.36* microns
H-band 1.50 - 1.80 microns
Ks-band 2.00 - 2.32 microns
* The long wavelength cutoff in this band is defined by atmospheric H2O and CO2 absorption. Unsaturated water absorption also occurs at 1.14 microns. The data pipeline routinely tracks smooth zeropoint variations as large as 0.1 magnitude in the course of a night in this band, presumably due to variation in atmospheric water vapor content.

The detector quantum efficiency, the anti-reflection coatings, and transmission of the dichroics, window, and optical elements are relatively flat across the bandpasses, so the band response functions are believed to be square to a good approximation. These values are all based on manufacturer's specifications. The completed camera optical assembly (Figure 3, below) forms a modular unit which attaches to the cryostat (Figure 1, below) cold plate adjacent to the three detector arrays.

The optics relay approximately 90% of the energy of a laboratory point source onto one 40 micron NICMOS3 pixel. When mounted on the 2MASS telescopes (Figure 4, below) each pixel subtends approximately 2.0" on the sky. Platescale varies by <1% between the three bands.

ii. Optical Response Functions

iii. Electronics

The 2MASS readout electronics sample each pixel with 16-bit precision with a pixel dwell time of 3.0 microseconds. A complete readout of the array requires approximately 51 milliseconds. In order to preserve a constant integration time spatially across the array, the reset cycle clocks with precisely the same timing as a readout cycle. A 2MASS frame results from the following sequence of events:

Figure 5 (below) shows the integration cycle timing vs. the position of the telescope secondary mirror.

The gain of the 2MASS electronics is approximately 8 electrons per analog-to-digital count. The array bias voltage has been set to 1.00 V to produce a dynamic range of 50,000 counts or 400,000 electrons. The system read noise in a doubly correlated difference is 5 counts or 40 electrons. In the standard 1.3 sec integration, sky photon noise dominates the read noise in all three band, although under extremely low-airglow conditions the read noise and photon noise in the J-band become nearly equal.

Figure 1Figure 2Figure 3

Figure 4Figure 5

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