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, p. 2).

i. Cryostat and Optical Configuration

Each 2MASS camera consisted of a liquid nitrogen cryostat (Figure 1) which contained three 256x256 NICMOS3 arrays. A raytrace (Figure 2) of the system shows that each array viewed the same region of the sky via beamsplitting dichroics. The light passed through both dichroics to the Ks-band array. The first dichroic reflection fed the light to the J-band array. All three optical paths shared the same first element located behind a cryogenic field stop. Each optical path had six other lenses. This set of six lenses was identical for each band. The lenses were composed of water-free fused silica (Infrasil) or calcium fluoride. All lens surfaces were spherical. All optical elements were anti-reflection coated and the integrated optical assembly transmitted ~70% of the incident light. A band-limiting interference filter located near a pupil image established the system bandpass. Figures 3, 4, and 5 show the system throughput (independent of the atmosphere), based on manufacturer's test data for the windows, optical coatings, dichroics filters, and detector quantum efficiency. Section III.1.b1 presents characterizations of the individual components of the system as well as the total instrumental throughput and characteristic atmospheric transmission across the bandpasses. Table 1 summarizes the Survey's bandpasses:

III.1.b1 - 2MASS Component and System Transmission Data

Table 1 - 2MASS 50% Transmission
BandOptics and CameraSystem + Atmosphere
J 1.11 - 1.40 um 1.12 - 1.36* um
H 1.51 - 1.79 um 1.51 - 1.78 um
Ks 2.00 - 2.31 um 2.02 - 2.30 um
*The long-wavelength cutoff of the J-band was defined by atmospheric H2O and CO2 absorption. Unsaturated water absorption also occurs between 1.10 and 1.14 microns. The data pipeline routinely tracked 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. Linear interpolation of the system zeropoint between frequent calibration observations (see III.2c) limited the impact of this zeropoint variation to less than 1% in the ultimate calibrated source photometry.

The completed camera optical assembly (Figure 6) forms a modular unit which attaches to the cryostat (Figure 1) cold plate adjacent to the three detector arrays. For a laboratory point source, the optics concentrated approximately 90% of the image's energy onto a square area defined by one 40 micron NICMOS3 pixel. When mounted on the 2MASS telescopes (Figure 7) each pixel subtended approximately 2.0" on the sky. Platescale varies by <1% between the three bands. In Tables 2 and 3 below "north1" refers to the northern camera prior to the H-band array replacement in 1999 August. The change of array left the platscales unchanged.

Table 2 - Array platescales (arcseconds per pixel)
North1 and 2 1.997 2.006 1.984
South 1.989 1.992 1.980

The array mounts could translate on three axes (focus and "x-y" on the sky) and could rotate about the optical axis. The x-y adjustment permitted the three arrays to have a common center to within 10" (Figure 8). "Figure 8 shows the actual projected alignment on the sky of the three arrays in each camera. The second "North Array" panel shows the alignment after the replacement of the H-band array in 1999 August. The rotational adjustment enabled array orientation such that a star crossed one or two columns as it was stepped in (largely) declination to each of 6 positions. This column crossing, combined with precise in-scan stepping of the telescope, permitted sampling of the PSF at consistent sub-pixel locations.

Table 3 - Array rotation angles (degrees relative to North-South)
North1 0.372+/-.020 0.440+/-.0220.326+/-.019 15814
North2 -0.303+/-.015 -0.420+/-.0160.265+/-.015 9132
South 0.225+/-.006 0.252+/-.0050.175+/-.005 34785

ii. Electronics

The 2MASS readout electronics sampled each pixel with 16-bit precision with a pixel dwell time of 3.0 microseconds. A complete readout of the array, obtained by simultaneous readout of the four independent 128x128 quadrants of the NICMOS3 device, required approximately 51 milliseconds. In order to preserve a constant integration time spatially across the array in every readout, the reset cycle clocked with precisely the same timing as a readout cycle. A 2MASS exposure resulted from the following sequence of events:

Figure 9 shows the integration cycle timing vs. the position of the telescope secondary mirror.

The gain of the 2MASS electronics was approximately 8 electrons per analog-to-digital count. The array bias voltage was 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 was 5 counts or 40 electrons. In the standard 1.3 s integration, sky photon noise dominated the read noise in all three bands, although under extremely low-airglow conditions the read noise and photon noise in the J-band were nearly equal.

iii. Operations and Maintenance

In order to maintain system stability the cameras were kept at cryogenic temperature for as long as possible. The Southern hemisphere camera was cooled in 1998 March and maintained cold for the three-year duration of Southern observations. The Northern hemisphere camera was warmed annually each Summer monsoon season.

The Northern hemisphere H-band array began suffering pixel outages with each thermal cycle early in Northern operations. By the 1999 monsoon season these outages combined to threaten survey coverage despite the six independent images of each point on the sky. The array was replaced in 1999 August and the "original" vs. "new" array distinguishes between "Period 1" and "Period 2" in Figure 8 below. The other five arrays were stable and used throughout the Survey.

The Northern array replacement represents the most substantial change in the overall observing system. The readout electronics were changed on both the Northern and Southern systems during operations. The replacement electronics were specified to have identical clocking and sampling timing and the preamplifier electronics were not replaced. The electronics change was thus hardly distinguishable in the data. The 2MASS Hardware and Operations Signficant Events Log in Table 4 identifies other modifications to the observing system. Some of these changes triggered the definition of new hardware periods for the evaluation of dark frames and flat field response.

Table 4 - Hardware and Operations Significant Events Log

Figure 1Figure 2

Figure 3Figure 4Figure 5

Figure 6Figure 7

Figure 8Figure 9

[Last Updated: 2008 September 3; by M. Skrutskie & R. Cutri]

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