III. 2MASS Facilities and Operations
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:
|Band||Optics and Camera||System + 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 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.
|North1 and 2||1.997||2.006||1.984|
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.
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:
- A 51 ms reset was taken during the telescope secondary mirror flyback period.
- A second 51 ms reset occurred after the telescope secondary mirror began its linear scan motion, thus stabilizing the telescope on a new position on the sky. This reset defined the beginning of the "READ1" or "R1" integration period. Each pixel begins integration immediately following its reset. Since the reset flows through a quadrant of array in the course of 51ms, integration times are staggered within each quadrant with the first pixel integrating 51 ms "ahead" of the last. Identical readout timing preserves a constant integration time across a quadrant.
- A 51 ms readout of the array was taken immediately following the second reset. This readout preserved a 51 ms integration time image of the sky for recovering bright sources and was the first readout in the doubly correlated sample that produced the final 1.3 s integration. Photometric extractions from this frame, used when bright stars saturated in the longer integration, are designated as "READ1" photometry and receive rd_flg=1.
- A 51 ms readout occurred after the 1.3 s frame integration time had passed. The difference between this readout ("READ2" or "R2") image and the previous readout constituted the final doubly-correlated difference frame, referred to as the "READ2-READ1" or "R2-R1" frame. Photometric extractions using this integration period receive rd_flg=2.
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.
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.
|Figure 1||Figure 2|
|Figure 3||Figure 4||Figure 5|
|Figure 6||Figure 7|
|Figure 8||Figure 9|
[Last Updated: 2008 September 3; by M. Skrutskie & R. Cutri]
Previous page. Next page.
Return to Explanatory Supplement TOC Page.