III. 2MASS Facilities and Operations

1. Facilities

1. The layout of the camera is shown in Figure 1, with J, above, H, below, K_s, straight through. After the two reflections in the telescope, the first camera element, proceeding from right to left, is the dewar window, followed by a field stop and the first of the seven lenses. Corresponding lenses are identical among the three cameras. Following the first lens (the only one common to all cameras) are the two dichroic mirrors, first J and then H. The straight-through light path leads to the K_s camera, the upper path to the J camera, and the lower one to H.

   Figure 1

2. Figure 2 shows the telescope mirror reflectivity M(µ). This is for a single reflection of the two in the telescopes, so it must be squared for the total.

   Figure 2

   Main Telescope Mirror Reflectivity Data Table

3. Figure 3 shows the dewar window transmission D(µ).

   Figure 3

   Dewar Window Transmission Data Table

4. Figure 4 shows the camera lens coatings L(µ). The factor L(µ) is for each of the seven lens elements, in each camera lightpath. The total transmission for all lenses combined is over 80% (ie, 0.97⁷ approximately), across each of the three camera bands.

   Figure 4

   Lens Coating Data Table

   J Camera

   Besides telescope mirrors, dewar transmission, and lens coatings, the J-band IR must be reflected by the J dichroic mirror, pass the J filter, and register in the J array:

5. Figure 5 shows the J dichroic reflectivity J_dr(µ).

   Figure 5

   J Dichroic Reflectivity Data Table

6. Figure 6 shows the J filter transmission J_f(µ)

   Figure 6

   J Filter Data Table

   H Camera

   Besides telescope mirrors, dewar transmission, and lens coatings, the H-band IR must be transmitted by the J-band dichroic mirror, be reflected by the H dichroic mirror, pass the H filter, and register in the H array:

7. Figure 7 shows the J dichroic transmission J_dt(µ).

   Figure 7

   J-band Dichroic Mirror Transmission Data Table

8. Figure 8 shows the H dichroic reflectivity H_dr(µ)

   Figure 8

   H-band Dichroic Mirror Reflection Data Table

9. Figure 9 shows the H filter transmission H_f(µ).

   Figure 9

   H-band Filter Transmission Data Table

   K_s Camera

   Besides telescope mirrors, dewar transmission, and lens coatings, the K_s-band IR must be transmitted by the J-band and H-band dichroic mirrors, pass through the K_s filter, and register in the K_s array:

10. J dichroic transmission J_dt(µ)
    See above for this information, in H-band camera section.

11. Figure 10 shows the H dichroic transmission H_dt(µ)

    Figure 10

    H-band Dichroic Mirror Transmission Data Table

12. Figure 11 shows the K_s filter transmission K_sf(µ)

    Figure 11

    K_s Filter Transmission Data Table

    Detectors

13. NICMOS3 detector array quantum efficiency D_q(µ)

    Figure 12 shows a representative NICMOS3 array similar to those used in the 2MASS cameras.

    Figure 12

    NICMOS3 Detector Array Quantum Efficiency Data Table

    Total System Response

    The following curves are all normalized to unity at the peak.

14. J-Band Total Response R_J(µ)

    Figure 13 shows the product of all the J-band factors, including representative atmospheric transmission at the observatory sites. The rapidly-varying structure is mostly due to atmospheric absorption lines and bands, as can be seen in the atmospheric curve below.

    Figure 13

    J-Band Total Response Data Table

15. Figure 14 shows the H-Band Total Response R_H(µ), which is the product of all the H-band factors, including representative atmospheric transmission at the observatory sites.

    Figure 14

    H-Band Total Response Data Table

16. Figure 15 shows the K_s-Band Total Response R_K(µ), which is the product of all the K_s-band factors, including representative atmospheric transmission at the observatory sites.

    Figure 15

    K_s-Band Total Response Data Table

    Atmospheric Effects:

17. Effect of Atmospheric Water on J-Band Response.

    The red edge of the J-band, as defined by the 2MASS filters, is somewhat sensitive to the amount of precipitable water in the atmosphere. Figure 16 illustrates the magnitude of the effect for 0.5 mm and 5.0 mm of water vapor, as computed by the ATRAN code, and an atmospheric model generated by the PLEXUS code, which incorporates also aerosols and particulates appropriate to the sites.

    Figure 16

    We thank Martin Cohen for providing us with the NICMOS3 data (originally from the manufacturer Rockwell, as obtained by Leslie Hunt), the atmospheric models, and the resulting total response curves.

[Last Updated: 2000 June 27 by W. Wheaton. Modified 2000 Sep 13 by S. Van Dyk.]

Return to Section III.1b.