III. 2MASS Facilities and Operations

1. Facilities

b1. System Transmission

Below are laboratory measurements of the various transmission, reflection, and response data needed to calculate the absolute response of the 2MASS cameras as a function of wavelength.

Summary of Figures and Data
1. Camera Optical Configuration
2. Telescope Mirror Reflectivity
3. Dewar Window Transmission
4. Camera Lens Coatings
5. J Dichroic Reflectivity
6. J Filter Transmission
7. J Dichroic Transmission (through to H and Ks bands)
8. H Dichroic reflectivity
9. H Filter Transmission
10. H Dichroic Transmission (through to Ks band)
11. Ks Filter Transmission
12. JHKs NICMOS3 Array Response
13. JHKs Total Instrumental Throughput (no atmosphere included)
14. JHKs Atmospheric Transmission
15. Effects of water vapor on J band
16. JHKs Total Instrumental+Atmosphere Throughput

  1. Camera optical configuration
  2. Light enters the camera following passage through the atmosphere and reflection off of the telescope primary and secondary mirrors. The first camera element, proceeding from right to left, is the dewar's ZnSe window, followed by a field stop and the first of the seven lenses. This first lens is common to all three bands. Two dichroic mirrors then split the optical paths for the J, H, and Ks bands. J-band experiences only a reflection off of the "J-band dichroic." H-band must traverse the J-band dichroic and then experience a reflection off of the "H-band dichroic." Ks band traverses both dichroics. Each band then encounters 6 further lenses and a band-limiting filter. The lens sets are identical in each band. In Figure 1, the straight-through light path leads to the Ks array, the upper path to the J array, and the lower one to H.

    Figure 1

  3. Telescope Mirror Reflectivity

    Figure 2 shows the telescope mirror reflectivity for a single surface reflection. Starlight encounters two reflections before entering the camera.

    Figure 2

    Main Telescope Mirror Reflectivity Data Table

  4. Dewar Window Transmission

    Figure 3 shows the transmission of the anti-reflection coated ZnSe dewar window.

    Figure 3

    Dewar Window Transmission Data Table

  5. Camera Lens Coatings

    Figure 4 shows the transmission of a single anti-reflection coated camera lens. 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

  6. J dichroic reflectivity

    Figure 5 shows the J dichroic reflectivity

    Figure 5

    J Dichroic Reflectivity Data Table

  7. J filter transmission

    Figure 6 shows the J filter transmission

    Figure 6

    J Filter Data Table

  8. J dichroic transmission

    Figure 7 shows the transmission of H and Ks light through the J dichroic.

    Figure 7

    J-band Dichroic Mirror Transmission Data Table

  9. H dichroic reflectivity

    Figure 8 shows the H dichroic reflectivity

    Figure 8

    H-band Dichroic Mirror Reflection Data Table

  10. H filter transmission

    Figure 9 shows the H filter transmission

    Figure 9

    H-band Filter Transmission Data Table

  11. H dichroic transmission

    Figure 10 shows the H dichroic transmission. This transmission, in combination with the J dichroic transmission is a factor in determining how much light is passed by the Ks band optical train.

    Figure 10

    H-band Dichroic Mirror Transmission Data Table

  12. Ks filter transmission

    Figure 11 shows the Ks filter transmission

    Figure 11

    Ks Filter Transmission Data Table

  13. NICMOS3 detector array quantum efficiency

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

    Figure 12

    NICMOS3 Detector Array Quantum Efficiency Data Table

  14. Total Instrumental Throughput (in the absence of the atmosphere)

    Figures 13, 14, and 15 show the product of all the relevant factors above that yield the total instrumental throughput of the camera+telescope system.

    Figure 13
    J band
    Figure 14
    H band
    Figure 15
    Ks band
    Data table Data table Data table

  15. Atmospheric Transmission

    Figures 16 and 17 show atmospheric transmission data for the north (Mt. Hopkins) and south (CTIO) sites, based on the USAF PLEXUS code (provided by M. Cohen). In addition to atomic and molecular absorption, this code accounts for Rayleigh and site-specific mean aerosol scattering.

    Figure 16
    North - Mt. Hopkins
    Figure 17
    South - CTIO
    Data table Data table

  16. Effect of varying water vapor on J band transmission

    The 2MASS J-band is sensitive to the amount of precipitable water in the atmosphere. The following plots illustrate the magnitude of the effect for 0.5 mm and 5.0 mm of water vapor, as computed by the ATRAN code. Figure 18 compares the total system transmission obtained by the two ATRAN models (Figures 19 and 20) and overlays the result using the PLEXUS code for comparison.

    Figure 18
    0.5 vs 5.0mm water
    Figure 19
    0.5mm ATRAN
    Figure 20
    5.0mm ATRAN
      Data table Data table

  17. Total Throughput (including atmospheric absorption)

    Figures 21, 22, 23, and 24 show the product of all the relevant factors above, including the atmospheric transmission, that yields the total throughput of the camera+telescope+atmosphere system.

    N.B. - These are not "relative spectral response curves" (RSRs) suitable for the derivation of synthetic photometry. RSR curves from Cohen et al. 2003 (AJ, in press) are tabulated in Section VI.4.a.

    Figure 21
    J band (0.5mm water)
    Figure 22
    J band (5.0mm water)
    Figure 23
    H band
    Figure 24
    Ks band
    Data table Data table Data table Data table

[Last Updated: 2005 June 7, by W. Wheaton, B. Nelson, and M. Skrutskie.]

Return to Section III.1b.