III. 2MASS Details

1. Facilities

b. Camera and Detectors

ii. Optical Response Calibration Data

Here are given 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 µ.

1. The layout of the camera is as follows:
   

   Camera Layout.
   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.

2. 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.

   

   Main Telescope Mirror Reflectivity Data Table

3. Dewar Window Transmission D(µ)
   

   Dewar Window Transmission Data Table

4. Camera Lens Coatings L(µ)

   The factor L(µ), plotted below, 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.

   

   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. J dichroic reflectivity J_dr(µ)
   

   J Dichroic Reflectivity Data Table

6. J filter transmission J_f(µ)
   

   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. J dichroic transmission J_dt(µ)
   

   J-band Dichroic Mirror Transmission Data Table

8. H dichroic reflectivity H_dr(µ)
   

   H-band Dichroic Mirror Reflection Data Table

9. H filter transmission H_f(µ)
   

   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. H dichroic transmission H_dt(µ)
    

    H-band Dichroic Mirror Transmission Data Table

12. K_s filter transmission Ks_f(µ)
    

    K_s Filter Transmission Data Table

    Detectors

13. NICMOS3 detector array quantum efficiency D_q(µ)

    The following is for a representative NICMOS3 array similar to those used in the 2MASS cameras.

    

    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)

    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.

    

    J-Band Total Response Data Table

15. H-Band Total Response R_H(µ)

    The product of all the H-band factors, including representative atmospheric transmission at the observatory sites.

    

    H-Band Total Response Data Table

16. K_s-Band Total Response R_K(µ)

    The product of all the K_s-band factors, including representative atmospheric transmission at the observatory sites.

    

    K_s-Band Total Response Data Table

    Atmospheric Effects:

17. Here are atmospheric transmission data for the north (Mt. Hopkins) and south (CTIO) sites, plus some information regarding the effects of aerosols and variability in atmospheric water on the J-band response. Generated using the USAF PLEXUS code, by M. Cohen.

    North Observatory Atmospheric Transmission:

    
    North Observatory Atmospheric Transmission Table

    South Observatory Atmospheric Transmission:

    
    South Observatory Atmospheric Transmission Table

    Effect of Aerosols and Atmospheric Water on J-Band Response.

    The red edge of the J-band, as defined by the 2MASS filters, is sensitive to the amount of precipitable water in the atmosphere. The following graph 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.

    

    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 Sept 23, by W. Wheaton.

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