EALFA memo 28Aug2004 (updated 5 Nov 2004)           Martha Haynes & Riccardo Giovanelli

Array rotation and beam spacing for off-meridian E-ALFA drift observations.    

As described in Germán's memo, the 6 exterior ALFA beams move on an elliptical path in the sky, the major axis of which is elongated in the azimuth direction. As explained on Phil's page, the array must be rotated to track the parallactic angle, if the outer beams are to remain fixed on the the sky through the duration of the scan. Parallactic angle tracking will probably be exploited by staring observations including those run commensally with the P-ALFA surveys. See also Wolfram's documentation on the AUDS Drift and Chase technique.

The various long- and short- drift mapping projects under consideration by E-ALFA should consider the issues associated with array rotation in designing their optimal strategies. The ellipticity of the array footprint on the sky has two important consequences for drift mapping schemes which desire beams tracks across the sky equally spaced in declination:
  • Drifts should be conducted with ALFA configured at the array rotation angle that produces adjacent beams equally spaced in Declination on the sky at the azimuth and zenith angle corresponding to the time of the observations.
  • The separation of the beams in the equal-spacing configuration is not fixed. It is maximal (~2.1 arcmin) for sources at transit (on the meridian). Thus drifts obtained at different azimuths will be characterized by different beam spacings, requiring the regridding of combined datasets.

    The E-ALFA transit drift precursor experiment A1946 has helped to clarify the orientation of the beams on the sky and confirmed the predicted array rotation angle for equally spaced beams for targets observed on the meridian. Those have been crudely illustrated (ignoring the ellipticity) in our previous memo available on the E-ALFA drift precursor program A1946 web site (and submitted to NAIC's ALFA memo series).

    In order to illustrate the result of such parallactic/array rotation angle tracking, we explore here the requirements for equally spaced (in declination) tracks for mapping the large galaxy NGC 2903, a source at declination +21, which is the target of one of the E-ALFA precursor experiments. The following table gives, for the path of NGC 2903 on 28Nov2004, the Atlantic standard time, local mean sidereal time, the hour angle (in decimal hours), azimuth and zenith angle (in degrees) of N2903 at that time, the mean separation in the Decl. direction of adjacent beams, in arcminutes, and the corresponding optimal (producing equally spaced beams) array rotation angle (input to CIMA), in degrees. The algorithm used to identify the optimal beam separation finds the rotation angle for which the **maximum** spacing (in Decl.) of adjacent beams (as depicted on our beam layout cartoon diagram) is **minimized**. These results have been obtained using a modified version of Phil Perillat's IDL routine ALFABMPOS. Note that these results are based on current understanding of the ALFA system and may need to be adjusted as more information on ALFA characteristics becomes available.

    NGC 2903: 09h32m10s +21d20'03"
    Sample almanac for date: 2004 11 28
    JD at midnight AST: 2453337.67
    dDec is the average spacing of adjacent beams, in the Decl. direction, in arcminutes
    This table has been produced using the eggidl routine optang.pro, and is based on various routines available in Phil's aoidl package, particularly alfabeampos.
    J.D. AST LMST HA az za dDec OptAng
    2453337.840 04h10m 08h13m -1.32 257.26 18.87 1.80 -64.0
    2453337.847 04h20m 08h23m -1.15 256.40 16.54 1.80 -62.0
    2453337.854 04h30m 08h33m -0.98 255.09 14.22 1.80 -60.0
    2453337.861 04h40m 08h43m -0.81 253.13 11.93 1.81 -56.0
    2453337.868 04h50m 08h53m -0.65 250.04 9.66 1.82 -52.0
    2453337.875 05h00m 09h03m -0.48 244.82 7.45 1.85 -45.0
    2453337.882 05h10m 09h13m -0.31 235.10 5.37 1.90 -34.0
    2453337.889 05h20m 09h23m -0.15 214.66 3.65 2.00 -13.0
    2453337.890 05h21m 09h24m -0.13 211.60 3.52 2.01 -10.0
    2453337.890 05h22m 09h25m -0.11 208.32 3.40 2.02 -7.0
    2453337.891 05h23m 09h26m -0.10 204.80 3.29 2.04 -3.0
    2453337.892 05h24m 09h27m -0.08 201.05 3.20 2.05 0.0
    2453337.892 05h25m 09h28m -0.06 197.09 3.12 2.06 4.0
    2453337.893 05h26m 09h29m -0.05 192.92 3.06 2.07 8.0
    2453337.894 05h27m 09h30m -0.03 188.60 3.01 2.07 11.0
    2453337.894 05h28m 09h31m -0.01 184.16 2.98 2.08 15.0
    2453337.895 05h29m 09h32m 0.01 179.65 2.97 2.08 19.0
    2453337.896 05h30m 09h33m 0.02 175.15 2.98 2.08 23.0
    2453337.897 05h31m 09h34m 0.04 170.69 3.01 2.08 27.0
    2453337.897 05h32m 09h35m 0.06 166.35 3.06 2.07 31.0
    2453337.898 05h33m 09h36m 0.07 162.16 3.12 2.06 34.0
    2453337.899 05h34m 09h37m 0.09 158.16 3.20 2.04 38.0
    2453337.899 05h35m 09h38m 0.11 154.38 3.29 2.03 41.0
    2453337.900 05h36m 09h39m 0.12 150.83 3.40 2.02 45.0
    2453337.901 05h37m 09h40m 0.14 147.51 3.52 2.00 48.0
    2453337.901 05h37m 09h41m 0.16 144.43 3.65 1.98 51.0
    2453337.908 05h48m 09h51m 0.32 124.01 5.38 1.89 73.0
    2453337.915 05h58m 10h01m 0.49 114.41 7.45 1.85 84.0
    2453337.922 06h08m 10h11m 0.66 109.26 9.67 1.83 91.0
    2453337.929 06h18m 10h21m 0.82 106.18 11.94 1.82 95.0
    2453337.936 06h28m 10h31m 0.99 104.22 14.23 1.80 98.0
    2453337.943 06h38m 10h41m 1.16 102.94 16.55 1.81 101.0
    2453337.950 06h48m 10h51m 1.33 102.06 18.88 1.80 103.0

    This single example is meant merely to demonstrate that the various E-ALFA mapping programs that plan to exploit drift modes should pay careful attention to feed rotation issues in devising their optimal strategies.


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    Last modified: Fri Nov 5 18:10:33 EST 2004