Focus tests of the northern 2MASS telescope were carried out by Howard McCallon on the nights of 970710 and 970711 UT. For these tests, one degree scans were made at a range of telescope focus settings. These data were ftp'd to IPAC and processed the following mornings. For each scan, composite images were constructed from all high snr point sources in each band. From the composite images, second image moment ratios were evaluated, and ellipses were fit to produce a true aspect ratio and position angle. The tables below list the scan number, scan direction, telescope temperature (C) at the start of the scan, telescope focus setting, J, H and Ks image moment ratios, and aspect ratio (b/a) and position angle (phi) for J, H and Ks, respectively, on the two nights. The telescope focus and temperature are read from the telemetry at the start of each scan.
/o2/temp/970710n scan dir TelT focus jrat hrat krat Jba Jphi Hba Hphi Kba Kphi ------------------------------------------------------------------- 003 n 17.8 -9533 0.833 0.865 0.814 0.77 -90. 0.77 80. 0.67 85. 004 s 17.7 -9533 0.816 0.827 0.800 0.75 -90. 0.77 80. 0.67 85. 005 n 17.6 -9448 0.827 0.769 0.713 0.77 -75. 0.70 -90. 0.60 -80. 006 s 17.5 -9450 0.859 0.819 0.741 0.75 -75. 0.80 85. 0.57 -85. 007 n 17.4 -9480 0.826 0.789 0.731 0.75 -80. 0.72 85. 0.65 -80. 008 s 17.3 -9480 0.833 0.805 0.753 0.75 -80. 0.72 85. 0.65 -85. 009 n 17.4 -9510 0.830 0.830 0.769 0.82 -70. 0.85 80. 0.62 -85. 010 s 17.6 -9510 0.857 0.864 0.814 0.77 -85. 0.82 80. 0.75 -80. 011 n 17.2 -9540 0.873 0.896 0.870 0.85 -90. 0.92 0. 0.87 0. 012 s 17.1 -9540 0.877 0.906 0.885 0.87 0. 0.80 75. 0.80 80. 013 n 17.0 -9570 0.968 0.996 1.030 0.90 0. 0.90 0. 0.90 0. 014 s 17.1 -9570 0.953 0.996 0.994 0.97 0. 0.92 0. 0.97 0. 015 n 17.0 -9600 1.075 1.117 1.205 0.92 0. 0.77 20. 0.65 15. 016 s 17.0 -9600 1.069 1.106 1.176 0.85 15. 0.85 25. 0.70 15. 017 n 16.1 -9630 1.159 1.200 1.284 0.75 10. 0.65 15. 0.65 15. 018 s 16.8 -9630 1.163 1.230 1.328 0.77 15. 0.65 15. 0.52 15. 019 n 16.7 -9660 1.168 1.286 1.389 0.65 15. 0.60 20. 0.40 15. /o2/temp/970711n scan dir Tel_T focus jrat hrat krat Jba Jphi Hba Hphi Kba Kphi -------------------------------------------------------------------- 075 n 13.3 -9450 0.908 0.876 0.761 0.95 0. 0.75 55. 0.65 -90. 076 n 13.1 -9480 0.879 0.861 0.760 0.87 0. 0.85 55. 0.67 -90. 077 n 13.4 -9510 0.880 0.881 0.829 0.80 75. 0.77 85. 0.80 -85. 078 n 13.4 -9540 0.916 0.946 0.931 0.77 75. 0.82 50. 0.85 80. 079 n 13.0 -9570 0.994 1.005 1.072 0.95 0. 0.97 0. 0.87 0. 080 n 13.4 -9600 1.035 1.074 1.162 0.80 30. 0.90 0. 0.75 25. 081 n 13.2 -9630 1.113 1.178 1.298 0.85 10. 0.67 15. 0.60 20. 082 n 13.2 -9660 1.153 1.203 1.310 0.67 0. 0.65 10. 0.52 25.
Figure 1 shows a plot of image moment ratio versus focus setting for 970710n. A clear optimal focus is located between -9570 and -9580, and image shape deviates from symmetry rapidly as the focus differs from this point. The optimal focus is not the same for the three bands, as has been recognized for some time. Previous to these tests, the nominal best focus was taken to be -9530 based on focus tests in April and May when the mean nightly temperature was much cooler. Notice that a focus deviation of as litle as ~20 microns are measureable using this method. As borne out by the position angles in the Table above, the image distortion rotates through focus Note also that the Ks images appear to deviate from roundness faster than J or H.
Figure 1 and the table for 970710n indicate that there is a small difference between the image moment ratios measured for north and south-going scans. The difference may be systematic in the sense that north-going scans tend to have slightly smaller moment ratios (more elongated) than south going scans for focus settings larger than optimal (>-9570). For focus setting less than the optimal, the north going scans tend to have larger moment ratios than south going scans. This trend is violated by the final scan pair, 17 and 18, though.
Figure 2 shows the same plot as in Figure 1, but includes the semi-minor/semi-major axis ratios derived from the ellipse fitting. Axis ratios are shown by open symbols. The formal ellipse fitting confirms the image moment analysis.
Figure 3 shows the image moment ratio versus focus setting for the night of 970711n. The telescope temperature was nearly 4C cooler than the previous night, so a slight focus change is expected. Curiously, the optimal focus settings appear to be quite close to those measured on July 10th at J and H, with the only measurable difference at Ks. In addition, the maximum elongation of the images (min and max image moment ratios) do not appear to be as severe for extreme focus values as they were on the previous nights.
The table below gives the optimal focus setting for each band on the two nights. Also listed is the implied focus versus temperature slope. The best focus position was calculated using a linear fit to the 3 points nearest unit aspect ratio for each band.
Best Focus (um) J H Ks
970710n -9579 -9569 -9565
970711n -9579 -9566 -9556
dF/dT (um/C) 0 -0.8 -2.3
As stated, there is very little change in J and H focus. The slope of
the Ks focus curve is much smaller than predicted from comparison of
the best focus measured in May to these focus setting.