2MASS North Focus Test Results for 980919 UT

Two sets of one-degree-long focus scans were obtained on the night of 980919 UT at the 2MASS North facility at Mt. Hopkins. This test was the seventh of several iterative tests to recheck telescope focus and collimation at the start of post-summer shutdown.

Two focus runs (Run 13-14) were obtained in a similar fashion to those obtained on 980914n, 980915n, 980916n, 980918n (Runs 1-12):

• Run 13 (scans 052-058) -- Focus was changed in coarse 30 um steps from +1953 to +2134.
• Run 14 (scans 059-065) -- Same set-up as Run 13 above, except focus was varied along a narrower range of settings +2001 to +2092 with finer increments of 15 um.

The raw data for the focus test scans were processed during the 24hr QA/Tape validation processing at IPAC on 980919 (PDT) using 2MAPPS v2.1. Scan 59 of Run 14 failed processing so only pfrac values are available.

• The composite psf images are shown in the figures below. The three rows show, respectively, the J, H and K_s images. Scan number is shown in each J image.

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• The psf images binned as a function of cross-scan position are shown in the figures below. Plotted are all focus positions, grouped by band, for run 13.

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  J Band (Run 13)

  
   
   

  H Band (Run 13)

  
   
   

  K Band (Run 13)

  
   
   

• The psf images binned as a function of cross-scan position are shown in the figures below. Plotted are all focus positions, grouped by band, for run 14.

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  J Band (Run 14)

  
   
   

  H Band (Run 14)

  
   
   

  K Band (Run 14)

  
   
   

• The table below lists the scan, focus setting, telescope and head ring temperatures reported in the parameter files for each scan. Also listed are the second image moment ratios(*rat), seeing shapes (*sh), and fractional encircled energy parameters (PFRAC) derived for each of the focus test scans. Scan 013 failed processing so there will be no second moment ratios or shape parameters for this scan.



scn focus Tel_T Rng_T  Jrat  Hrat  Krat   Jsh   Hsh   Ksh   Jpfrc Hpfrc  Kpfrc 
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Run 13:
052 +1953 18.9  18.1  1.018 1.045 1.170  1.323 1.312 1.267 0.3656 0.3647 0.3764
053 +1982 18.9  18.1  0.994 1.031 1.174  1.147 1.143 1.122 0.4856 0.5088 0.5235
054 +2013 18.9  18.0  0.976 0.994 1.114  1.077 1.068 1.035 0.5640 0.6165 0.6260
055 +2043 18.9  18.0  0.962 0.971 1.002  1.077 1.053 1.016 0.5755 0.6267 0.6583
056 +2073 18.8  17.9  0.944 0.945 0.917  1.105 1.069 1.023 0.5340 0.6015 0.6395
057 +2103 18.8  17.9  0.916 0.914 0.829  1.147 1.103 1.053 0.4684 0.5607 0.5730
058 +2135 18.9  17.9  1.028 0.891 0.724  1.221 1.190 1.124 0.3606 0.4483 0.4485

Run 14:
059 +2001 18.9  17.9  UNDEF UNDEF UNDEF  UNDEF UNDEF UNDEF 0.5622 0.5974 0.5985
060 +2018 18.9  18.0  0.983 0.998 1.089  1.094 1.075 1.050 0.5669 0.6203 0.6296
061 +2031 18.9  18.0  0.964 0.977 1.047  1.080 1.056 1.024 0.5726 0.6368 0.6488
062 +2046 18.8  17.9  0.965 0.968 0.993  1.133 1.099 1.064 0.5202 0.5819 0.6130
063 +2061 18.8  18.0  0.950 0.950 0.948  1.138 1.101 1.050 0.5065 0.5710 0.6131
064 +2077 18.9  18.0  0.949 0.941 0.910  1.160 1.112 1.065 0.4882 0.5628 0.5981
065 +2092 18.9  18.0  0.932 0.934 0.869  1.189 1.144 1.082 0.4557 0.5414 0.5666

• The figures below show second image moment ratio, seeing shape, and PFRAC plotted as a function of focus setting for the coarse spacing scans 052-058 with solid lines and the fine focus spacing scans 060-065 (059 failed processing) with (somewhat difficult to see) dashed lines. J data are plotted in blue, H in green and K_s in red.


 
 

• The figures below show the delta mags as a function of cross-scan position for all scans in Run 13 (52-58).

  Scan 52:

  

  Scan 53:

  

  Scan 54:

  

  Scan 55:

  

  Scan 56:

  

  Scan 57:

  

  Scan 58:

  

• Summary

  The focus table and curves indicate that best focus during Run 13 (coarse steps) occurred near a focus of 2045 in H and Ks (scan 055), and approximately 2035-2040 in J (between scans 054 and 055). Note that the image symmetry is maximized (i.e. second moment ratio closest to unity) in the Ks band in scan 055, but at a slightly lower focus setting for J and H. Although there remains some measureable image elongation in the Ks band, as manifested in the image moment ratio plots, the slope of the second moment ratio curves are significantly flatter than measured any time before. The psf maps show that there is image elongation in all three bands, but the position angle is +/-45 deg, which makes the second moment ratio a poor diagnostic. In any case, the effective "depth-of-focus" to image elongation appears significantly improved.

  The second, finer stepped focus test results are problematic. The first three scans of Run 14 are consistent with Run 13. However, there appears to be a jump to systemmatically poorer values of PFRAC and shape for scans 062-065. This is most likely due to a degradation in the seeing. We note that the image moment ratios track perfectly in the two tests. Becuase of the jump in the curves, it is not possible to estimate a best-focus position from this scan test, except to say that the image symmetry profile is consistent with the coarser stepped test.

  The cross-stepped psf-maps show the psf to be highly consistent across the focal plane in all three bands, even when the telescope is far from optimal focus. This is confirmed by the psf-fit/aperture mag difference versus cross-scan position plots. The worst cross-scan photometric bias even +/-30 microns out of focus is probably <5%, and at best focus, it is negligible in all three bands.

  In general, the image elongation problem in the Ks band has been vastly improved. It has come at the cost of slightly reduced image quality in the J band. How much worse the J images are is difficult too gauge because the seeing quality during these tests was not ideal. However, the new collimation setting has provided much more tolerance in focus setting to image elongation in all three bands. Another focus test should be made in the near future under better seeing conditions.