2MASS North Focus Test Results for 980913 UT

A set of one-degree-long focus scans was obtained on the night of 980913 UT at the 2MASS North facility at Mt. Hopkins. This test was the first of several iterative tests to recheck telescope focus at the start of post-summer shutdown.

Two sets of focus scans were taken: Scans 003-009 were taken with focus settings ranging from +807 to +1108 microns in increments of +50 microns and were taken to simulate a situation where the night assistant, late for work, opens the dome and almost immediately begins data taking. Scans 011-017 were taken with the same focus range and 50-micron increments but were obtained an hour after the first scans and after dome and air temperature were more equilibrated. This memo summarizes the point source focus performance from these scan sets.

The raw data for the focus test scans were processed during the 24hr QA/Tape validation processing at IPAC on 980913 (PDT) using 2MAPPS v2.0x. (Note that the severe image elongations in scan 011 caused POSFRM to fail, so psf images, second moment ratios, and seeing shape values are not available for this scan.)

• The composite psf images generated for all of the focus test scans are shown in the figure below. The three rows show the J, H and K_s images for the zenith focus test scans (scan number is shown in each J image), respectively.  The best apparent focus occurs between scans 005-007 and 013-015 for all three bands.




 
 

• 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.

 
scn focus Tel_T Rng_T  Jrat  Hrat  Krat   Jsh   Hsh   Ksh   Jpfrc  Hpfrc  Kpfrc
--------------------------------------------------------------------------------
003 +0807 19.1  18.0  1.366 1.249 1.377  1.455 1.490 1.561  0.2641 0.2520 0.2696
004 +0857 19.0  18.1  1.328 1.211 1.494  1.253 1.281 1.345  0.3682 0.3312 0.3297
005 +0908 19.0  18.0  1.122 1.113 1.251  1.127 1.147 1.197  0.5404 0.5136 0.4323
006 +0958 19.0  18.0  0.976 0.995 1.047  1.080 1.074 1.115  0.5766 0.6044 0.4919
007 +1008 19.0  18.0  0.831 0.844 0.827  1.113 1.093 1.117  0.5164 0.5633 0.4715
008 +1058 19.0  18.0  0.751 0.741 0.700  1.289 1.251 1.268  0.3432 0.3859 0.3728
009 +1108 19.0  18.0  0.756 0.776 0.721  1.504 1.488 1.519  0.2570 0.2897 0.3259

011 +0807 18.9  18.0  UNDEF UNDEF UNDEF  UNDEF UNDEF UNDEF  0.2673 0.2589 0.2713
012 +0860 18.9  18.0  1.366 1.342 1.517  1.256 1.291 1.367  0.3630 0.3239 0.3270
013 +0908 18.9  18.0  1.209 1.198 1.348  1.094 1.133 1.189  0.5457 0.5176 0.4265
014 +0957 18.9  18.1  1.006 1.022 1.119  1.023 1.034 1.080  0.6540 0.6621 0.5215
015 +1007 18.9  18.0  0.830 0.845 0.845  1.061 1.045 1.081  0.5721 0.6188 0.4918
016 +1058 18.8  18.0  0.699 0.699 0.662  1.208 1.174 1.195  0.3885 0.4336 0.3904
017 +1108 18.9  17.9  0.695 0.731 0.676  1.441 1.391 1.440  0.2798 0.3143 0.3434
 
 
• The figure below shows second image moment ratio, seeing shape, and PFRAC plotted as a function of focus setting for scans 003-009 (solid lines) and scans 011-017 (dashed lines). J data are plotted in blue, H in green and K_s in red.


 
 

• Examination of the table and plot above indicates that the optimal focus (i.e. minimum shape and maximum PFRAC) occurs near +950 microns for all three bands, with J-band being very near +950 microns (assuming symmetry about optimal focus), H-band between +950 and +1000 microns, and K_s-band between +950 and +1000 microns.

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D. Kirkpatrick - IPAC
Last update - 13 September 1998