WISE Software Interface Specification; qua01; (WSDC-DI103) Version 1.1: April 28, 2009 Interface Name: PSFmoments/QA Interface Type of Interface: ASCII text file output by PSFmoments Read By: WISE QA Application Framework __________ D. Kirkpatrick/H. Brandenburg Written By: PSFmoments______________________________ T. Jarrett Description: This program characterizes the shape of the "PSF" image created from WISE scan-synchronization frames. The input "PSF" image is provided by JWF (SIS = TBD). Two kinds of PSFs are characterized: (1) average PSF over the focal plane, and (2) 5x5 family of PSFs that represent the focal plane differences. Consequently, two different tables are created; parameters are described below. Algorithm Notes: The "moments" represent intensity-weighted moments (normalized summation over all pixels with the radius multiplied by the intensity). The "rotating" moment is computed by rotating the image axis plane and computing the 2nd moment for the source at center. The plane is rotated through all angles, with the optimum angle (parameter == rPHI) corresponding to the perpendicular maximum semi-major axis (from the 2nd moment) and minimum semi-minor axis. The ratio between the minor-to-major represents the axis ratio (parameter == rbovera). It is important to set the input profile "size" such that the moment corresponds to the full profile (at least 90% of the profile). The other method used to estimate the axis ratio and position angle is to fit an elliptical profile to the stellar (PSF) isophote. The designated isophote is an input parameter, and is currently set to 10% of the star peak. An ellipse is fit to the 0.1*Peak isophote, deriving the semi-major axis (parameter == bovera) and the position angle (parameter == PHI). Another handy metric that is derived from the PSF is the semi-major axis corresponding to the elliptical fit at the 0.5*Peak, or half-power points. Note regarding the position angles: The PA solution becomes degenerate when the stellar profile approaches perfect symmetry (i.e., 'round'). Hence the values become chaotic as b/a approaches unity. The technical description for this phenomenon is summarized by John Fowler as: the eigenvalues of a circular distribution are degenerate and thus have arbitrary eigenvectors. When deviations from circularity are at the noise level or below, the direction angle arising from the eigenvectors is not a meaningful metric. Noise pixels is a one-dimensional metric that effectively gauges the width of the PSF. The formalism and equation (Eq. 10) used to compute the noise pixels is described in http://web.ipac.caltech.edu/staff/fmasci/home/wise/noisepix_specs.pdf where the PSF is integrated over a circular area that captures the PSF pixels, corresponding to a radius of 11 pixels. Additional Notes: It remains to be seen which characterization metric, the rotated-axis moment or the fit to the elliptical isophote, is the more robust measure. For now, both metrics should be carried along. The namelist (nl.PSFmom) parameters: raper = 10. >> is the active radius for characterization, which should be large pscale = 0.91668 >> pixel scale (2.75" / 3) isolim = 0.1 >> isophotal threshold relative to peak power +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Table 1 -- Single PSF that represents the average PSF over the focal plane example: \ list_1x1 \ iso-threshold (relative to peak): 0.10 \ moments in pixel units \ r_05p, r_01p in arcsec \ rPHI, PHI in deg EofN | frame | x | y | mom1x| mom1y| mom2x| mom2y |r_major| rbovera | rPHI | r_05p | r_01p | bovera | PHI |NoisePix| | char | r | r | r | r | r | r | r | r | r | r | r | r | r | r | imgpsf1 14.0 14.0 0.0000 0.0183 2.7353 3.2978 5.4468 0.6851 4.00 3.1496 6.1537 0.7900 0.00 11.647 imgpsf2 14.0 14.0 0.0000 0.0280 2.7111 3.3430 5.5941 0.6559 4.00 3.1496 6.4900 0.7300 7.00 11.659 Table 2 -- 5x5 family of PSFs example: \ list_5x5 \ iso-threshold (relative to peak): 0.10 \ moments in pixel units \ r_05p, r_01p in arcsec \ rPHI, PHI in deg EofN | frame | x | y | mom1x| mom1y| mom2x| mom2y |r_major| rbovera | rPHI | r_05p | r_01p | bovera | PHI |NoisePix| | char | r | r | r | r | r | r | r | r | r | r | r | r | r | r | img5x5psf1 14.0 14.0 0.0233 0.0221 2.6644 3.4886 6.0972 0.5802 176.00 3.4646 6.7750 0.7000 175.00 11.647 img5x5psf1 41.0 14.0 0.0000 0.0195 2.7037 3.3161 5.4982 0.6647 180.00 3.1496 6.3384 0.7400 2.00 11.659 Specification of output variables: cols Name Description Units Type Format ---- ---- ------------------------------------------------- ------- ---- -------- 1- 19 frame PSF image name -- char*19 a 20- 26 x x-pixel location pix R*4 f7.1 27- 33 y y-pixel location pix R*4 f7.1 34- 41 mom1x 1st moment, x-axis pix R*4 f8.4 42- 49 mom1y 1st moment, y-axis pix R*4 f8.4 50- 57 mom2x 2nd moment, x-axis pix R*4 f8.4 58- 65 mom2y 2nd moment, y-axis pix R*4 f8.4 66- 73 r_major major axis, rotated-axis 2nd moment pix R*4 f8.4 74- 84 rbovera axis ratio from rotated-axis 2nd moment -- R*4 f8.4 85- 93 rPHI position angle (EofN) from rotated-axis 2nd moment deg R*4 f9.2 94-101 r_05p semi-major axis of the half-power points asec R*4 f8.4 102-109 r_01p semi-major axis of the 0.1-power points asec R*4 f8.4 110-120 bovera axis ratio from the 0.1-power points -- R*4 f8.4 121-129 PHI position angle (EofN) from the 0.1-power points deg R*4 f9.2 130-138 NoisePix effective number of noise pixels in the PRF pix R*4 f9.3