= PSPHOT Kron Measurements Investigation = This page describes some investigations comparing the Kron radius and magnitudes measured by psphot == Overview == On April 12 Peter Draper pointed out that the Kron radii measured by psphot using the default recipe are much lower than those measured by SExtractor. He found that if the gaussian weighting function is removed, more sensible results are obtained. See plot at http://ps1sc.ifa.hawaii.edu/PS1wiki/index.php/KP12videocon_page#Kron_radii_-_possible_improvement Using a set of 40 simulated images provided by Peter we have done many experiments with psphot to investigate the accuracy of the kron measurements. We reported April 30 at the DRAVG meeting that results closer to SEXtractor can be achieved with a prototype version of psphot that smooths the input image with a 1.7 pixel Gaussian prior to measuring the kron radius and flux. http://ps1sc.ifa.hawaii.edu/PS1wiki/index.php/DRAVG_telecon#Kron_radii_improvements We have more carefully implemented the smoothing function and the code is now included in the ipp-trunk. There are several recipe values that now may be used to control the psphot kron measurements. Current defaault values are listed. * PSPHOT_APPLY_WEIGHT T apply gaussian weight of sigma == windowRadius * PSPHOT_APPLY_WINDOW T apply window function to downweight subtracted sources * PSPHOT_KRON_SMOOTH F when true smooth the input image before measuring the kron radius * PSPHOT_KRON_SIGMA 1.7 smoothing parameter * PSPHOT KRON_NSIGMA 2 smoothing parameter Upon reviewing the following results the differences between applying the weighting and window functions are not significant. We suggest turning them off. The smoothing yields kron radii closer to SExtractor with these simulated images. We need to investigate the performance impact on single frame psphot before deciding to turn it on. == Results == === Kron Radius Histograms === The following images contain histograms of the measured Kron Radius for various recipe combinations. * Green "Expected" Kron Radius for the simulated galaxy * Grey Kron Radius measured by SExtractor (provided by Peter Draper) * Red Kron Radius measured by psphot Default recipe: Measure on smoothed image, gaussian weighting, window function [[Image(kr.default.png)]] Window function no weighting [[Image(kr.noweight.png)]] Weighting no window [[Image(kr.nowin.png)]] No Window No weight [[Image(kr.noweight.nowin.png)]] No window no weighting no smoothing [[Image(kr.nosmooth.noweight.nowin.png)]] === Difference In Kron Radius === Default Recipe [[Image(dkr.v.kr.default.png)]] [[Image(dkr_kr.v.kr.default.png)]] Window Function no weighting [[Image(dkr.v.kr.noweight.png)]] [[Image(dkr_kr.v.kr.noweight.png)]] Weighting, no window [[Image(dkr.v.kr.nowin.png)]] [[Image(dkr_kr.v.kr.nowin.png)]] No window, no weighting [[Image(dkr.v.kr.noweight.nowin.png)]] [[Image(dkr_kr.v.kr.noweight.nowin.png)]] No window, no weighting, no smoothing [[Image(dkr.v.kr.nosmooth.noweight.nowin.png)]] [[Image(dkr_kr.v.kr.nosmooth.noweight.nowin.png)]] === Difference in Kron Magnitude === Default Recipe [[Image(dkm.v.kr.default.png)]] [[Image(dkm.v.km.default.png)]] Window Function no weighting [[Image(dkm.v.kr.noweight.png)]] [[Image(dkm.v.km.noweight.png)]] Weighting, no window [[Image(dkm.v.kr.nowin.png)]] [[Image(dkm.v.km.nowin.png)]] No window, no weighting [[Image(dkm.v.kr.noweight.nowin.png)]] [[Image(dkm.v.km.noweight.nowin.png)]] No window, no weighting, no smoothing [[Image(dkm.v.kr.nosmooth.noweight.nowin.png)]] [[Image(dkm.v.km.nosmooth.noweight.nowin.png)]] === Statistics === {{{ Statistics for various recipe combinations ------------------------------------------ Fractional Difference in Kron Radius ( KR_SExtractor - KR_psphot ) / KR_Sextractor All KR Recipe mean stdev min max median Npts ------------------------------------------------------------------------------------------- default -0.201895 0.490706 -20.0476 0.924053 -0.103559 113705 nowin -0.2036380 0.689935 -165.129 0.909 -0.253494 113701 noweight -0.245435 0.83742 -129.597 0.900042 -0.143933 112929 noweight.nowin -0.245726 0.834698 -129.597 0.948736 -0.226174 112930 nosmooth.noweight.nowin -0.123882 0.830385 -101.242 0.921227 0.00175791 113087 Mean and meidan Kron raddii are closest to SExtractor with all of our various recipe choices off but the standard deviation is lowest with our default recipe. This set of objects includes small values of KR which psphot does not measure well (Minimum KR_psphot in this sampe is 7.1) ( KR_SExtractor - KR_psphot ) / KR_Sextractor KR_SE > 10 Recipe mean stdev min max median Npts ------------------------------------------------------------------------------------------- default -0.0250245 0.22567 -4.66826 0.924053 -0.032233 42142 nowin -0.0257794 0.2263675 -4.576 0.909 -0.0291643 42140 noweight -0.0402723 0.287004 -3.22148 0.900042 -0.0437856 42074 noweight.nowin -0.0410538 0.287949 -4.44391 0.948736 -0.0435108 42078 nosmooth.noweight.nowin 0.0934499 0.53906 -101.242 0.921227 0.00175791 42138 Considering sources with SE kron radii above 10 the smallest stdev is also with the default recipe. The window function appears to have has little effect on the statistics. The biggest improvement is provided by the smoothing. Without smoothing the psphot KR is systematically low. --------------------------------------------------------------------------------------------------------------- Kron Magnitude KM_SExtractor - KM_psphot All KR Recipe mean stdev min max median Npts -------------------------------------------------------------------------------- default 0.0144785 0.192932 -5.144 4.495 0.003226 113705 nowin 0.0150963 0.1902736 -3.777 4.584 0.002172 113701 noweight -0.0860446 0.700536 -8.964 4.529 -0.004648 112929 noweight.nowin 0.0131 0.220402 -5.862 4.514 0.00044 112930 nosmooth.noweight.nowin -0.014707 0.215786 -5.733 4.725 -0.03339 113087 KM_SExtractor - KM_psphot KR_SE > 10 pixels Recipe mean stdev min max median Npts -------------------------------------------------------------------------------- default -0.0229234 0.172541 -5.144 4.139 -0.001218 42142 nowin -0.0217864 0.1680391 -3.777 4.584 -0.0061890 42140 noweight -0.12206 0.674331 -7.844 3.337 -0.006119 42074 noweight.nowin -0.0268161 0.187884 -4.832 4.514 -0.000118 42078 nosmooth.noweight.nowin -0.065724 0.175386 -4.684 4.725 -0.035954 42138 }}}