Sigma image
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| + | == Sigma image creation == | ||
| + | |||
| + | * Number of photo-electrons: | ||
| + | N<sub>e</sub> = (F<sub>gal</sub>+F<sub>sky</sub>) × N<sub>comb</sub> × G | ||
| + | |||
| + | N<sub>comb</sub> - number of images | ||
| + | |||
| + | F<sub>gal</sub> [ADU] - flux of the galaxy | ||
| + | |||
| + | F<sub>sky</sub> [ADU] - flux of the sky background | ||
| + | |||
| + | G [e/ADU] - gain parameter | ||
| + | |||
| + | σ<sup>2</sup>(N<sub>e</sub>) = N<sub>e</sub> + N<sub>comb</sub> × RON<sup>2</sup> | ||
| + | |||
| + | RON<sup>2</sup> [e] - readout noise | ||
| + | |||
| + | If there is a substantial dark current, then average value can be added in quadrature: | ||
| + | |||
| + | σ<sup>2</sup>(N<sub>e</sub>) = N<sub>e</sub> + N<sub>comb</sub> × RON<sup>2</sup> + <DC> × N<sub>comb</sub> | ||
| + | |||
| + | <DC>[e] - dark current of the same exposure time as the images | ||
| + | |||
| + | σ = σ(N<sub>e</sub>) / (N<sub>comb</sub> × G) | ||
| + | |||
== SDSS sigma image == | == SDSS sigma image == | ||
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To get those errors into nanomaggies, you simply apply back the calibration: | To get those errors into nanomaggies, you simply apply back the calibration: | ||
img_err = dn_err*cimg | img_err = dn_err*cimg | ||
| - | |||
== Spitzer S4G sigma image == | == Spitzer S4G sigma image == | ||
| - | Ex. dwarf galaxy DDO 053: | + | Ex. dwarf galaxy DDO 053: SKYDRKZB = 0.035142 and FLUXCONV = 0.1088 |
| - | + | ||
| - | SKYDRKZB = 0.035142 | + | |
| - | FLUXCONV = 0.1088 | + | |
imcopy ddo053_v7.phot.1 ddo53.phot.1 | imcopy ddo053_v7.phot.1 ddo53.phot.1 | ||
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output='ddo53_sigma_final.fits' | output='ddo53_sigma_final.fits' | ||
| - | // This last number c is iterstat ddo53_sigma_decom.fits ( | + | // This last number c is iterstat ddo53_sigma_decom.fits (it's last average value). Simply |
replace zeros in the sigma image by some resonable values, doesn't matter since galfit | replace zeros in the sigma image by some resonable values, doesn't matter since galfit | ||
won't see those given the bad pixel mask... | won't see those given the bad pixel mask... | ||
Revision as of 13:33, 12 June 2018
Sigma image creation
- Number of photo-electrons:
Ne = (Fgal+Fsky) × Ncomb × G
Ncomb - number of images
Fgal [ADU] - flux of the galaxy
Fsky [ADU] - flux of the sky background
G [e/ADU] - gain parameter
σ2(Ne) = Ne + Ncomb × RON2
RON2 [e] - readout noise
If there is a substantial dark current, then average value can be added in quadrature:
σ2(Ne) = Ne + Ncomb × RON2 + <DC> × Ncomb
<DC>[e] - dark current of the same exposure time as the images
σ = σ(Ne) / (Ncomb × G)
SDSS sigma image
http://data.sdss3.org/datamodel/files/BOSS_PHOTOOBJ/frames/RERUN/RUN/CAMCOL/frame.html
There is gain + rdnoise depending on camcol (for each filter) in the table (same link)!
To get sigma image go from Nmgy to DN: sky-subtracted image as well as calibrated in nanomaggies/pixel
img = mrdfits(framename,0,hdr) nrowc = (size(img,/dim))[1] sky = mrdfits(framename,2)
Sky image must be of the same size as the frame image, in units of counts
simg= interpolate(sky.allsky, sky.xinterp, sky.yinterp, /grid)
a calibration image the same size as the frame image, in units of nanomaggies per count
calib = mrdfits(framename,1) cimg= calib#replicate(1.,nrowc)
Original image in DN
dn = img/cimg + simg
Corresponding sigma image
dn_err= sqrt(dn/gain+darkVariance) ; in counts
To get those errors into nanomaggies, you simply apply back the calibration:
img_err = dn_err*cimg
Spitzer S4G sigma image
Ex. dwarf galaxy DDO 053: SKYDRKZB = 0.035142 and FLUXCONV = 0.1088
imcopy ddo053_v7.phot.1 ddo53.phot.1
! SIGMA IMAGE must not have ZEROS, which we will solve by replacing 0 -> -1 in the WEIGHT image.
imarith ddo053_v7.phot.1_wt.fits + 0. test imexpr expr='(a = b) ? c : a' a='test.fits' b='0' c='-1' output='ddo53.phot.1_wt.fits' imarith ddo53.phot.1.fits + 0.035142 ddo53.phot.1_sky.fits imarith ddo53.phot.1_sky.fits / 909.9265 ddo53.phot.1_tmp.fits // 1./0.1088 *30*3.3 = 909.9265 imarith ddo53.phot.1_wt.fits / 10 ddo53_W.fits imarith ddo53.phot.1_tmp.fits * ddo53_W.fits ddo53_Ne.fits imarith ddo53_W.fits * 213.16 ddo53_ron.fits // 14.6*14.6 = ron^2 iz Salo2014_S4G imarith ddo53_Ne.fits + ddo53_ron.fits ddo53_sigma2.fits stsdas toolbox imgtools imcalc ddo53_sigma2.fits ddo53_sigma.fits "sqrt(im1)" nullval="0" imarith ddo53_sigma.fits * 0.0011 ddo53_sigma_tmp.fits imarith ddo53_sigma_tmp.fits / ddo53_W.fits ddo53_sigma_est.fits imarith ddo53_sigma_est.fits * 0.9 ddo53_sigma_decom.fits imexpr expr='(a = b) ? c : a' a='ddo53_sigma_decom.fits' b='0' c='0.00266506' output='ddo53_sigma_final.fits'
// This last number c is iterstat ddo53_sigma_decom.fits (it's last average value). Simply replace zeros in the sigma image by some resonable values, doesn't matter since galfit won't see those given the bad pixel mask...
