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This is a (very) old version, potentially with bugs.

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Source code for simage_add_noise.pro:

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pro simage_add_noise, filename, time=time, IMAGE=image, HEADER=header, $ SEED=seed, HDF=hdf, SMOOTH=smooth, NOSMOOTH=nosmooth, $ COSMIC_RAYS=cosmic_rays ;$Id: simage_add_noise.pro, v1.0$ ; ; Copyright 2004 Richard Massey and Alexandre Refregier. ; ; This file is a part of the Shapelets analysis code. ; www.ast.cam.ac.uk/~rjm/shapelets/ ; ; The Shapelets code is free software; you can redistribute it and/or ; modify it under the terms of the GNU General Public License as published ; by the Free Software Foundation; either version 2 of the License, or ; (at your option) any later version. ; ; The Shapelets code is distributed in the hope that it will be useful, ; but WITHOUT ANY WARRANTY; without even the implied warranty of ; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the ; GNU General Public License for more details. ; ; You should have received a copy of the GNU General Public License ; along with the Shapelets code; if not, write to the Free Software ; Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA ; ;+ ; NAME: ; SIM_ADDNOISE ; ; PURPOSE: ; Adds a model of observational noise to a simulated image. ; ; CATEGORY: ; Astronomical image simulation. ; ; INPUTS: ; filename - Name of input image, with path but w/o extension. ; time - Equivalent exposure time on WFPC2, in ks. ; Ought to be < 100, since that is the depth of the HDF! ; ; OPTIONAL INPUTS: ; [image] - Image array, to save time loading it from disc. ; [header] - Image FITS header ; [seed] - Random number seed. ; ; KEYWORD PARAMETERS: ; [/HDF] - Cuts out WFPC L shape, for fun. ; [/NOSMOOTH] - Explicitly set to -1 to force correlated background noise. ; DEFAULT is now unsmoothed, uncorrelated pixels. ; [/SMOOTH] - Set in order to enable correlated background noise. ; [/COSMIC_RAYS] - Add cosmic rays, using model by Jodi Lamoureux. ; ; OUTPUTS: ; Image with noise added is written to file image_t100.fits. ; The updated image, updated header and the inverse variance map ; are also returned in variables image, header and image_noise ; respectively. ; ; NOTES: ; The current implementation consists of a noise model including ; photon counting noise plus Gaussian background noise, with their ; levels set to roughly match those in the HDF. ; ; MODIFICATION HISTORY: ; Oct 03 - Jodi Lamoureux's cosmic ray model incorporated. ; Sep 03 - Inverse variance map output created by RM. ; May 03 - Default behaviour for correlated bground noise ; changed to unsmoothed by RM. ; Mar 03 - Program name changed to protect the innocent. ; Jul 02 - Exposure time glitch corrected by RM. ; Feb 02 - add_noise.pro written by Richard Massey. ;- ; ; Parse input requirements ; if n_elements(filename) eq 0 then filename=get_path(1)+'hst/simulation/synth2' if not keyword_set(seed) then seed=101L if not keyword_set(time) then time=100. if not keyword_set(nosmooth) then nosmooth=1 ; ; Set default noise levels ; background = 0. ; HDF-N is roughly 1.33150e-05 shot_noise = 4e-3/2. ; rms of photon shot noise (estimated from HDF) gauss_noise = 7e-5/2. ; rms of background (calculated from mean of inv. variance map) if keyword_set(smooth) then gauss_noise = 12e-5 ; compensate for smoothed noise gain = 14.0 ; [DN/e-] ? sat_level = 4096.0 ; [DN] saturation level ? read_noise = 5.0 ; [e-] ? DQE = 0.40 ; 40% maximum at 7000A ? e- to photon ratio? bias = 300.0 ; [DN] ? Fuck knows! hdftime = 100. ; [ks] (HDF-N is 100.3ks, HDF-S is 123.6ks) CCDwidth = 10.5 ; CCD width in microns CCDthickness= 200.0 ; CCD thickness in microns CRdiffusion = 0 ; Cosmic rays diffusion 0=off, 1=on CRflux = 4.5 ; Cosmic rays per second per cm^2 CRn_elec = 70.0 ; Electrons emitted per micron of silicon CRsigma = 3.5 ; Microns diffusion for 200 micron drift kernel = 2.0 ; [pixels] to simulate DRIZZLE and seeing (sky background is colvolved with this) ; The pixel scale will affect the noise smoothing kernel ; - or if images are to be drizzled, there should be no kernel at all! ; ; Read in image (NB: rd_shapecathdf has normalised things to HDF-S) ; if not keyword_set(image) or not keyword_set(header) then begin message,'Reading in noise-free image '+filename+'.fits',/info fits_read,filename+'.fits',image,header endif ; ; Determine key image properties ; im_size=size(image,/dimensions) pixel_scale_line=where(strmid(header[*],0,9) eq "PIXSIZE =") if pixel_scale_line[0] eq -1 then pixel_scale=1. $ else pixel_scale=float(strmid(header[pixel_scale_line],9,22)) ; ; Update header with parameters of the noise model ; message,'Adapting .fits header to incorporate noise parameters',/info for i=0,n_elements(header)-1 do begin ; Could use sxpar.pro in astlib case strmid(header[i],0,9) of "EXPTIME =": eline=i "PHOTOZP =": header[i] = "PHOTOZP = "+string(22.08-2.5*alog10(hdftime/time))+" /" "DATAMAX =": header[i] = "DATAMAX = "+string(float(max(image)))+" /" "DATAMIN =": header[i] = "DATAMIN = "+string(float(min(image)))+" /" else: endcase endfor if keyword_set(eline) then begin header[eline] = "EXPTIME = "+string(float(time)*1000.)+" / s Normalised to HST throughput" cortext="NOISECOR= 0 / Uncorrelated between adjacent pixels" if nosmooth eq -1 then cortext="NOISECOR= "+string(float(kernel))+" / pixels Smoothing length of background noise" header=[header[0:eline],$ ;"COMMENT = / Parameters of applied noise model",$ "NOISEPOS= 1 / Positive-definite constraint imposed",$ "NOISELEV= "+string(float(background))+" / Sky background level",$ "NOISESKY= "+string(float(gauss_noise))+" / Gaussian sky noise rms",cortext,$ "NOISEPHT= "+string(float(shot_noise))+" / Photon counting noise rms x sqrt(flux)",$ "NOISECR = 0 / No cosmic rays",$ "NOISESAT= 0 / No CCD saturation limit",$ header[eline+1:*]] endif ; Positive-definite constraint message,'Imposing positive-definite constraint',/info ;image = abs(temporary(image)) ; Impose constraint but conserve clumpiness image = temporary(image) > 0.; (truncation was used for Massey etal 2004) ; but both of these ideas might affect shearing if time ne hdftime then $ ; renormalise level for exposure time image=temporary(image)*time/hdftime ; Create inverse variance noise map image_noise=image*(shot_noise)^2+(gauss_noise)^2 ; Construct header for inverse variance noise map ;image_noise = temporary(image)*shot_noise^2+gauss_noise^2 header_noise = header for i=0,n_elements(header_noise)-1 do begin case strmid(header_noise[i],0,9) of "OBJECT =": header_noise[i] = "OBJECT = 'Inverse variance map'" "BUNIT =": header_noise[i] = "BUNIT = 'photons^(-2)' / '" "DATAMAX =": header_noise[i] = "DATAMAX = "+string(float(max(image_noise)))+" /" "DATAMIN =": header_noise[i] = "DATAMIN = "+string(float(min(image_noise)))+" /" else: endcase endfor ; Write inverse variance noise map to disc and free space in memory message,'Writing inverse variance noise map to file '+$ filename+'_t'+strmid(strtrim(time,1),0,5)+'_noise.fits',/info writefits,filename+'_t'+strmid(strtrim(time,1),0,5)+'_noise.fits',$ 1./image_noise, header_noise delvarx,image_noise ; Free up memory for later calculations delvarx,header_noise ; Add Photon counting noise to original image (stdev = sqrt(n)) message,'Adding Gaussian photon counting noise (should be Poisson)',/info ;b=sqrt(image)*randomn(seed,im_size[0],im_size[1])*shot_noise image=image+sqrt(image)*randomn(seed,im_size[0],im_size[1])*shot_noise ; ; *** Slow but memory-efficient method *** ;for y=0,im_size(2)-1 do for x=0,im_size(1)-1 do $ ; image(x,y) = image(x,y)+sqrt(image(x,y))*randomn(seed)/400. ; Getting y,x the right way round speeds it up a bit (see RSI website) ; However, 500 is a fudge factor because I don`t know the absolute ; background level in the HDF! LATER NOTE: SHOULDN`T DEPEND ON THIS! ; Add sky background THE SMOOTHING TAKES LOADS OF MEMORY! message,'Adding Gaussian sky background',/info ; NB: This really should be Gaussian. The background level is very high, but ; pre-subtracted from my image. s_noise = randomn(seed,im_size[0],im_size[1])*gauss_noise ; Scale to level observe in HDF if nosmooth eq -1 then s_noise = smooth(s_noise,kernel,/edge) ; Only top-hat, but quick (actual kernel produced by dithering is on web) s_noise = temporary(s_noise)*sqrt(time/hdftime) ; Exposure time calibration image = temporary(image)+s_noise delvarx,snoise ; Add a constant background level if background ne 0 then begin message,'Adding constant background level',/info image=temporary(image)+background endif ; Add cosmic rays (NB: should also add these to the inverse variance map) if keyword_set(cosmic_rays) then begin message,'Superimposing cosmic rays',/info cnoise = fltarr(im_size[0],im_size[1]) dummy = call_external(shapelets_paths(9,/silent)+'simages/cosmic_rays/CosmicRay.so',$ 'cosmicRayIDL', cnoise, im_size[0], im_size[1],$ CCDwidth, CCDthickness, time, $ CRdiffusion, CRflux, CRn_elec, CRsigma) image = temporary(image)+cnoise/time delvarx,cnoise endif ; Chop out a HST-type mask for fun! if keyword_set(hdf) then begin image[0:80,*]=0. ; left image[4015:4095,*]=0. ; right image[*,0:80]=0. ; bottom image[*,4015:4095]=0. ; top image[0:1000,2049:4095]=0. ; L1 image[0:2048,3050:4095]=0. ; L2 endif ; Write image to file message,'Writing noisy image to file '+$ filename+'_t'+strmid(strtrim(time,1),0,5)+'.fits',/info writefits, filename+'_t'+strmid(strtrim(time,1),0,5)+'.fits', image, header end

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Last modified on 02nd May 2008 by Richard Massey.

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