The software package is currently being dramatically improved and updated.
This is a (very) old version, potentially with bugs.

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Help for "simage_add_noise" routine:

Adds a model of background and observational noise to a simulated image.

This takes a noise-free simulated image, and adds "observational" noise. It is called automatically at the end of simage_assemble_image.pro. The image plus noise, and the inverse variance pixel weight map, are written to disc. If the simulated image is called image.fits, and noise is added to represent a 100ks exposure, the output files will be image_t100.0.fits and image_t100.0_noise.fits. The headers of these files contain details of the noise levels imposed.

Currently, the noise model includes

By default, the noise is uncorrelated in adjacent pixels. This will not be true in real life, especially if multiple exposures are stacked using the DRIZZLE or MULTIDRIZZLE algorithms. As a very quick approximation to this scenario, the noise model (only) can be smoothed prior to its superposition on the noise-free simulated image. This is done using the /SMOOTH flag, but be aware that it is rather unsophisticated.

See also the header below, which has been extracted from the source code for this routine.

; 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.

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Last modified on 02nd May 2008 by Richard Massey.

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