Image simulation instructions:

Creating your first image:

These instructions will guide you through the creating of your first simulated image using this code. There are three stages in the creation of a new image: the decomposition of a suitable real galaxy population into shapelets, the generation of a catalogue of new galaxies, and the placing of those objects into an image. A flowchart describing an overview of these stages is available (in various formats).

• Real image decomposition
  A sample of real galaxies, from a regime similar to that being simulated, are decomposed into shapelets using the IDL routine shex.pro. This can be invoked from a shell prompt via
  > shex HDF-N_f814
  > shex HDF-S_f814
  This has already been performed for the HDF, which will prove adequate for our first run. The shapelet catalogues of HDF objects are the files HDF-N_f814.shape and HDF-S_f814.shape in shapelets_shims.tar.gz. These can be read into memory using the IDL routine shapelets_read_shapecat_hdf.pro.
  
  
• New catalogue generation
  The shapelet coefficients of the real galaxies populate a multi-dimensional parameter space equivalent to the Hubble tuning fork. As described in Massey et al. (2004), this population can then be smoothed and resampled using a Monte-Carlo technique. The amount of smoothing is tuned to create genuinely new, yet realistic galaxy morphologies.
  This step uses the IDL routine simage_generate_shapecat.pro, or can be invoked from a shell prompt via
  > generate_catalogue new_catalogue 1234
  where the first parameter is the file name of the shapelet catalogue to be created, and the second parameter is a seed for the random number generator.
  The file Masseyetal2004.shape in shapelets_shims.tar.gz is catalogue containing objects used in Massey et al. (2004), whose morphologies were compared to HDF galaxies using concentration, asymmetry and clumpines statistics, and verified to be realistic.
  
  
• Image generation
  The new galaxies are pixellated, and layed down in a new image by the IDL routine simage_assemble_image.pro. This can be invoked from a shell prompt via
  > generate_simage new_catalogue
  This script automatically adds observational noise, according to the desired exposure time, and using a noise model in the IDL routine simage_add_noise.pro. The output image file name can be specified but, by default, incorporates information about the exposure time, PSF, and input shear etc.

More information about these routines, and about their various subroutines, can be found on the shapelet help pages.

Input parameters:

The file input_parameters in shapelets_shims.tar.gz specifies the various input parameters for a new simulated image during the third stage above. These parameters are

"To do" list:

• Generate source galaxy catalogue from HST Ultra-Deep Field
• Create multicolour simulations
• Add redshifts (relate to HDF galaxies' measured redshifts, or just their sizes and magnitudes?)
• Smooth coefficient parameter space only after performing PCA
• Vary PSF across the field of view
• Vary shear across the field of view?
• Improve background noise model? Zodie etc?
• Check cosmic ray code is working properly
• Link to galaxy morphology indices?
• Adapt analytic models to simulate ground-based observations
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