NOTE: Hires Simulator mode is currently not available.

The resolution-enhancement algorithm used in the YORIC module of the HIRES processor is based upon detailed detector response functions and contains a mechanism to simulate Infrared Astronomical Satellite (IRAS) observations of any model of the sky. The HIRES simulator mode takes an input (FITS) image and produces simulated IRAS scan data which faithfully reproduce what IRAS would have seen if the input image is an accurate representation of the actual target. The simulator mode can be used to help determine if the IRAS data are compatible with a given model of the "real" infrared sky by applying the IRAS limitations to that model. For example, an optical or radio map in FITS format can be fed to the simulator to determine what IRAS would have seen if the IR morphology were identical to the morphology of the source in the input map.

The simulated scan data can also be run through HIRES, and results used to determine if HIRES could pull certain types of structure out of the IRAS data, were that structure present. The simulator is a particularly powerful tool when used with HIRES, and, in many cases, may be the only way to "get a handle on" what HIRES is doing.

The simulator can also be used to compare the structures in different bands. For example, a HIRES image at 60 µm could be scanned by the (100 µm) simulator and rerun through HIRES at 100 µm. The resulting output could then be compared with the actual 100 µm survey data to see if the structure was the same at both wavelengths. Similarly, cross-band simulation can be used to attempt to produce identical resolution in two different wavebands for constructing ratios.

How to Use the Simulator

A request to do a simulation is made as if it were a HIRES request, stating in the Special Instructions that this is a simulation request. A field center and size is still required, since YORIC uses detector files from LAUNDR to determine the coverage, scan geometry and detector response functions to use for the simulation. A standard FITS format image with an odd number of rows and columns between 15 and 513 must also be supplied as the input model.

Simulations do not take a great deal of CPU time (only 3--4 minutes on a SPARC 1), but the simulated and actual detector files take up a great deal of disk space. Additionally, although the simulator is able to quickly make the simulated detector data, YORIC must still be rerun in order to generate simulated HIRES output, and this takes as long as running HIRES normally using real data. It is envisioned that ultimately YORIC will be exported, along with the appropriate IRAS data, to the user for running on his or her own machine.


The most noticeable limitation of the simulator is the lack of noise in the output data. A capability to add noise exists but carries restrictions and requires special handling. The other limitation to be aware of is the imperfect modeling of the detector response functions. This seldom causes significant problems. However, the detector response functions are cut off sharply at 1% of peak, which implies an infinite spatial frequency response. Since the achievable resolution is usually limited to spatial frequencies below the diffraction limit of the telescope, this is usually a negligible effect.


It is important that the header in the input FITS image be set up correctly. Unless the header keywords CTYPE, CRVAL, CRPIX1, CRPIX2, CDELT1, and CDELT2 are set correctly, YORIC has no way to understand the geometry of your field, and you will not get correct results. In addition, BUNIT must also be set correctly if you want YORIC to produce output in the same intensity units as your input. Further, you will need to include the FBIAS value from the image in the header if you will use default FBIAS processing.

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