Interpreting HIRES Images

The information in this section is a set of notes about issues of concern to HIRES users. The topics included below are:

• How to assess the resolution of a HIRES.
• The effect of image dynamic range
• HIRES photometry and flux stability with integration
• HIRES positional accuracy
• Dealing with saturated data
• Making ratio maps from HIRES images

Achieved Resolution

The resolution in a HIRES image not only varies from band to band, but also from point to point within a single map. The variation from band to band is due mostly to the inherent point spread function differences between IRAS bands. The variation within a map is due primarily to coverage variations from point to point and secondarily to signal-to-noise variation from source to source. The rate at which a source approaches the limiting resolution depends, in part, upon the source strength relative to the background. In a map containing both strong and weak sources, the weak sources will appear noticeably broader than the strong sources.

The beam sample maps, which are among the standard HIRES Image Types, are particularly valuable in estimating the achieved resolution for a given source at a given point in the map. You can specify a source flux for the beam sample maps. Flux estimates from the PSC2 are adequate for this purpose.

To get an idea of the typical performance of HIRES, the following table (W. Rice, 1993,AJ,105,69) gives the mean beam width of 13 stars. The quoted number is the full-width half-maximum cited as (cross-scan x in-scan) in arcseconds.

Dynamic Range

The dynamic range of HIRES is limited on small spatial scales (on the order of 10') to about 1:100. You should be careful in interpreting faint structure near bright sources. At small spatial scales, it is best to work at intensities within 5% of the peak value.

Photometry

An important thing to consider about HIRES images is the flux calibration. In general, fluxes measured from the intensity maps agree with those of the Point Source Catalog (PSC2) to within 20%.

Background estimation when doing aperture photometry with HIRES can be tricky, particularly when ringing is present.

In addition you must consider the impact of the AC/DC effect (see material on IRAS Data Characteristics); for extended sources you may want to apply a source-size-dependent flux correction.

Flux Stability

It has been found that fluxes measured from HIRES images vary by up to 20% over the course of iteration. Flux is not conserved until most of the resolvable structure has been resolved. The following numbers relating iteration number and flux have been found :

NOTE: All HIRES images are made using the Level 1 Archive CRDD, which has the most recent calibration ("Pass 3").

Positional Accuracy

Testing with HIRES to determine the positional accuracy of point sources was carried out. It was found that the HIRES centroid positions are within 20" of the PSC2 positions on average and the peak positions are within 30". Since the HIRES processing makes much more complete use of the position information than the PSC processing did, the HIRES positions may be the more accurate ones, but no studies to show this have been done to date. Comparison to WISE data may be helpful in some cases.

Saturation

The detectors saturated at about 1000 Jy. Saturation can be diagnosed in the cfv maps (see HIRES Image Types), which show high variances on source that do not decrease with further iteration. In theory, HIRES may be able to reconstruct saturated sources by excluding data from the detectors which passed directly over the saturated source, using only those detectors that passed near the saturated source. HIRES would, essentially, reconstruct the peak from the wings of the point source.

Ratio Maps

Many users would like to make high resolution color or ratio maps from HIRES maps. Be warned though, the resolution in an image not only varies from band to band but also from point to point within a single map.

Return to HIRES Instructions .