The information in this section is an evolving set of notes about issues of concern to HIRES users, the kinds of things users might want to do with HIRES images, tricks to try and pitfalls to avoid. Some included topics are:

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

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 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 output maps, are particularly valuable in estimating the achieved resolution for a given source at a given point in the map. You can specify both source fluxes and positions for the beam sample maps. Fluxes and positions from the PSC2 or FSC 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); 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 (G. Laughlin, D. Engler and W. Rice, 1990, IPAC IOM 701-90-077):

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

Mosaics

You may want to attempt to mosaic HIRES images to make one big image. This is a somewhat complicated problem since LAUNDR fits baselines to the data in each separate field. There is no provision in LAUNDR to ensure that image boundaries will match.

One possible solution is to LAUNDR a large field, then run 2 degree by 2 degree sub-fields through HIRES separately. LAUNDR can be used on a field up to 6 degrees by 6 degrees (about the size of Level 1 Archive plates), so mosaickable sub-fields up to this size may be made if the target is strategically located. LAUNDR can only operate on a single Level 1 Archive plate at a time, so taking the overlap into account, the largest field size guaranteed to be available to LAUNDR for mosaicking would be 3 degrees by 3 degrees.

Baseline removal de-striping is less effective for large regions; in large busy fields, cross-scan offset de-striping may work better.

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.

Ratio Maps

Many users would like to make high resolution color or ratio maps from HIRES maps. Be warned: This is a non-trivial task! As discussed above, the resolution in an image not only varies from band to band but also from point to point within a single map.

There are several possible solutions to this dilemma. The first is to smooth both maps a lot in order to overwhelm resolution differences. However, this approach tends to defeat the purpose of using HIRES in the first place. A second possibility is to examine intermediate iterations for the better-resolved band to try to find a point-spread function which nearly matches that of the less-resolved band. A more sophisticated approach is to use the IRAS simulator mode of YORIC to cross-simulate the bands. For example, if a 12/100 µm ratio map is desired, one could use the 12 µm HIRES map (presumably at better resolution than the 100 µm map) to generate simulated 100 µm data by "scanning" the 12 µm HIRES image using the 100 µm simulator (that is, using the 100 µm detector files as the input to YORIC in simulator mode, so that the coverage and detector response functions for the 100 µm observations are used the simulation). In essence this produces an output image of the 12 µm sky at the 100 µm resolution. Tests of this technique have been encouraging.

Saturation

The detectors saturated at about 1000 Jy. Saturation can be diagnosed in the cfv maps (see HIRES output maps), 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.

Potential users wishing to test this approach should contact IRSA Support at irsadmin@ipac.caltech.edu