Overview

FRESCO employs the Maximum Correlation Method (MCM) (H.H. Aumann, J.W. Fowler and M. Melnyk, 1990,AJ, 99,1674) to construct coadded images. This is the same algorithm used to make enhanced-resolution images (HIRES) at IPAC.

FRESCO is a three-step process:

1. A program called SNIPSCAN is used to retrieve the raw survey data. The IRAS raw scan data are known as CRDD (Calibrated, Reconstructed Detector Data) and the full set of survey CRDD is stored in a FITS table format merged with the pointing (boresight) information. The collection of the raw data in FITS table format is known as the Level 1 Archive.
2. A program called LAUNDR cleans the data up
3. A program called YORIC applies the MCM algorithm to the LAUNDR'd data. In FRESCO processing, YORIC does not iterate, but stops after it has built the ``iteration 1" image. At this point, the image is a simple gridding of the raw scan data into a coadded map, without any resolution enhancement.

As of March 31, 1993, FRESCO is AC calibrated. The IRAS detectors exhibited dwell-time dependent responsivity changes, especially at 12 µm . This is the IRAS data feature known as the "AC/DC effect". Previous IRAS image products have been calibrated on the DC scale, which is correct for structure > 30'. The AC scale gives flux densities commensurate with the Point Source Catalog (PSC2) for point sources.

FRESCO Processing Defaults

The default configuration for FRESCO is:

• Either a 1 degree by 1 degree or a 2 degree by 2 degree field with 15" pixels
• Baseline removal de-striping
• Data for all four wavelength bands (12, 25, 60 and 100 µm) processed
• All available survey data coadded
• Only intensity maps produced.

Commonly Used Processing Options

Only commonly used FRESCO processing options are discussed below. Since FRESCO is essentially the first iteration of HIRES, the documentation for HIRES may also be of interest.

Field and Pixel Size and Wavelength

FRESCO fields can be up to 2 degrees on a side, and need not be square. Pixels must be square and are usually 15" or 30". Since the resolution of FRESCO images is dominated by the size of the IRAS detectors these pixel sizes are small compared to the achieved resolution.

Fields are normally processed for all four IRAS wave bands but a subset may be specified.

Coverage Options

IRAS surveyed most of the sky with three separate hours-confirming observations HCONs. HCONs 1 and 2 were concurrent, HCON 3 was done several months later and only covers about 2/3 of the sky; because there is generally a larger time difference between HCON 1 or 2 and HCON 3, there is a greater difference in the zodiacal background. Because the IRAS data display flux-dependent non-linear response variations of the detectors this can lead to a calibration difference for HCON 3 which can be significant.

FRESCO can process data from any combination of HCONs. Popular combinations are 1 and 2 together, 3 alone, which can be useful in examining the effects of the background on processing, or 1, 2 and 3 all together.

Output Options

There are several different types of images available. The default processing produces only intensity maps, one per band. Coverage and photometric noise maps, described below, may also be specially requested for FRESCO.

Intensity Map

Intensity in the map is in units of MegaJanskys per steradian. Intensity maps are named with the convention "img_name_bx" where "name" is the object name supplied by the requester, x is the band number. (Bands 1, 2, 3, and 4 refer to the 12, 25, 60, and 100 µm wavebands, respectively).

Coverage Map

The coverage map gives the sum of the weighted detector responses at each image pixel over all detector samples. The actual number of detectors at a given point is roughly twice the value of the pixel in the coverage map. Examination of the coverage map can indicate non-uniformities in detector coverage. The naming scheme for coverage maps is "cvg_name_bx".

Photometric Noise Map

The photometric noise map indicates the internal photometric error of the detector samples resulting from the averaging of those that overlap, and is essentially the standard deviation of contributions to each pixel. It does not include absolute errors such as calibration errors. These maps can thus be used to show the relative noise across an image, but not the absolute level of the photometric noise. The naming scheme for these maps is "phn_name_bx".