The MIPS cycle 2 data

54 hours of observations with the MIPS camera have been dedicated in Cycle 2 to cover:

• the entire 2 sq.deg. COSMOS field with a shallow survey (16hr) at 24 µm, 70 µm , 160 µm;
• a small "test" area (0.16 sq.deg.) with very deep observations (38hr) at 24 µm, 70 µm , 160 µm.

Additional observing time has been obtained in Cycle 3 to expand the very deep MIPS coverage (see Figure below) to the entire 2 sq.deg. COSMOS field.

Observing strategy

The entire COSMOS field has been covered using slow-scan mapping for the small "test" area and a medium-scan mapping for the main field. We used a scan leg of length 1.5 deg with 148'' cross scan offsets between the forward and return scan legs. The field-of-view for one exposure varies from about 5x5 arcmin at the shortest wavelength to about 0.5x5 arcmin at the longest wavelength. The three bands 24 µm, 70 µm , 160 µm are observed simultaneously. The spatial coverage (Cycle 2) is shown below where we can easely distinguish between the current shallow and deep areas.

Data reduction

The S-COSMOS survey inherits the data reduction methodology and analysis softwares developed for the GOODS, SWIRE, and xFLS surveys. Our team is in the process of optimizing these analysis tools for use in reducing the S-COSMOS data.

The Spitzer Science Center produced the basic calibrated data (BCD) from the raw data. The BCD images are flux calibrated and the well-understood instrumental signatures are removed. The BCDs are droop corrected, dark subtracted, corrected for non-linearity and divided by the flat field. Work is in progress to improve the BCD at 24µm (e.g. self-flatten the data as a function of scan mirror and AOR) and at 70µm and 160µm (e.g. filtering techniques optimized for COSMOS data).

Second step: the mosaic

The BCDs are coadded and combined using MOPEX software. The BCDs are corrected for array distortions and projected onto a common sky grid. Using the redundancy of the MIPS data, spurious data are masked and rejected from the coaddition. We also found ~115 asteroids in the deep field area, half of which where undetected so far. These asteroids were removed from the final image, shown bellow. In this figure, the small deep "test" area is outlined by the black rectangle, superimposed on the shallow 2 sq.deg. COSMOS field.



The figure below shows a zoom at the limit between the deep "test" region (left) and the shallow region (right). This figure illustrates a clear improvement in terms of the number density of detected objects in the deep area.

The figure below shows an RGB image which combines IRAC (3.6 and 8 micron) and MIPS (24 micron) within the deep MIPS "test" area. The deep "test" area covers a field where a Large Scale Structure at redshift z=0.73 has been detected (Guzzo et al., Scoville et al., Finoguenov et al., 2006 in preparation).