8.5    Things to Check and Important Notes for Data Analysis

When working on and analyzing Spitzer/IRAC data, there are several important things to keep in mind. We have collected the most important things to keep in mind here so that the user can go through this list before starting to work on IRAC data, and come back to it during data reduction and analysis.

 

· Before you start using IRAC data, we recommend that you familiarize yourself very carefully with this document, and specifically Chapter 7, which discusses the various artifacts in IRAC data.

 

· In general, we caution the user to be extremely careful when using data from PC1 − PC4 campaigns as the calibration was constantly changing during this time period (2009 July 28 - September 23).

 

· The CBCD frames contain the artifact-corrected observations, and should usually be a starting point for your further data reduction and analysis. However, you always have the option of going back to the BCD frames if you are not happy with how artifacts were corrected in the CBCDs, and perform your own corrections.

 

· Point source photometry should be performed with aperture photometry, unless the targets lie in an area of sky that has an extremely high surface density or a strongly spatially variable background.

 

· The user should read the caveats in Section 8 and think carefully before publishing any results about extended emission flux densities and surface brightnesses.

 

· For diffuse surface brightness measurements, we recommend making differential measurements among at least two sky positions, preferably from the same campaign.

 

· The saturation estimates were conservatively computed from the worst case in which the PSF is directly centered on a pixel.

 

· The lack of an isolated measurement of the dark current and bias offset during shutterless operations limited the ability of IRAC to measure the true celestial background.

 

· The flat-field data were generated from the zodiacal background, and are appropriate for objects with that color. There was a significant color term, of order 5%-10%, for objects with a Rayleigh-Jeans spectrum in the mid-infrared (such as stars); see Section 4.5 for more information.

 

· For deep survey observations and other data sets with a large number of frames and a good dithering strategy, the system gain could be determined by the actual survey frames themselves, rather than using the standard set of dedicated observations of some other part of the sky.

 

· Our definition of the color correction looks slightly different from that in the IRAS Explanatory Supplement, because we used the system spectral response R in electrons/photon, instead of ergs/photon.

 

· The array location-dependent photometric correction images for the cryogenic mission and the warm mission are different, and users should use the appropriate mission corrections. See Section 4.5 for more notes about these corrections.

 

 

· It is important to apply an aperture correction to flux densities measured through aperture photometry or PRF fitting, unless the exact same aperture and background radii and annuli were used as for the calibration stars.

 

· The spatial extent of the PSF in channels 3 and 4 was much larger than the subarray area. In other words, a large amount of the total power in the PSF is scattered onto arcminute size scales. As a result, special care needs to be taken when measuring fluxes in these channels, since accurate measurement of the background is difficult. Proper application of aperture corrections is very important.

 

· The post-BCD mosaics available from the Spitzer Heritage Archive use pixels that correspond to exactly 0.6 arcseconds * 0.6 arcseconds.

 

· PRF fitting does not work in image mosaics where the information from the PSFs has been blended together. Aperture photometry is the correct way to perform point source flux density measurements in image mosaics.

 

· The extended PSF should not be used for PSF-fitting photometry and source extraction of non-saturated point sources.

 

· There is a problem with the skydark subtraction from the 58th frame in the subarray observations which leaves the background level in that frame different from the rest.

 

· Because of the “first-frame effect,” the first frame of every Astronomical Observation Request (AOR) has a different delay time and it cannot be calibrated perfectly.

 

· The pipeline cannot flag slew residuals, as there is no reasonable way of tracking the appearance of bright sources relative to the moving telescope pointing.

 

· Users should be aware of the uncertainties resulting from banding, specifically when attempting measurements of faint sources near the affected rows or columns.

 

· The aperture fluxes reported by APEX are always made using the median background, and hence may be inaccurate for faint sources.