4.8 Photometric Variations in Data Taken with Full Array, Subarray, Dithering and Staring Modes
Differences in the photometric flux densities of the same targets in IRAC channel 1 and 2 images taken using various modes (full array, subarray, staring mode, dithering) were found. To investigate these differences, data from special calibration PIDs 1336 and 1367 were employed. A set of ten stars were observed using varying frame times, subarray and full array modes, staring (taking images successively at the same pointing position) and dithering. The stars used were IRAC calibration stars that are known to not have astrophysical variation in their flux densities. About 80000 photometry points in each of the channels 1 and 2 were measured and used in this study.
Staring mode data in the Spitzer Heritage Archive were processed through the pipeline in the same manner as dithered data, including the subtraction of a skydark image that was made by dithering. For this study, data from special calibration PID 1345 was used to make a staring mode skydark suite. Subtracting an appropriate staring mode skydark from the staring mode data, after having first undone the dithered dark subtraction, does have a measurable effect on staring mode photometry. It is therefore recommended that anyone performing absolute photometry with staring mode data use a staring mode skydark instead of the pipeline-provided dithered dark. Instructions for how to make an appropriate skydark and subtract it are available in the Contributed Software section of IRSA’s Spitzer web pages:
For the comparison of the flux densities of the same star taken via various observing modes, all of the photometric corrections recommended in this chapter must be implemented first. To compare photometry on all the ten calibration stars on the same plots below, we used a normalization by dividing all the photometry points of a given star by the median flux of the same star in all of its measurements.
Figure 4.7 shows all the calibration star measurements divided into four subcategories: full array vs. subarray, and short frame times vs. long frame times. The division between short and long frame times is at 0.3 seconds (in terms of actual exposure time). There is no good way to compare flux density measurements of equal exposure times, because the exposure times are significantly different for the full array and the subarray. Longer frame times are in blue and green and shorter frame times in red and orange. There is an effect with both frame time and array read mode. The effect is stronger in channel 1 than in channel 2. For the stars which have full array observations with both short and long frame times, the median difference in flux density is 3.3% in channel 1 (for six stars) and 1.0% in channel 2 (for ten stars). For subarray, the median difference in flux density between short and long frame times is 2.9% in channel 1 and 1.7% in channel 2.
Figure 4.7: Distribution of normalized corrected calibration star flux densities. There is a difference in the absolute photometry of stars taken in different observing modes (shown here in the different colored distributions). The legend lists the different observing modes (full array vs. subarray, and long exposure times vs. short exposure times) as well as the number of photometry points per distribution. Channel 1 is on the left, channel 2 is on the right.
For three calibration stars observations exist in all the modes for the 2 second frame times (subarray staring, subarray dithering, full array staring, full array dithering). Remember that the 2 second subarray observations are really 1.92 second exposures and the 2 second full array observations are really 1.2 second exposures, so these frame times are not equivalent, but they are the closest there are between the subarray and full array. Figure 4.8 shows the median and standard deviation of the corrected flux densities for observations in the different modes for each star. The legend also lists the number of the data points per mode. Again, one can see a difference in photometry between subarray and full array observations (blue points vs. red/orange points). The staring mode absolute photometry is within one sigma of the dithered data absolute photometry in both channels.
Figure 4.8: Cleveland dot plot showing medians and one sigma uncertainties of the distributions of photometry from four different modes shown in Figure 4.7 (subarray vs. full array and staring vs. dithering). The names of the three calibration stars considered here are shown on the y-axis, and the data points for different modes of photometry for each star are offset slightly in the y-direction for ease of viewing. Staring mode observations in the subarray and full array modes were taken at the same locations on the array (on the sweet spot pixel).