In this section, we describe how to reduce 70 micron data when you are interested in bright extended sources. See also a graphical representation of this process in Figure 8.6. This example uses NGC 300; the corresponding science paper is Helou et al. (2004, ApJS, 154, 253). NB: The NGC 300 data as shown were taken before the bias change in March 2004. Data taken after the bias change may be sufficient for most purposes, without additional offline processing.
Figure 8.13 shows a 70 micron mosaic of NGC 300 made from the default non-filtered BCDs. The dark stripes at the left of the image are due to the variation of the residuals in the slow response. The bright stripes at the right are due to stimflash latents. Figure 8.13 also shows the mosaic made from the temporal filtered BCDs (*fbcd files). Although the drifts of the slow response and stim latents are removed, the filtering process removes extended source flux and introduces negative sidelobes near bright emission regions.
For extended sources, we recommend you use the default BCDs, not the filtered BCDs. The filtering techniques are optimized for point sources and remove a significant fraction of the emission from extended sources. For example, note how much the extended source flux changes in Figure 8.14. To remove data artifacts for extended sources, observers need to observe enough off-source regions for good background subtraction. We recommend that observers linearly interpolate the measured background levels in the BCDs across the target on a pixel basis. The interpolated background corrections should be subtracted from the BCDs before coadding the data and making the mosaic. The additive background corrections as described here have been shown to yield good results for extended galaxies. To derive the true sky level for extended regions, observers may want to consider TPM observations.
Figure 8.13: Mosaic of 70 micron BCDs from NGC 300: left is unfiltered and right is filtered (DO NOT DO THIS!). On the left, note the variations in the background due to stim latents (bright) and the slow response residuals (dark). On the right, note the filtering process has removed extended source flux and introduced negative sidelobes near bright regions.
Figure 8.14 shows examples of response corrections made via offline reprocessing with the GeRT for NGC 7331. The ''before'' and ''after'' compares the automatically-produced (''online'') mosaic and the mosaic created from GeRT-reprocessed BCDs. With the GeRT, we removed stim pixels affected by the bright source and stitched the scan legs together. Variations in response can occur when the stim flash and/or stim_background DCE occurs on bright sources. After correcting for response effects, filtering corrections are derived with the GeRT by masking source regions. The final GeRT produced image is shown on the right.
Figure 8.14: Example of offline processing using the GeRT using a series of images of NGC 7331. Going from left to right the images are the automatically produced (''online'') mosaic, the mosaic made from BCDs with corrected stim response, the mosaic produced using filtered BCDs (DO NOT DO THIS FOR EXTENDED SOURCES!), and the final mosaic made using a high-time pass filter after masking out sources.