Spitzer Documentation & Tools
MIPS Instrument Handbook

8.2.2        Point vs. extended sources, or why there are 2 BCDs

The goal of BCD pipeline processing is to produce the best possible data products that can be derived from automated processing.  Currently, the SSC produces two BCD products: (1) bcd.fits which is the standard calibrated BCD and (2) fbcd.fits which is a ''filtered'' bcd product designed for point sources.  The fbcd is produced by subtracting off a median of the surrounding DCEs as a function of time per pixel.  This filtering technique significantly mitigates the accumulation of stimflash latents and the residual background drifts due to variations of the slow response as a function of time (see below for more information).


The application of the median filter removes the background from both the sky and the residual detector effects (i.e., loses information about the extended background level in the field).  Early tests show that the application of a median filter maintains point source calibration for scan maps of fields with uniform backgrounds, but the fbcds do not preserve calibration for extended sources or for bright (>0.2 Jy) point sources within complex emission regions.  For extended and/or bright sources, you may achieve better results with offline custom filtering using the GeRT (see section 8.2.3). 


A mosaic constructed from some sample unfiltered MIPS-70 scan map data appears in Figure 7.20; a mosaic constructed from the same data but filtered is in Figure 8.5.  The dark horizontal (in-scan) stripes are fast/slow response variations, which are removed from the data using a temporal median filter per pixel in the current automatic pipeline.  The bright vertical stripes are stim latents, which are removed in the pipeline using column median filtering.  These sample FLS data date from early in the mission, e.g. they were taken with the old bias settings (see ''stim latents'' below).  Most data were taken with the new bias setting, so the magnitude of both of these effects should be lessened in a typical observation. 


Figure 8.6 shows a flowchart of possible analysis steps, discussed in much more detail below. To the BCD level, our pipelines yield quantitatively similar results to those described in Gordon et al. (2005, PASP, 117, 503). 


Real-life specific examples of extended source work with MIPS can be found, for example, in the ApJS special issue; see, e.g., the paper by Engelbracht et al. (2004, ApJS, 154, 248) on NGC 55, Helou et al. (2004, ApJS, 154, 253) on NGC 300, and/or Regan et al. (2004, ApJS, 154, 204) on NGC 7331.




Figure 8.5: Default mosaic from filtered MIPS-70 BCDs, constructed from 4 AORs.



Figure 8.6: A flowchart describing possible pathways for analysis of scan map post-BCD Ge data processing.  If your source of interest is not detected or higher S/N is required, then we recommend doing the steps above, otherwise the default BCDs may be sufficient.  For extended sources of intermediate sizes, you may not want to correct for the slow response via an illumination correction but instead by an additive correction to the background.