Spitzer Documentation & Tools
MIPS Instrument Handbook


7.2.7        Offline Enhancements to MIPS 70 Fine Scale or 70/160 large-field

The observing strategy implemented for the MIPS-70 Fine Scale AOT and the 70/160 large-field photometry differs from both the MIPS default photometry and scan modes in that alternating on- and off-source DCEs are obtained during each cycle.  The distance between the centers of the on- versus off-source positions is roughly 4'.  Consequently, the two fields of view overlap and the online pipeline currently creates a single mosaic consisting of both on- and off-source BCDs. 


The acquisition of the off-source frames provides the opportunity for additional offline post-processing.  While in most cases the pipeline reduced BCDs are of good quality, we have found that the data can be improved to varying degrees by different methods of off-source ''background'' subtraction.  However, there is no general recipe that works equally well for all types of observed sources and the observers are cautioned to carefully investigate the appropriate method for their particular datasets, especially in the case of bright and spatially varying extended emission. 


Several offline approaches can be applied to clean up these data without filtering, which allows users to maintain proper calibration for extended sources.  The best technique depends on the details of a particular data set.  You can subtract the ''off-source frame'' from the preceding ''on-source'' frame (e.g., with IRAF) and mosaic the resulting difference images with MOPEX.  You can take the median of the ''off-source'' frames, subtract the resulting median image from each ''on-source'' frame, and mosaic the resulting difference images.  Or, you can do some variation/ expansion/ combination of these two approaches depending on the particular data set.  In using any of these, watch out for sources in the ''off-source'' frames! 


Figure 7.27: Examples of 70 micron fine scale reprocessing discussed in text.


Figure 7.27 shows examples of several cases. 


a)  The mosaic of all on-source BCDs without any additional post-processing.  On-source BCDs are the odd-numbered BCDs in each cycle (1,3,5,7).  The even-numbered BCDs represent the off-source BCDs and additional stim and stim background observations. 


b) The mosaic of paired on-source minus off-source BCDs, i.e., for each on-source BCD, subtract the off-source BCD immediately following it. This method usually provides the visually smoothest mosaic.  However, the noise is usually slightly increased over methods where the off-source BCDs are first combined before subtraction (e.g., case c).


c) The mosaic of on-source BCDs after subtraction of the median-combined off-source BCDs from each BCD.


d) The mosaic of all off-source BCDs.


Before performing any off-source subtraction, we recommend that you always inspect the mosaic of the off-source BCDs to prevent introduction of off-source structure into the on-source mosaic.


Figure 7.28: Applying the same technique to a small raster map observation; see text.


In the raster map example shown in Figure 7.28, the target was positioned so that it appears both in the ''on-source'' and ''off-source'' frames, hence neighboring ''on'' and ''off-source'' frames cannot be subtracted.  Instead, all frames were median combined and the resulting image subtracted from each BCD.