Spitzer Documentation & Tools
MIPS Instrument Handbook

 

3.1.7        Photometry - 160 Ám, Enhanced (Small Field)

One of the conclusions from analysis of the first three years of 160 Ám calibration data, plus that of some technically challenging science programs (e.g., new planets in the Solar System or Kuiper Belt objects), was that to improve both the photometric accuracy and repeatability of the 160 Ám small field photometric observations, the AOT needed to be modified.á This led to the development of an ''enhanced'' 160 Ám small field photometric mode by the Instrument Team and SSC staff, to take full advantage of the 160 Ám array capabilities.

 

The 160 Ám "enhanced" photometric mode relies on the same principles as the small field photometry, but provides a larger field of view and a more uniform coverage at a given single nodding position.á This is accomplished by increasing the number of DCEs, modifying the stim cycle, and optimizing the scan mirror dither pattern. This improved sensitivity by 15% over the original 160 um small field AOT. This mode represented a significant improvement for point source photometry at 160 Ám and should have been the preferred mode for observers in the last year of Spitzer cryogenic operations. Figure 3.11 and Figure 3.12 show a sample of observations obtained using the 160 Ám ''enhanced'' mode of two red sources, the asteroid Harmonia and the extragalactic source 3C371, both part of the MIPS 160 Ám calibration plan.á

 

The 160 Ám "enhanced" photometric mode takes 30 DCEs per cycle, in comparison with the 20 DCEs obtained by the small field photometry mode. Table 3.6 includes some of the integration times and overheads for one and eight cycles.

 

harmonia_160um_enhanced

Figure 3.11: Asteroid Harmonia observed using the 160 Ám enhanced mode during MC42 (July 2007). This asteroid is bright (approximately 3 Jy) and samples the MIPS 160 Ám upper limit point source calibration (see áStansberry et al. 2007). The enhanced mode FOV clearly encloses the first Airy ring (the green circle is 1.5┤), allowing for a good PSF fitting and/or a better aperture photometry with background subtraction.

 

Enhanced_160um_3c371

Figure 3.12: A comparison of photometric observations of 3C371 at 160 Ám using the enhanced mode (left) and the ''default'' small-field mode (right), obtained during MC41 (2007 June).á The bottom panels are the corresponding coverage maps.á The observations utilized 4 cycles and are displayed to the same greyscale level.á The green circle corresponds to 1.5┤ and encloses the 160 Ám first Airy ring (see Figure 3.11). The mean coverage is about the same (10% higher for the small-field mode); however, to cover a similar FOV the small field would need twice as much time as the enhanced mode, and the coverage would not be as uniform.