Spitzer Documentation & Tools
MIPS Instrument Handbook

3.2.4        Fast Scan Mode

Instrument operations at the fast scan rate (nominally about 17''/sec) are slightly different from those at the lower rates.  Because the efficiency would be undesirably low at this scan rate if full sampling were obtained at 160 µm, that option is not supported and full coverage at this wavelength requires a second scan.  The scan mirror flyback is set to four 160 µm pixel widths (64''), with no alternation of flyback amplitudes.  As a result, there is alternating one-pixel-wide gaps in 160 µm maps acquired in fast scan mode.  Given that the pixels are 0.4  wide and separated by the same amount, any reasonably bright sources will be ''sighted'' at 160 µm even in the partial coverage obtained in a single pass.  A general requirement for reliable scan maps is that the same area be scanned a second time (at a later date/time) to screen out asteroids and other transient phenomena.  If a rescan is carried out very soon after the first, it can be approximately interleaved such that both scans together provide a nearly full 160 µm map. 

 

Note that the SSC strongly encouraged all observers to obtain at least 4 images of a 160 µm target if the 160 µm data was important, e.g., a forward and back scan leg over the same area with 2 map cycles, and with a 5-pixel offset in the cross-scan direction for fast scan (5 px compensates for the inoperable readout; see section 3.1.

 

The total integration time per point on the sky per pixel in a single scan leg is provided in Table 3.5; overscan times are discussed further below in section 3.4.1.

Table 3.5: Total single-pass MIPS scan map integration time per pixel and overscan time.

Scan Rate 24 µm (sec) Total / [time per frame] 70 µm (sec) Total / [time per frame] 160 µm (sec) Overscan time (min)
Slow      (2.6''/sec) 100 / [10] 100 / [10] 10 1.2 min
Medium (6.5''/sec) 40 / [4] 40 / [4] 4 3.2 min
Fast       (17''/sec) 15 / [3] 15 / [3] 3 * 7.9 min

* Only 1/2 of map region is covered by the 160 µm pixels at the fast scan rate in one scan leg.

Note: Each source appears in 10 consecutive frames for slow and medium scans at 24 and 70 µm, each source appears in 5 consecutive frames for fast scans at 24 and 70 µm, and each source appears in one frame at slow and medium scans at 160 µm.  See Figure 3.17 and Figure 3.18.