5.4.2 Frame Times

Having set our field of view, and pixel scale, all that remains is to set the total integration time. The observer sets a frame (exposure) time, and number of repeats.

• 24 micron array: there are three selectable exposure times: 3, 10 and 30 seconds. For this example, since we are searching for intrinsically faint objects, we take the longest frame time, 30 seconds, and build up our desired total integration time as follows:

  Accounting for the 10 pointings per exposure with the 24 micron array and large field size, plus some housekeeping factors which introduce two extra exposures per AOR, the total integration time is

  $$t = [ (N \times 10) + 2 ] \times ({\rm Exposure \; Time}) - 2 \;{\rm sec}$$ (5.2)

  
  
  where $N$ is the number of cycles (i.e., the number of times the exposure sequence is executed). This is the exposure time on the science field. However, since we selected our sky field to be only very slightly shifted $(10^{\prime\prime})$, these images effectively double the depth.

  Hence to reach a 1-sigma sensitivity of better than $20 \; \mu{\rm Jy}$, which requires 500+ seconds integration, we use 1 cycle of the 30 second frame time (yielding a total on-source integration of 720 seconds). However, just to make things confusing, the timing in MIPS has been adjusted to be synchronous with the computer oscillator, and other potential sources of synchronous noise. Hence a MIPS second is 5% longer than a conventional one. Hence the total time on target in conventional seconds will be slightly larger than the estimate here. The time estimates returned by Spot are in real seconds.

• 70 micron array: The goal is to obtain 500s total integration. One cycle yields 10 exposures on the science field. Hence selecting the 10 second frame time, we require $\simeq 5$ cycles to reach the required depth.

• 160 micron array: one cycle of the large field, 160 micron array AOT yields 10 exposures, with depth unity, covering a $4^\prime \times 5^{\prime}$ area on the science field. Hence the reach a depth of 20 seconds, with the 10 second frame time, requires two cycles. However, the SSC strongly recommends at least four cycles redundancy with this AOT, for artifact removal, etc., and hence we will set the number of cycles to be four. See SOM, v7.0, § 8.2.1.2.6.