Spitzer Documentation & Tools
MIPS Instrument Handbook

3.4  Overheads and Best Observing Practices

3.4.1        Sources of Observing Time Overhead

We provide the following information on observation time overhead to provide the interested data user with an overview of the factors that contribute to the observing. It should also provide a rough guide as to what percentage of the time during an observation a user should expect to have actually been taking data on the chosen field.  Note that the observing overheads that appear here are only approximate.

 

Observing time overheads are incurred as a result of both spacecraft and MIPS operations constraints.  Spacecraft operations that result in overheads are primarily related to the time required for offsetting, slewing, and settling/pointing acquisition.  As a result, programs that minimize the number of pointings will be more efficient than those with many pointings.  Some programs will be able to get around this limitation by using scan map mode, which is designed to survey large areas without incurring excessive overheads associated with slew and settle.  However, scan map mode has integration-time limitations, and may not be suitable for particularly faint sources.  In photometry/super resolution mode, telescope offsets are required in order to switch between the three MIPS bands; each such offset and settle requires roughly 40 seconds to accomplish.  In addition to slew and settle times specifically required within a particular program, some amount of slew time must be included for switching between observing programs.  A telescope slew overhead of 215 seconds is also added to each AOR (an additional 300 sec overhead is charged for moving targets).

 

There are two primary types of observing time overhead resulting from MIPS operations.  The first of these has to do with the operation of the on-board calibration sources, the stimulators (or 'stims').  The stims are employed very frequently (several times per observation cycle) for both the 70 µm and 160 µm arrays in order to track the detector response carefully (see, e.g., sections 2.3.2, 3.1.3, 3.1.5, 3.2.3, and 4.1.1).  This is true for all of the MIPS observation modes.  Observers have no direct control over the frequency of stim flashes.  Observations made in the 24 µm band are not currently subject to a stim flash overhead.  The second type of overhead associated with MIPS operations involves dither motions.  In photometry mode, all observations include dithers to place the source of interest (or the desired pointing position) at a number of places on the array.  Most of the dither motion is accomplished through the use of the scan mirror mechanism, which moves virtually instantaneously relative to MIPS integration and readout times.  However, the dithers also involve at least one nod of the telescope in the direction perpendicular to the scan mirror motion direction, and these nods carry an overhead.

 

The photometry mode dithers cannot be circumvented by observers.  These required dither patterns are designed to improve the accuracy of MIPS photometry by placing sources at several positions on the detectors (both 70 and 160 µm arrays), and to provide a filled 2-D image for the 160 µm array.  While the dithering does not result in an observing overhead per se, it does mean that the minimum integration time on a source is a function of the product of the observer-selected exposure time and the number of dither positions.  For bright sources, these minimum integration times might exceed the integration time required by a significant amount.