5.4.1 Field Size and Resolution

MIPS photometry observations are taken as a series of exposures on the prime science target, and at offset ``sky'' fields. The `field size' option in the AOT entry dialog refers to the amplitude of the offsets that will be applied to sample the sky, and also has some consequences on how large the dithers will be for the on-field images.

In general terms, the basic idea in the design of the field size option is that the ``small'' field size is appropriate for imaging sufficiently compact target objects so that taking an exposure offset from the target by $\simeq 1/2$ array is sufficient to determine the sky. For large sources, the ``large'' field size is required to sample the sky, and, for the 24 micron array, the offset can be specified manually. How large is large, and how small is small? A generic rule-of-thumb is that objects smaller than about $2^\prime$ should be imaged with the ``small'' field size; objects with size $2^\prime - 4^\prime$ are observed with the ``large'' field size, and objects larger than about $4^\prime$ should be observed using the raster map option, or the MIPS Scan Map AOT.

This works well for objects that are extended. However, for this example, we are looking essentially for point sources (for the pixel scale of the MIPS arrays anyway). That is to say that the sky will be sampled with the science images themselves, and so both the on-source and sky images are useful for scientific purposes. Our choice for field size will simply determine the overlap of the ``science'' and ``sky'' images.

The IRAC observations in §4 covered a $\sim 5\hbox{$.\!\!^{\prime}$}2 \Box$ FOV uniformly at 3.6, 4.5, 5.8 and 8.0 microns, with offset fields imaged once in either of the 3.6/5.8 or 4.5/8.0 micron arrays, respectively. We will design the MIPS observations to image the same central area in all of the MIPS bands, and use the MIPS offset (sky) fields to image the part of flanking fields viewed in the IRAC observations.

To accomplish this, we will use the ``large'' field size for both MIPS 24 and 160 micron wavebands, and manually configure the offsets to the sky images, where appropriate. Let's describe in detail how the observations will be carried out in each array:

• 24 micron array: The basic $24 \; \mu {\rm m}$ large field photometry observations acquires 10 separate images of a source, dithered around the center of the array. The nominal pattern is a $1 \times 5$ set of images along the scan direction, separated by 1.5 pixels = $3\hbox{$.\!\!^{\prime\prime}$}83$ for each, followed by a cross-scan spacecraft offset of 4.5 pixels $(11\hbox{$.\!\!^{\prime\prime}$}48)$, and a repeat $1 \times 5$ set of images. This 10-point imaging sequence is also executed for the offset sky field. In addition, for calibration purposes, two extra exposures will be obtained at both the object and sky positions per AOR (as opposed to per cycle). The details of this pattern are given in the Spitzer Observer's Manual (in v7.0, see § 8.2.1.2.2):
  http://ssc.spitzer.caltech.edu/documents/som/

  Thus, for the 24 micron band large field observations, entering a small offset for the sky observations, say $10^{\prime\prime}$, will mean that for a single cycle observation, 24 images will be taken centered on the target position. For $N$ cycles, the total number of on-source exposures will be

  $$N_{\rm exp} = 2 \times[ ( N \times 10) + 2]$$ (5.1)

  
  
  where, since the sky and science fields are effectively the same, we effectively get double the depth (and hence the factor of 2 at the front of equation 5.1.

  A visualization of these observations (arbitrary frame time) on a DSS + IRAS 12 micron image (see § 4.8) of the Abell 2218 field is shown in Figure 5.4.

  Figure 5.4: The overlay of the MIPS 24 micron, large field Photometry/Super Resolution observations on the Abell 2218 field. One cycle of the 30 second frame time was observed, with sky offset of 10 arcseconds.

  

• 160 micron array: Imaging the field at 160 microns is straightforward. Selecting the 160 micron array with large field size will yield a complete, uniform coverage map over a $\simeq 4^\prime \times 5^\prime$ field. Note that this large-source AOT cycle provides only one fully sampled image of the field. Observers are strongly encouraged to specify at least four cycles so that the redundancy can be used to reject artifacts in the images.

  The overlay on the Abell 2218 field (4 cycles, arbitrary frame time) is shown in Figure 5.5.

  Figure 5.5: The overlay of the MIPS 160 micron, large field Photometry/Super Resolution observations on the Abell 2218 field.

  

Hint: Keeping track of the number of exposures on-source is somewhat complicated within the MIPS Photometry and Super-Resolution AOT; a good summary table of the frames/cycle and the integration time per pixel per cycle is given in the SOM (in v7.0, see Table 8.11).

• 70 micron array: With the $70 \; \mu {\rm m}$ array, because of the smaller effective size of the array compared to pre-flight expectations, there is a recommended work-around to image a $\simeq 5 \Box^\prime$ FOV: first, we create a Fixed Cluster-offsets target with offsets of $(0^{\prime\prime}, 80^{\prime\prime})$, $(0^{\prime\prime},-80^{\prime\prime})$ in array coordinates observing ``offsets only'' (see Figure 5.6). Then, we use the MIPS Photo/Super-Resolution AOT, select the 70 micron array, default pixel scale, and small field size. See § 8.2.1.2.4 of the SOM, v7.0.

  The result is 10 exposures ``on-source,'' covering the 5 arcminute field around the target. A visualization of this on a DSS image of the Abell 2218 field is shown in Figure 5.7.

  We need to do this manually for each of the targets on our program.

  Figure 5.6: The ``cluster offsets'' target for the 70 micron Photometry/Super Resolution observations on the Abell 2218 field.

  

  Figure 5.7: The overlay of the MIPS 70 micron, small field, default pixel scale for ``cluster offsets'' target and the Photometry/Super Resolution observations on the Abell 2218 field.