MIPS : Saturation

Nonetheless, in some circumstances, saturation cannot be avoided. The MIPS electronics provide a short-exposure look at each source along with the requested long exposure, so that information can be recovered that would otherwise be lost due to saturated signals. These short exposures provide a measure of source brightness within the first second after the array reset that begins a DCE. Measured fluxes for any source that causes saturation in <1 sec will therefore be compromised. Saturation by extended sources is a more severe problem. If many pixels attached to a single readout saturate in a given DCE, the response of the amplifier can be seriously impacted. As a result, it is important to avoid saturation on extended sources or background emission at any point within a DCE. The 1 sec saturation limits for point sources, and the 10 sec limits for extended sources, are given below. These limits include the effects of pixel-to-pixel responsivity variations for all three arrays, and expected responsivity changes between thermal anneals for the 70 and 160 micron arrays. Observations of targets or regions that approach these limits must be carefully considered and planned.

(table copied from SOM)

The general case for observations involves a combination of extended and point-like emission. The implications of saturation can be computed by assuming a combination of the effects in the first table above, as shown in the second table above. For example, if the estimated background at 24 microns is 100 MJy/ster, it uses up 40% of the dynamic range in a 20 second integration. Without the background, a source of 0.5 Jy would not saturate in a 10 second integration. However, only 60% of the dynamic range is left on top of the background, so the brightest measurable source in a 10 second integration is 0.4 Jy (from the 10-second formula above).

During the Cycle-1 proposal process, we received many questions regarding these two tables, specifically from observers attempting to derive the first table from the second. This is not possible! The values for truly (ideal) cases are in the first table. The formulae in the second table actually describe the general case, which includes a combination of point and background flux, so they break down in the limiting cases. Further complicating matters, the second table also has incorporated into it subframes that are part of the observing sequence AND the impact of some substitution of values for point sources - there is a half-second data frame taken at the beginning of the observing sequence, and we can use the first difference to substitute in values for point sources that are saturated later in the exposure. This doesn't work for extended sources, so far as we know from our testing to date. The values in the first table are meant to be single-point values, with nothing hidden. The values in the second table are a better approximation to reality: a mixture of point and background sources, subframes that are part of the observing sequence, and the use of some of these subframes to substitute saturated pixels.

For extended source 24 micron saturation limits, the 10-second limit from the first table (260 MJy/sr) can be scaled by a factor of 9.5 /(exposure time - 0.5).

At 70 microns, some targets slightly brighter than these limits can be usefully observed. However, the consequences for the immediately following reads will be significant, such as latent images and a degradation of linearity. Such observations will need to be manually scheduled near a thermal anneal, and this should be described and justified in the observing proposal. Note that, since our calibrators are not this bright, observations of objects this bright may not be as well-calibrated as fainter objects.

At 160 microns, the saturation limit for a 3 second integration is about 1 Jy. For sources brighter than this level, up to about 4 Jy, useful data will be obtained on the first few reads, but the brightest pixels will saturate before the end of the integration. As a result, there will be some degradation of the results in the readouts immediately following, but the recovery will be relatively fast.

Note that there is no equivalent to the second table above for 70 and 160 microns; the equivalent effects (ramp fitting to just the points before saturation) are already included in the values given in the first table above.

The sky background should be considered carefully when planning MIPS observations in all bands and modes. For example, a significant fraction of the extended emission in the Galaxy can saturate the 160 micron detectors in 10 seconds or less.

• A 160 micron saturation map is here.

Soft Saturation with MIPS 24 Micron:

A handful of programs had severely compromised signal/noise because the observers chose long exposure times (30 sec) in regions where the background due to zodiacal light was very high. In these cases, the slope image becomes saturated during the exposure and a large fraction of the array has had the "soft" saturated 30 sec slope image replaced with pixels from the unsaturated but much noisier difference image. The difference image represents only about 0.5 sec of exposure time. Thus, the use of the 30 sec exposure time has instead resulted in BCD images dominated by only 0.5 sec of exposure time! A better designed program to detect faint sources superposed on high backgrounds would make use of coadding many shorter exposures (for photometry mode, 3 or 10 sec exposures).

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