There are three types of MIPS-24 latents: (a) ~0.8% bright latents that last from seconds to 10s of seconds, decay exponentially, look like PSFs, are easily recognizable by eye, and are flagged (but not corrected) by the pipeline; (b) ~2% dark splotchy effects from very bright objects (>50 Jy) that last for hours; and (c) 0.5% bright latents from extremely bright objects that can last for days. (Saturated sources will also produce cosmetically ugly ''jailbars'' (see above), but these are easily recognizable.)
Short-lived bright latents
These are the easiest to recognize by eye or automatically because they always occur in the in-scan direction, and the pipeline does in fact flag them in the mask file (though the pipeline does not remove them). Scan maps leave a trail of latents behind a bright source with spacing which reflects the imaging cadence, Photometry will typically leave latents about 1 arcminute ''above and below'' (i.e. +/- Y) the central source. Examples of this effect can be found in Figure 7.8, as well as Figure 7.1, Figure 7.7, and Figure 7.9.
Bright latents are typically not removed by outlier rejection in scan mode because the scan mirror produces repeated overlapping patterns. This is why you may see a trail of latents (often on both sides of a bright source) along the scan direction, particularly for fast scans.
Figure 7.8: Example of short-lived bright latents - note trail of bright dots above the bright source.
Examples of dark latents from bright sources can be found in Figure 7.2, Figure 7.9, Figure 7.10, and Figure 7.12. This reduction in response of 1-2% comes from very bright sources, those >50 Jy. Low-level dark latents have been seen for sources as low as 18 Jy. They can last a long time, with a timescale of about 10 hours. If your observations have these features, it may be a result of a bright object seen in observations prior to yours. The impact of this is usually reduced by redundancy (e.g., additional dithered frames). If you encounter these kinds of latents in scan mode, because the latents last longer than the maximum length of an AOR, they are stable and can be removed using your own data, called self-calibration (see below and section 8.1.2). If you have these kind of latents in data taken in photometry mode, the pattern of latents may be different (because the dithers are different), and you might not have enough data to self-correct it. (However, see section 8.1.2.)
The effects of dark latents AND any residual but static imperfections in the flat fielding can be removed quite effectively by division of each BCD affected by dark latents by a normalized median of all of the BCDs affected by the dark latents within the AOR (excluding very bright pixels); see section 8.1.2. This effectively gives an improved flat field and is generally recommended in regions of low background where subtle (1-2%) effects may be important to remove. After correcting the BCDs, you then have to make a new mosaic (using the SSC's MOPEX or other software).
The dark latents are removed by thermal anneals, which are a rare event for the 24 micron array, and are generally only done at the start of a campaign. Currently, we try to limit the impact of these latents by manually scheduling already-known bright sources (known to be bright from 25 micron IRAS data) at the ends of campaigns.
Figure 7.9: Initial BCD affected by bright sources; note ''jailbars,'' bright (point-like) latents, and dark (splotchy) latents.
Figure 7.10: BCD after additive jailbar correction.
Figure 7.11: BCD after ''self-calibration'' to correct dark latents.
Figure 7.12: Example of dark latents all by themselves. Even trails from slewing the telescope over the bright object can also clearly be seen.
Long-lived bright latents
Finally, examples of long-lived bright latents can be found in Figure 7.13 and Figure 7.14, as well as Figure 8.1. These bright 0.5% latents remain until the next anneal or the end of the campaign, whichever comes first.
Just like for long-lived dark latents, the impact of these latents is usually reduced by redundancy (e.g., additional dithered frames, inherent to MIPS observing modes). If you encounter these kinds of latents in scan mode, because the latents last longer than the maximum length of an AOR, they are stable and can be removed using your own data, called self-calibration (see below, and section 8.1.2). If you have these kind of latents in data taken in photometry mode, the pattern of latents may be different (because the dithers are different), and you might not have enough data to self-correct it. Just as above, you can remove these effects by dividing each of the affected BCDs by a normalized median of all of the affected BCDs (excluding very bright pixels). This effectively gives an improved flat field, and is generally recommended in regions of low background where subtle (1-2%) effects may be important to remove. After correcting the BCDs, you then have to make a new mosaic (using MOPEX or other software).
Figure 7.13: Example of long-lived bright latents with newer dark latents.
Figure 7.14: BCDs from AORs later in the campaign where the data in Figure 7.13 were obtained. Note that the dark latents seen above have turned into bright latents here. The figure on the right has additional bright latents obtained in still later observations.