FAQs: MIPS

See also MIPS web pages.

Q:   Now that only half of the 70 micron array is working, how can I cover a field of about 5'x 5' using photometry mode?

A:   There is a paragraph in the SOM on this very topic. Now, the 70 micron large field fine pixel scale AOT by default covers a region about 2.5' x 1.25'. To cover a larger region, you need to use cluster mode targets to cover the area -- keep visualizing the observations to be sure you're covering what you think you're covering. (In cluster mode targets MIPS is doing the entire photometry observing sequence at each of the points that you request.) You can also use the 'raster map' option -- see the examples in the SOM.

Q:   More generically, how can I cover a field of about 5'x 5' using photometry mode?

A:   See the above question and answer as well. For all of the bands, you can do this via cluster-mode targets; for 24 and 160 (and eventually 70), you can do this via raster mapping. There is a raster map example and discussion in the current Spitzer Observer's Manual (SOM) that might help you. Raster maps are also mentioned (and used) in the two examples in the SOM.

As with cluster mode targets, in raster maps, MIPS is doing the entire photometry observing sequence at each of the points that you request.

Now, if you create a raster map, you can (and should) visualize the whole sequence in Spot to be sure that it's doing what you think it should be doing, but be aware that the depth of coverage maps (see the Spot User's Guide) are not sophisticated enough to understand the specific placements of the map grid - they work for plain photometry, but for raster maps, they can display confusing results. You will have to imagine the depth of coverage for one cycle replicated at each map position, and summed up in the places they overlap.

Keep visualizing the observations to be sure you're covering what you think you're covering.

Q:   I want to create a cluster target, but I don't see this as an option in the MIPS/IRS/IRAC AOT window.

A:   Cluster targets are a TARGET definition, not an AOT definition. First, create the target in Spot, then create the AOT for that target.

Be careful when using map center offset or cluster array coordinate offsets for an asymmetric pattern about the map or cluster center, because it may yield differing results in different visibility windows. (see how this carefully-planned map changes 6 months later -- the small map on the far right has a separate target, but the others are controlled by map center offset. The AORs are exactly the same in each figure.

Q:   Can I control the rate or placement of stimulator flashes in the MIPS AOT?

A:   No; the rate and placement of stim flashes in the MIPS AOT observational sequence is controlled by the SSC, and is rarely changed.

Q:   How come I can't make more than 20 cycles in a single photometry AOR?

A:   The maximum cycles for MIPS photometry is limited to 20, even though it's not close to the 3hr limit for an AOR. It's not totally arbitrary...it has to do with the logic of placing stim cycles in the photometry observing sequence -- it just gets more complicated when there are more than 20 cycles. Creating two (or more) AORs that will sum up to the total integration time you want is not the end of the world -- chances are excellent that they will get scheduled together even without constraints. But, if you want to force the issue, you'll need to constrain the AORs and justify it in your proposal.

Q:   Can I propose to use SED mode in GO-3? What about TP?

A:   Yes, you may propose to use either (or both). However, note that TP observations will not be scheduled until the mode is officially commissioned. Spot will currently allow you to select observing parameters and get accurate observing time estimates for TP, just as for any other mode.

Q:   I found SED point source sensitivities, but what are the extended source sensitivities for SED mode?

A:   At this time, the SED flux calibration is being done using point sources, while wavelength calibration relies on emission line objects, usually extended. An analysis of point source and extended source sensitivities is being carried out, but presently we don't have a final report. To first order, however, we don't expect enormous difference in their sensitivities. At this point, make the best estimate you can as to the integration time you will need. Be conservative; any changes to extended source SED sensitivities are not expected to be more than a factor of 2, so plan your integration times accordingly. If your proposal is accepted, in general, only minor modifications (see Observing Rules) will be permitted. If the extended source SED sensitivities change by MORE than a factor of two, you will be notified and allowed to change your AORs accordingly.

Q:   Why can't I derive the 24 um saturation values in table 8.13 (from the SOM) by sticking either 0 or large background levels into table 8.14?

A:   Trying to compare Tables 8.13 and 8.14 is like comparing apples and oranges. The values for truly (ideal) cases are in Table 8.13. The formulae in Table 8.14 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, Table 8.14 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 Table 8.13 are meant to be single-point values, plain vanilla, with nothing hidden. The values in Table 8.14 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.

Q:   How do I best get started with my MIPS data analysis, and how do I make sense of it all?

A:   You can get a quick understanding of the various features in your MIPS data by consulting the MIPS Data Features/Caveats page. If you're analyzing MIPS-Ge (70 and 160 micron) data and want to maximize your science return, we also encourage you to check out the GeRT, which allows you to carry out custom offline data reduction. (However, the GeRT should be used carefully; if used improperly, you can corrupt the quality and calibration of your data.) Ultimately, you should consult the MIPS Data Handbook for further details on MIPS data and analysis.

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