Spitzer Documentation & Tools
MIPS Instrument Handbook

3.4.3        Best Observing Practices

Below are summaries of the recommended Best Observing Practices at the time of the data acquisition. This does not imply that data taken in another way are inferior in any way. In many cases these general rules either did not apply to a particular observation or made no difference one way or another. Often the practices merely ensured time efficient data acquisition, which would be of little relevance to an archive user. However, if Best Observing Practices were not followed it is possible the data might not be as sensitive or well calibrated as other comparable data, so we recommend the user try to understand why (or why it does not matter) the Best Observing Practices were ignored. Also be sure to check Data Features and Artifacts for other possible issues the data may contain.

Best Observing Practices: General MIPS

For far infrared Ge detectors, observers should make sure they take enough background (source-free) data.  For large sources we recommend using the chopping large-field photometry mode or scan mapping.  At 160 µm we recommend using the new 160 µm enhanced photometry mode for small sources.  At least four images of a 160 µm target are recommended to ensure reliable photometry. 

 

Images of objects near the ecliptic plane are likely to include asteroids and/or Kuiper Belt objects; a second observation at a later time is necessary to ensure that moving targets have been removed. 

 

Observations at MIPS wavelengths may be limited by noise (including confusion noise; see section 2.5.1) but observers should ensure sufficient data redundancy for cosmic ray rejection, even if the confusion limit is reached with shorter exposures.

 

It is best to build up long integrations with long exposure times, e.g. large total integration times built from 3 s exposures will not produce as high-quality a final product as fewer cycles of 10 s exposures.

Best Observing Practices: MIPS Photometry

In addition to the items mentioned immediately above for MIPS in general, observers should also consider the following items relevant specifically to MIPS photometry. 

 

Super-resolution data needs to be post-processed to take advantage of the sub-pixel sampling inherent in the AOT; the Spitzer pipelines do not provide this service. 

 

In photometry observations, the dither pattern is such that the region of highest-quality S/N and best coverage is less than a full FOV; be sure to visualize the exposures to determine the region of sky actually covered by all pointings for any given AOR.

 

Because of the smaller effective size of the 70 µm array compared to original expectations, to obtain the same coverage as pre-launch expectations for large-field photometry, some users needed to efficiently map the approximately 5“x5“ (2.5“x2.5“ fine scale) area covered by the original AOT.  In order to do this, we suggested that the user create a cluster target with offsets of (0, +80““), (0, -80““) [for fine scale, (0, +60““), (0, -60““)] in array coordinates observing ''offsets only.''  This allowed recovery of the full area of the original AOT while saving the slew tax from mapping with 2 AORs.

 

Best Observing Practices: Scan

In addition to the items mentioned immediately above and for MIPS in general, observers should also consider the following items relevant specifically to MIPS scans. 

 

Step by at most 1/2 array steps to obtain full sky coverage at all 3 wavelengths.  For the best data at 70 µm we recommend cross-scan steps of  <= 148““.  For highest S/N coverage, step by at most ¼ array steps.  The AOT provides inherent redundancy in all wavelengths, but there are more images of a given source at 24 and 70 µm than at 160 µm.  Fast scans provide only half coverage at 160 µm, so the scans must be repeated to approximately fill in the gaps.  To achieve the recommended minimum 4 images of the same patch of sky, observers should plan additional scans with small cross-scan steps.

 

Bright objects may leave latents in the data; to limit these effects, an observer should arrange to cover the same region of the sky in two directions (e.g., forward and back).  A user may want to isolate extremely bright objects in a separate AOR.

 

Note that the first several frames of a scan leg necessarily have extrapolated stim backgrounds (see section 7.2.3 for more information on this effect).  In general, but especially in the case of fast scans over bright objects, an observer should be sure to have enough background measurements around your source so that extrapolated stims are not an issue.

 

Best Observing Practices: SED

In addition to the items mentioned above for MIPS in general, observers using MIPS SED mode need to consider their exposure times and likelihood of saturation.  Since the saturation limits are so high for the 70 µm array, it's rare to saturate SED mode.  Thus, except in situations where extremely bright objects are being observed, one should always use the 10 second exposure time.