Spitzer Documentation & Tools
MIPS Instrument Handbook

Chapter 4              Calibration

For all information on calibration of MIPS data one should refer to the published MIPS calibration papers: 24 microns (Engelbracht et al. 2007, PASP, 119, 994), 70 microns (Gordon et al. 2007, 119, 1019), 160 microns (Stansberry et al. 2007, PASP, 119, 1038), and MIPS SED (Lu et al. 2008, PASP, 120, 328).

4.1  Imaging

4.1.1        Methodology

MIPS 24 and 70 micron arrays are calibrated using primary (routine) stars and secondary calibrator stars. See the individual calibration papers for specific methodology for each camera. Because of the near infrared filter leak at 160 µm, asteroids (primary calibrators) and red extragalactic sources (secondary calibrators) are used instead of stars (see Stansberry et al. 2007).

 

The routine stars are used to monitor stability of the array in the absolute calibration. For routine stars, we use HD159330 (which is a K star) at 24 microns, and HD163588 and HD180711 star for 70 and 160 microns (for repeatability).  These stars are observed at least once every 3 days.

 

The secondary stars have been chosen from our MIPS calibrator list of about 150 stars.  Star on this list are bright enough (S/N > 100) and well within the linear range of the detector, i.e., less than 1/4 of the saturation limit.  Most of the calibrators are located in regions of low cirrus background, but there are a few stars with a high background in order to test photometry in high background conditions.

 

We present a large representative sample composed of all calibration observations through the 42nd MIPS campaign (~2/3 of the total cryogenic mission) in Table 4.1. Note that the 160 micron stars are affected by the spectral leak, and thus used for stability and templates, not flux calibration. A complete list of MIPS calibrators can also be found with the Calibration and Data Analysis Files on this website.

 

To retrieve actual calibration observations, if you don't know specific names of targets, then you need a program id (pid) to retrieve the entire MIPS calibration program for a given campaign.  MIPS calibrations are assigned a PID = 1700+MIPS campaign number.  For example, MC 7 corresponds to 1710 and MC 18 to 1721, etc.  These campaigns should include calibration for everything (all MIPS modes), so you have to investigate the specific AORs to find ones that match your needs.  MIPS calibrations are done every campaign, so you should be able to start with downloading just one.  If the data are not accessible from a given observation, it is most likely because there is an approved science program that is also observing that target in that mode. 

Table 4.1: A Representative Sample of Calibrators Used.

Mode Total Unique Flux range (Jy) Types (A/G/K/M/F/B)
24 micron 372 103 0.086-4.0 34/25/39/0/2/3
70 micron default 384 93 0.02-19 9/8/54/16/4/2
70 micron fine scale 50 17 0.3-19 1/1/8/6/1/0
SED 130 27 0.3-19 1/0/16/9/1
160 micron stars 260 40 0.005-0.7 5/1/22/7/4/1
160 micron asteroids 177 50 0.07-3 N/A


As described in section 2.3.2, the internal calibration sources, or stimulators, play an all-important role in calibrating the MIPS data.  The stimulators are used as relative calibrators, tracking drifts in the responsivity of the detectors.  The brightness of the stimulators is tied to periodic observations of well-calibrated celestial standards.  At a minimum, the celestial standards are observed at the start and end of each MIPS campaign using the standard Photometry AOT (see section 3.1).

 

The fundamental MIPS flux calibration is against normal stars. Except for the occasional dedicated calibration AOR of celestial standards, an individual science AOR does not depend on any other science AORs for obtaining calibration.  Each MIPS AOR is internally calibrated.  Because of the use of the stimulators as relative calibration sources, this method is robust and stable over an instrument campaign.  This relative calibration method also allows the varying instrument response to be frequently referred to the signals from the celestial standards.  For the Ge:Ga arrays, the stimulators are flashed approximately every 2 minutes or less as an integral part of the basic observation sequences.  Because of the inherent longer term stability of the Si:As array, that detector uses much less frequent stimulator flashes.  The frequency of stimulator flashes is determined by the SSC and implemented in the AOT design, and cannot be selected by the observer.

Calibration with Stimulators

The built-in stimulators (calibration light sources) are the heart of maintaining the germanium detector calibration throughout ground test and on orbit.  MIPS has five sets of dually redundant stimulators: 1) flood stimulator for the 70 µm array; 2) flat field stimulator for the 70 µm array; 3) flood stimulator for the 24 µm array; 4) flat field stimulator for the 24 µm array; and 5) flood/flat stimulator for the 160 µm array.  The flood stimulators are used exhaustively for calibration purposes.  Flat field stimulators offer extra redundancy for the flood stimulators, and are designed to provide a method for monitoring the flat field of the arrays, although operational flat fielding will be based on sky flats.  Because the calibration of the germanium detectors is dynamic, the stimulator activity is integrated with the AOTs and repeated flashes are interleaved with the data.  The 24 µm stimulators were used sparingly, because that array to maintained its calibration well.  

Stimulator Operating Principle

The stimulators operate on the reverse-bolometer principle.  That is, they have an emitter that is suspended with a small thermal conductance to the cold sink, and an electrical heater that can heat the emitter above the cold sink temperature. 

 

Stimulator Usage in MIPS

The flat field stimulator for the 70 µm array is at the focus of a projector that re-images it onto the scan mirror, which is at a pupil in the optical system.  The output can be directed through the wide-field 70 µm imaging optical train or through the SED optical train by suitably positioning the scan mirror.  The projector is required so flat fields can be obtained for imaging and spectroscopic modes independently (i.e., without light going through both optical trains). 

 

Geometric constraints prevent light from the flat field stimulator from being directed into the high-resolution 70 µm imaging train.  The 24 µm flat field stimulator is located in an integrating cavity and its output is conveyed through a machined aluminum light pipe to an exit pinhole at a pupil near the array.  The 160 µm stimulator is also in an integrating cavity with a small exit hole in a mirror at a pupil, placed right in front of the concentrators that convey the light to the detectors.  The flat fields obtained using the flat field stimulators within MIPS are periodically calibrated against observations of a uniform region of the sky.  The flood stimulators at 24 and 70 µm are close to the detectors and broadcast light in a less controlled manner than the flat field stimulators.  They are used primarily for relative calibration, while the 160 µm stimulator is used for both calibration and flat fielding.

 

For observations taken with the 70 and 160 µm arrays, frequent (~2 minute interval) brief (~2 second) stimulator flashes automatically occur during data taking, both during observations of reference flux standards and science observations.  The resulting signal from these stimulator flashes provides a continuous measure of the responsivity of each pixel in these arrays.  Observers could not control the rate or placement of the stim flashes.  Pipeline processing of the data uses the stimulator flashes to remove long-term changes in responsivity, and provides a relative calibration source for referencing all science observations to flux standards. 

Dark Current

As part of instrument calibration, dark current must be periodically measured for each array.  This is done at the beginning and end of each instrument campaign, similar to observations of the flux calibration stars.  The scan mirror has positions which do not allow light to fall on the arrays, and these positions are used, in part, for dark current measurements.  The dark current is measured immediately after an anneal, before the detector responsivity has had time to change due to cosmic ray hits.

 

Detector dark current measurements are made for each array individually by positioning the scan mirror such that the array views only the interior of the cold instrument.  The optical layout of MIPS is such that only one array at a time can be completely hidden from light entering through the telescope.  In fact, only about two thirds of the 70 µm array can be placed in the dark at once, so for that array the dark current measurement is made in two pieces.  Dark current measurements are expected to only be required at the beginning and end of each MIPS campaign.  To avoid any scattered light from nearby very bright sources affecting the measurement, the telescope is pointed toward a region known to be free of such sources during the dark current measurement sequence.

Summary

The MIPS calibration consists of measurements of a number of candidate calibration stars.  From these data, we have made a selection of fundamental calibrator stars.  At the beginning and end of a MIPS observing campaign, we observe one or more fundamental calibrator stars, establishing the absolute flux calibration for the internal stimulators.  Frequent observations of the brightness of the stimulators are used as relative calibrators, allowing the instrument response to be referred to the signals from the celestial standards while only observing those standards occasionally.