Flux Standard Screening and Linearity Check

Objective

The purpose of this task is to begin screening the MIPS flux standards to identify those that are unsuitable for calibration due to dust disks, background sources, binarity, etc., and to improve the flux linearity measurement on the three arrays.

Description

This task is designed to perform photometry observations of a large number of stars to begin screening the list of potential MIPS flux calibrators for suitable standards. The second goal is to measure the flux linearity of the data that has passed through the pipeline.

The data will be taken using the same observation design used for our regular standard-star observations, i.e., 2 cycles of small-field photometry at 24µm, 3 cycles of small-field photometry at 70µm, and 2 cycles of superresolution observations at 160µm. This will produce 28, 30, and 12 independent images of each star at 24, 70, and 160µm, respectively. The nominal 1-sigma sensitivity achieved when these images are combined is 0.1, 0.6, and 3 mJy at 24, 70, and 160µm, respectively. The DCE time used will be 3s unless the stars are faint enough to be below 1/4 the nominal 10s saturation level, in which case 10s DCEs will be used. Those thresholds are 127, 120, and 333 mJy at 24, 70, and 160µm, respectively. The nominal 1-sigma sensitivities achieved in the 10s observations are 0.05, 0.4, and 2 mJy at 24, 70, and 160µm, respectively.

Scheduling Notes

• The observations are deliberately targeted as separate AORs, even when the same target will be observed in multiple bands. The task should be scheduled such that the 160µm observations are obtained first, followed by 70µm, followed by 24µm. Since our ground testing indicates that the 160µm array requires more frequent anneals than the 70µm array, a single anneal done before execution of this task should be sufficient for the full run of the task.
• An attempt has been made to optimze the AOR order by minimizing the total distance the spacecraft must slew. Executing the AORs in the order listed in the AOR files is recommended, unless a better optimization is found.
• These notes are duplicated in comments in the AOR files.

Data Collected

As described above, this task will collect 12-30 images of each star in each band. In total, this is a lot of data - over 6000 DCEs each time the task is run. The data will all be collected in photometry mode and the reduction will be identical for each star.

As currently defined, the task requires 6.4 hours for the northern-target version to be run in campaign W. There is a smaller number of stars in the south that have not already been observed in other IOC tasks, so we could contemplate a smaller version of this task to be run in another campaign to get the southern stars, should the time be available.

Data Reformatting Requirements

Array Data Desired:

All Arrays

Data Reformatting Option:

• NORMAL
  1 FITS file per AOR per array.

Special Instructions:

Task Dependencies

• mips-920: 24µm flux standards - we must be able to perform the routine calibrator observations
• mips-922: 70µm flux standards - we must be able to perform the routine calibrator observations
• mips-924: 160µm flux standards - we must be able to perform the routine calibrator observations

Calibration Dependencies

• darks (all arrays)
• flats or illumination corrections (all arrays)
• linearity correction (Si:As)
• electronic linearity correction (Ge:Ga)

Output and Deliverable Products

• reduced data
• report on flux linearity
• report on suitability of individual flux standards

Data Analysis

The data will be reduced as for the other calibration standards, by running them through the DAT and extracting fluxes with IDP3 (see the routine calibration task cookbooks, MIPS-920, 922, and 924, for more detail). The measured DN/s should then be plotted against the predicted flux (in Jy, for example). Ideally, all the stars would fall on a straight line, indicating they were good standards and that the array was perfectly linear. The expected outcome is that some of the stars will not fall on the curve defined by most of the stars, due to circumstellar dust, binarity, background sources, etc. Those stars should be flagged as poor calibrators and ejected from our calibration list. In addition, the Ge:Ga array data are not expected to fall on a straight line, but rather along a curve that defines the flux nonlinearity not corrected by the pipeline. This curve will be used in conjunction with the data obtained in mips-600 to define a linearity correction for the Ge:Ga arrays.

Software Requirements

• MIPS DAT
• IDP3

Actions Following Analysis

• make report available to IT/IST

Failure Modes and Responses

If this task fails outright to collect any data, it should obviously be repeated. If it collects data, but there are not enough suitable stars to define the linearity curve, the task should be repeated with new stars.

Additional Notes

This task should be repeated peroidically throughout the first 6 months of the mission, using different stars as they become available. The ultimate goal is to measure all our potential calibration standards and screen out the ones that are not good standards.