Objective

To determine if the 24µm array is responding to illumination through the telescope, and to verify that basic instrument properties (e.g. throughput, image quality, CSMM operation) are as expected.

Description

Place a bright MIPS flux standard in the 24µm FOV. Execute 2 cycles each of the 24µm photometry small-field and large-field AOT using 3s exposures. The large-field AOT should be executed with a sky offset small enough to keep the star on the array during the background observation. The target definition for the large-field observation should include an offset half the size and in the opposite sense of the background field offset. This will center the star in the large-field observations and maximize the chance that the star will be on the array in all exposures. The target should be one of our focus-check standards in the CVZ, e.g., the K3III HD 53501, which is about 1.2 Jy at 24µm. This star was chosen because:

• The star is in the CVZ
• The observation provides a preview for a focus-check star
• The star is bright enough to stand out against any excess noise or background, and is faint enough not to saturate in a 3s exposure if the background is less than 15,000 - 25,000 electons per second
• The star is a MIPS calibration target, and thus has a predictable brightness that will allow us to determine an initial calibration for the instrument

Data Collected

The AOTs described will generate 74 DCEs on each array. The 24µm data are expected to look something like Figure 1, which shows simulated images of a 1 Jy point source against a moderate background that includes zodiacal light, background galaxies, and cirrus. Read noise, dark current, photon noise, and cosmic rays are included in the simulation.

Figure 1: Simulated images of 24µm first-light data.

Data Reformatting Requirements

Array Data Desired:

24 µm

Data Reformatting Option:

• NONE
  1 FITS file per DCE per array.

Task Dependencies

• mips-015: CE State Validation & Functional Tests - MIPS must be functioning correctly

Calibration Dependencies

• Linearity Correction
• Droop Correction
• Dark current image

Output and Deliverable Products

• 74 24µm BCDs
• 24µm mosaic
• 24µm flat field
• report to include:
  • assessment of intial image quality
  • assessment of intial system throughput
  • confirmation of expected PCS operation
  • confirmation of expected CSMM operation (e.g., is the scan mirror motion projected on the sky as we expected?)
  • confirmation of difference frame threshold functionality

Data Analysis

Some checks of the data need to be performed quickly to assess whether we'll need to delay campaign D2 due to the background being too high. The results should be reported to the instrument P.I.

• Compute background level: use IRAF's "imstat" (or some equivalent test) to measure the background level. The median or mode of the array (after rejection of invalid data values) values prior to running the data through the pipeline may be an appropriate statistic.
• Compute peak point-source levels: again, use IRAF's "imstat" (or some equivalent test) to measure the data values at the center of the star. This should be done on the raw data. If the star is saturated, the difference frame should be used.

Some quick checks can be performed to verify that certain properties of the data are as expected:

• Did the instrument go into the correct mode? (It should be photometry: AORTYPE=pht)
• Do we have the correct number of DCEs? (There should be 74: N_DCES=74)
• Are we observing the correct fields-of-view? (FOVID = 99 for DCEs 1-15, 100 for DCEs 16-30, 103 for DCEs 31-52, 104 for DCEs 53-74)
• Is the exposure time correct? (It should nominally be 3 MIPS seconds: EXPTIME=2.62 for all DCEs except 1, 16, 31, 42, 53, and 64, where it should be 1.57)
• Is there light on the array? (I hope so)

These data will be the first sky observations obtained by MIPS that will not be saturated. Thus, there will not yet be a proper flat field to support standard pipeline operation. A usable dark will be obtained from the scattered background measurement made in the same campaign. The data will be processed with the SSC pipeline to subtract the dark and to perform linearity and droop corrections.

A simple flat field will be generated by median-combining the small-field data, as shown in Figure 2. The large-field data should be avoided because the concentration of source positions near the center of the array will bias the flat field. This can be done using IRAF's "imcombine" task or using mips_enhancer. The data can then be processed again, this time using the flat field generated from the data themselves.

Figure 2: Simulated flat-field image.

Software Requirements

• IRAF
• mips_enhancer
• IDP3

Actions Following Analysis

• make updated flat field image available to development pipeline
• make sure any corrections to PCS or CSMM operation are propagated to future AOT tests
• supply mosaic image to focus-check personnel so they can evaluate the suitability of the target for the focus check

Failure Modes and Responses

• If no star is observed:
  • Confirm pointing
  • Ensure that noise levels are not too high to see star

Additional Notes