MIPS SED-Mode Flux Calibration

SED-mode Illumination Correction

This is an ongoing task. The illumination correction for SED-mode observations is constructed by observing fields near the Galactic plane. It is in these extremely bright regions of the sky that enough signal is obtained during each exposure of the dispersed light to yield sufficient statistics for individual determinations of the illumination correction (IC). Each IC measurement is the result of using the MIPS IOC task MIPS-299. This uses an 1x4 SED-mode map using the maximum 4.5-arcmin allowed telescope displacement in the spatial direction ("along" the slit) and a 2-arcmin "chop" of the CSMM for each of 4 positions observed during the task. One cycle of the standard SED-mode AOR is used with 10-second DCEs for each of the 4 slit positions. Usable IC data have been acquired during MIPS campaigns 11, 13, 15, 16, 20, and 21. The data from each campaign was reduced using the MIPS DAT with the following command sequence:

mips_sloper -q -n file.fits

mips_caler -P mips_caler70S.param -i file.red.fits

mips_enhancer -f file.cal.fits -o IC -r 1 -C -NA

Note that "-r 1" is used in mips_enhancer to ignore the DCE that follows a stim flash. This is done to avoid possible latents in the 70 micron array caused by the bright, undispersed stim flash. Differences in the IC at the level of about 10% are seen between illumination correction frames constructed with "-r 0" and "-r 1". Using "-r 2" results in no significant differences from the IC constructed with the switch set to skip only the first DCE after the stim flash. As a precaution against stim-induced latents, "-r 1" has been adopted for the SED-mode IC.

The resulting ICs from all 6 MIPS campaigns with usable IC observations were medianed to arrive at the current "master" illumination correction frame for SED-mode data. The ICs for all campaigns were scaled to the to median value found in the array region [5:12,10:20] to help account for the differing spectra of the various telescope pointings and then medianed. Any revision to the master SED-mode IC will need to be applied to the both the standards used for flux calibration and to the data to be calibrated. That is, the same IC needs to be used to derive a sensitivity function for SED-mode data as is used to reduce science data.

"Second-Order" Slit Illumination Correction

The MIPS 70-micron array suffers from sensitivity variations across the 4 column-wide array modules that the IC constructed above does not adequately rectify. That is, an object's flux measured while in the first slit position are 10-15% brighter than when measured in the second slit position. To correct this discrepancy and thereby allow for improved flux calibration of extended targets, the flux of NGC4418 was measured as it was stepped along the slit. At the spatial resolution of the MIPS SED mode, NGC4418 is a point source and its flux was measured at 15 positions along the slit (only covering the portion of the slit on Side A of the array). A 2-pixel extraction aperture was used, and the corrective factors to yield equivalent flux measurements at each position along the slit were determined. The flux correction factors were interpolated so that a value could be assigned to each of the 16 columns (listed in Table 1).

These corrective values were divided into the IC derived above to yield a modified SED IC that should be employed to process data. Due to the difficulty in measuring the flux when the object is close to the ends of the slit, no correction has been applied to the columns bounding the array. In any event, these columns are not useful for SED observations and should not be included in spectral extraction apertures that reduce the data to 1-dimensional spectra. Tests using the modified IC on Arcturus, HD98800, and TW Hya show that the discrepancy in the measured flux of a point source between the first and second slit positions is 2-3%.

Flux Calibration of MIPS SED Mode

Current flux calibration uses SED-mode observations of Arcturus obtained during MIPS campaigns 10 (AOR 11625216), 18 (AOR 13124096), and 22 (AOR 15248896). Six SED-mode cycles using 10s DCEs and a "+"1-arcmin "chop" to measure the local background were used for each observation of Arcturus. Over the SED-mode wavelength range, it is assumed that the spectrum of Arcturus is proportional to 1/(wavelength)^2 and is normalized to a value of 14.7 Jy at 70 microns.

Two problems occurred during the MIPS campaign 10 observation that were not seen in the subsequent observations of the standard star. First, a latent caused by the bright star is seen in the "off" object DCEs along column 5 of the 70 micron array when the object was centered near column 5 (the second position along the slit in the SED-mode AOR). Much more subtle latents may also be present as the background in columns 4, 6, and 7 (when the object was centered in the second slit position) are also elevated slightly compared to when the star is at the first slit position. However, column 5 shows a very strong latent as the background measurement averaged between lines 10 and 20 is about a factor of 4 higher when the star is centered near this column than when it is centered at the other slit position. To avoid over-subtracting the background, the background determined for the first slit position of the object was also subtracted from the second slit position. Second, rows 3 and 4 of DCE 19 were contaminated by a strong cosmic ray that hit the array at the position of the star's spectrum that was not removed in the DAT processing. This DCE was replaced by an average of DCEs 17 and 21 and the data were reprocessed.

The observations of Arcturus were processed by the DAT using the following commands:

mips_sloper -q -n file.fits

mips_caler -P mips_caler70S.param file.red.fits (where mips_caler70S.param points to the modified SED IC file; see above)

mips_enhancer -f file.cal.fits -o mosaicSED -r 0 -s 1.0 -RR

Background subtraction was then accomplished within IRAF by:

imarith mosaicSED_on.fits - mosaicSED_off.fits Arcturus.fits ,

and then the IRAF task noao.twodspec.apextract.apall was used to extract a 1-dimensional spectrum with a 5-pixel extraction aperture. The three observations of Arcturus were used to construct a sensitivity function to convert observed units (MIPS_70 units) to Janskys. The sensitivity file (SENS.fits) is an average of the sensitivities determined from the individual observations of Arcturus. The scatter about the mean suggests a level of repeatability of generally about 10%. The table below gives the average sensitivity function based on the average of the three observations of Arcturus obtained in MIPS campaigns 10, 18, and 22.

Note that the turn over in the sensitivity at pixel 28 is caused by the contamination of the first-order spectrum by second-order light. Therefore, SED-mode spectra are NOT properly flux calibrated at wavelengths longer than 97 microns, and the extracted spectrum should be truncated after pixel 27 (i.e., edit pixels 28-33 out of the spectrum). Also in Table 2, the last column gives the aperture correction that should be applied to the extracted spectrum if a 5 pixel-wide extraction aperture is used on a point source. The aperture correction is based on the report "MIPS SED PSF: Observations versus STinyTim" by C. Engelbracht. Adjustments to the aperture correction for a point source if other than a 5 pixel-wide extraction aperture is chosen are given in Table 3 (see the next section).

SED-Mode Data Reduction Example for a Point Source

Here, an example of how to process MIPS SED-mode data using the DAT is presented. The raw data is contained in file.fits.

mips_sloper -q -n file.fits

mips_caler -P mips_caler70S.param file.red.fits (using the proper modified SED IC to account for column-to-column sensitivity variations across the 70 micron array)

mips_enhancer -f file.cal.fits -o mosaicSED -r 0 -s 1 -RR

Subtract mosaicSED_off.fits from mosaicSED_on.fits and extract a 1-dimensional spectrum (1Dspec.fits) from the resulting background-subtracted 2-dimensional frame. From this step there are 2 possibilities:

(1) If the spectral extraction aperture is 5 pixels wide (HIGHLY RECOMMENDED for point sources):

Multiply 1Dspec.fits by SENS.fits (the sensitivity function).

Multiply the result by the aperture correction (APTcorr_5.fits). The resulting spectrum is the final, flux-calibrated result (in Janskys).

(2) If the spectral extraction aperture used is NOT 5 pixels wide:

Multiply 1Dspec.fits by SENS.fits (the sensitivity function).

Multiply the result by the aperture correction (APTcorr_5.fits).

Multiply the result by the factor in Table 3 appropriate for the width of the extraction aperture used. The resulting spectrum is the final, flux-calibrated result (in Janskys).

In both cases it is HIGHLY RECOMMENDED that pixels 28-32 be edited out of the final spectrum since they have little to do with reality.