The basic data processing steps for PUI are virtually identical to those for spectroscopy on the SL detector. These steps are described in Chapter 5. The sky darks and flatfields are created specifically for PUI but are applied together with the spectroscopy equivalents whenever the SL detector data are processed.
4.2.1 Sky Darks
As with spectroscopic observations, multiple observations of low background regions of the sky are combined into a calibration file to be subtracted, plane-by-plane in the SUR data, from the science observations. While this sky dark calibration is meant primarily to remove the detector bias voltage and the (relatively small) dark current that accumulates during the exposure, sky emission is also subtracted. Dark subtraction removes roughly 14 MJy/sr from the background - the value of the background where the SL darks are measured. However, the zodiacal background is a function of position on the sky and of the time when the data were taken. As a result, many calibrated observations will still have substantial residual sky background. Note that the variation of the zodiacal emission during the year may also result in observations in which the standard calibration will over-subtract the background.
Users whose science interests require accurate zodiacal measurements will need to add the zodiacal emission back into their data. For this purpose, we provide 2D dark images for each exposure time (6, 14, and 30 seconds; http://irsa.ipac.caltech.edu/data/SPITZER/docs/irs/calibrationfiles/peakupimagingdarks/). These dark images were run through the DARKDRIFT module (see Section 5.1.13) to remove jailbars and were flat fielded. Blue and red image cutouts were made, the image cutouts were multiplied by the appropriate zeropoint to convert from electrons/sec to MJy/sr, and the header keywords BUNIT and FLUXCONV were edited to match.
4.2.2 Sky Flats
Flat fields are generated from observations of regions of extended emission (e.g., high zodiacal light background). The flat fields are very uniform over the peak-up array fields of view, with a standard deviation in relative pixel response of about 1.2%.
4.2.3 Point Spread Function (PSF) and Point Response Function (PRF)
The IRS provided a 16 micron PSF FWHM of 3.8 arcseconds and a 22 micron PSF FWHM of 5.3 arcsec. Two dimensional images of the PSFs and their radial profiles are shown in Figure 4.18 and Figure 4.19.
Figure 4.18 Left: A 2D image of the 16 micron PSF, based on observations of the star HD 42525. The relevant data have AORKEYs 17080064 and 17082112, and were mosaicked using MOPEX. The image is approximately 54 arcsec on a side. Right: A radial profile of the Blue Peak-Up PSF.
Figure 4.19 Left: A 2D image of the 22 micron PSF, based on observations of the star HD 172728. The relevant data have AORKEYs 20552960 and 20678912, and were mosaicked using MOPEX. The image is approximately 54 arcsec on a side. Right: A radial profile of the Red Peak-Up PSF.
A PRF is the convolution of an oversampled PSF with the pixel response function, and is required by the point source extraction module (APEX) within MOPEX to perform PSF fitting. An explanation of the difference between PRFs and PSFs is given in the document PRF vs PSF (PDF, 41 KB).
The SSC developed three different kinds of PRFs, with varying pixel scales and interpolation methods (see the MOPEX User's Guide). Users should choose the PRF that best matches the data reduction technique used to create their mosaics. The choices are:
1. standard pixel scale (1.8 arcsec) and default interpolation
2. resampled pixel scale (0.9 arcsec) and default interpolation
3. resampled pixel scale (0.9 arcsec) and drizzle interpolation
The PUI calibration is set so that the flux of a point source on zero background measured through an infinite aperture will be correct. In practice, measurements are made with small apertures so these need to be corrected to infinite apertures. Standard star observations were processed by pipeline version S18.18. Aperture photometry was performed in a circular area of 12 and 13 pixel radius for blue and red PUI, respectively. A background value equal to the median in an annulus between 18 and 25 pixels was subtracted. Aperture corrections to infinite aperture were obtained using STinyTim 2.0 (see next section).
True fluxes of the standard stars were obtained from IRS spectra of the stars. An average spectrum was fit with a model and the model was integrated over the PUI bandpasses (to reduce the effect of noise). Thus the PUI magnitude zero-points are tied to that of the spectra. (See Figure 4.20).
A 2% uncertainty is seen between the zeropoints inferred from different standard stars. Observations of the same target are repeatable to 1-sigma 1-2% for both red and blue. A comparison between the MIPS 24 micron calibration and IRS calibration of extragalactic objects was found to be cross-consistent (see Teplitz et al. 2010, Sections 2.2 and 2.3.)
The flux conversion factor from electrons/sec to MJy/sr is 0.0114 (MJy/sr)/(electrons/sec) for blue PUI. For red PUI the factor is 0.0119 (MJy/sr)/(electrons/sec). These flux conversion factors were calculated assuming a source with the following spectral shape:
They were pegged to a monochromatic wavelength, at the effective wavelength of each filter, defined as:
Effective wavelength values were 15.8 microns for blue and 22.3 microns for red.
Figure 4.20 Photometric accuracy of the red (top) and blue (bottom) peak-up imaging. PUI fluxes are compared to the fluxes derived from spectra of each star, integrated over the PUI bandpasses. The red PUI calibrators are HD163466 (23.4 mJy), HD172728 (36.9 mJy), HR5467 (37.3 mJy), HD173511 (254 mJy), and HR6348 (240 mJy). The blue PUI calibrators are HD163466 (46.6 mJy), HD172728 (73.7 mJy), and HR5467 (74.3 mJy).
126.96.36.199 Aperture Correction
While PUI calibration assumes that the sources are measured within an infinite aperture, in practice it is often possible to measure them only within a small aperture. Multiplicative point source aperture corrections derived from STINYTIM are given below. PUI observations are diffraction-limited, so these corrections will be different for sources with different colors. In general, the user should derive his or her own aperture correction appropriate for the source of interest.
Table 4.11: Aperture Corrections to Infinity (1.8 arcsec/pix).
For PUI observations, the Spitzer Heritage Archive provides photometrically calibrated data, assuming the source spectrum has the following spectral shape:
The calculations take into account the full spectral response of the instrument. Due to the so-called “Red Leak” (a small increase in response at about 28 microns) some emission will be detected in the Blue Peak-Up for very cold sources that would not have been detected in a system without the red leak. This translates into large correction factors for very cold (T<50 K) sources observed with the blue filter. While these numbers are formally correct, observers should be very wary of blindly applying them to the data. Longer wavelength observations are advised in this case. To get the peak-up flux for a non-standard spectral shape, divide the flux by the appropriate color correction factor, as given in the tables below.
Table 4.12 Peak-up Imaging Color Corrections for Blackbodies.