This chapter summarizes the most common features and artifacts seen in IRS Peak-Up and spectroscopic data. We describe these features, show images with representative examples, and recommend methods for their removal from the data.
7.1 Peak-Up Imaging (PUI)
7.1.1 Saturated Data
Saturated samples in the data can be identified in raw DCEs or in intermediate BCD products (lnz.fits or dmask.fits). The IRS uses a signed 16-bit A-to-D converter, resulting in a DN range for science observations of -32747 DN to 32767 DN. Therefore, in raw DCEs, saturated samples have values of 32767 DN. For data taken in DCS mode, like peak-up acquisition data, the range is 0 to 65535 DN.
Because the gain is 4.6 electrons/DN, saturated samples in lnz.fits have values typically above 298,000 electrons. This is slightly less than 4.6×216 because of dark current subtraction. Saturated samples have mask values in dmask.fits with bit #3 set. That is, the bit-by-bit logical “OR” operation of the sample mask value with the value 8 is non-zero.
The presence of saturated samples does not necessarily invalidate the data of the corresponding bcd.fits pixel because saturated samples in a ramp are excluded from slope calculations. Therefore, the reduction in the signal-to-noise ratio due to saturation depends on how many samples are saturated in the ramp. Bit #2 is set in bmask.fits for BCD pixels in which at least one sample of the ramp was saturated. Bit #13 is set if all samples of the ramp are saturated, and bit #12 is set if all but the first sample are saturated.
7.1.2 Latent Images
Bright objects falling on the IRS SL detector can result in persistent charge that appears as a latent image of the object in subsequent exposures. For exposures with (1-5)x104 electrons pix-1 in the last sample, 0.04 to 0.1% will remain behind as latent images in the subsequent exposure. These latents decay rather slowly, with an e-folding time of ~1100 sec. For many successive exposures source flux can appear to increase by several percent. Given the parallel position of the peak-up fields-of-view to the SL slits, it is not uncommon for a bright object to fall on the peak-up arrays during science operations. Proper dithering strategies can mitigate the latent image problem, although with some loss of signal-to-noise on the affected pixels.
A second source of latent charge on the detector is simply the background. As a result, during very long Peak-Up Imaging AORs or those in very high background regions, the background level may be seen to rise slowly over time. This effect is stable and varies smoothly, so users may fit the (small) rise in the background and subtract it.
7.1.3 Rogue Pixels
A rogue pixel is a pixel with abnormally high dark current and/or photon responsivity (a “hot” pixel) that manifests as pattern noise in an IRS BCD image. The term “rogue” was used originally to distinguish pixels whose hotness was unpredictable, but now rogue pixel masks include those that are permanently as well as temporarily hot. Proper dithering will mitigate the effect of rogue pixels, but they should be masked before mosaicking. See also Section 7.2.2.