17.3 Instrument Artifacts

17.3.1 Non-Zero Zeroth Read Correction for Bright Sources

The first non-destructive read after a reset during a NICMOS exposure provides the reference bias level for the counts in each pixel of the science image. This is the zeroth read in a MULTIACCUM image, which is directly subtracted from the final readouts on-board the telescope for all the other readout modes so that only differences between non-destructive reads are sent back to the ground. Due to physical limitation in the readout speed, the zeroth read happens 0.203 seconds after the reset of the detector. When a bright source is being observed, a non negligible amount of charge will already have accumulated on the detector by the time the zeroth read is performed. (NICMOS doesn't have a shutter.) The consequences for the calibration of bright sources are obvious. Because the zeroth read subtraction from all subsequent readouts in a MULTIACCUM exposure is the first step of the calibration processing (and is automatically subtracted on-board the telescope from the final read in a ACCUM exposure), the handling of the detector nonlinear response will be inaccurate. At the time of this writing (August 1997) a modification of the calibration pipeline software calnica is being developed to correct the NICMOS observations for the non-zero zeroth read problem. The software fix requires that observations of bright sources be performed using the MULTIACCUM readout mode. The reason for this strategy is that all individual readouts are returned to the observer, and those can be used to extrapolate the counts back to the reset time (-0.203 seconds from the zeroth read) to recover the true bias level. Once the modification to the calibration pipeline software is in place, the problem will likely disappear. Then only the first few months of NICMOS on-orbit data taking will be affected (if bright sources have been observed), and those data may need recalibration.

17.3.2 Effects of Overexposure and the "Mr. Staypuft" Anomaly

Because each pixel of the NICMOS detectors is read individually, overexposure does not cause bleeding along the columns direction, unlike the case of CCDs. However, two artifacts result from the overexposure of one or more pixels:

An afterimage with excess dark current persists for many minutes (>> 10). Decay of this signal depends both upon elapsed time and, fairly strongly, upon the number of readouts performed (ongoing modifications to the autoflush procedure used between exposures may improve this situation). It is not unreasonable to expect signals of ~1 e^-/second up to an hour following a severe exposure.
Extremely bright targets can result in faint phantom images at the same pixel locations in the other three quadrants of the detector. NICMOS arrays are divided into four quadrants of 128 x 128 pixels each. When a very large number of counts are recorded in a source at pixel (i,j) of any one quadrant, a faint ghost of this image appears at pixel (i,j) of each of the other three quadrants. A faint band is seen running along detector rows which pass through these ghosts, which could be mistaken for a diffraction spike. The amplitude of the ghost images is of order a tenth of a percent of the real image count rate. It appears to be an electronic phenomenon and is not an optical ghost. Inside the NICMOS group, this effect is known as the "Mr. Staypuft anomaly."

17.3.3 Vignetting

Lateral shifts of the dewar resulted in vignetting in all three cameras. In the case of NIC1 and NIC2, the source of the vignetting is most likely the Field Divider Assembly (FDA) mask. The losses in throughput are relatively small (< 5%) and affect only the first 30 rows of the arrays. In NIC3, the region affected by vignetting is larger than in the case of the other two cameras; the first 60 rows are vignetted. In this case, the source of vignetting is likely to be a combination of FDA and fore-optics. At the time of this writing (August 1997), we are evaluating the use of the Field Offset Mechanism (FOM) to move the aperture of NIC3, in order to correct at least the part of vignetting caused by the fore-optics.

17.3.4 Amplifier Glow

Each quadrant of a NICMOS detector has its own readout amplifier, which is situated close to an exterior corner of the detector. When a readout is made, the amplifier injects a real signal into the detector, known as amplifier glow. This signal is largest closest to the corners of the detector where the amplifiers are situated, and falls rapidly towards the center of the detector. The signal is only present during a readout, but is repeated for each readout (e.g., a MULTIACCUM sequence or an ACCUM with multiple initial and final reads). Typically the extra signal is about 20-30 DN at the corners of the detector and 2-3 DN at the center, for each readout. The signal is highly repeatable, and almost exactly linearly dependent on number of reads (however, there may be a small non-linearity for reads made very close together in time; the amplitude of this non-linearity typically amounts to only a fraction of DN accumulated over an entire MULTIACCUM exposure in the brightest parts of the amplifier glow signal, and our detection of this non-linearity is, at the time of this writing, marginal).

The amplifier glow is a real signal and is subject to photon statistics, so it is a source of noise in NICMOS exposures. Thanks to the repeatibility of the signal, images calibrated with the appropriate dark frames (same MULTIACCUM sequence or same exposure time for ACCUM images) will have the amplifier glow removed. Its noise is propagated into the ERR array of the NICMOS calibrated images, thanks to its Poissonian nature.

17.3.5 Intra-Pixel Sensitivity Variations

As with many other modern array detectors, the sensitivity of the NICMOS detectors is lower near the edges of pixels then in their centers, causing reduced sensitivity along the intra-pixel boundaries. The response of a pixel to a source whose flux changes rapidly on a size scale comparable with or smaller than the pixel size will thus depend on where the center of the source lies with respect to the center of the pixel. Because the latter is not known a priori, this effect will introduce some uncertainty in the flux calibration for a point source. This uncertainty will be largest for Camera 3 at short wavelengths, for which the PSF is undersampled. We will try to measure the size of this effect on orbit and post updates on the NICMOS WWW pages; we expect it to be no more than a few percent uncertainty for Camera 3.

17.3.6 Hot Pixels, Cold Pixels, and Grot

The statistics on the cold and hot pixels present in each of the NICMOS cameras are presented in Table 17.1 below. In NIC2, the presence of the coronographic spot increases the number of cold pixels to 154.

In addition to these bad pixels which were already known from ground-based testing, more pixels have shown low measured quantum efficiency in orbit. These pixels are possibly affected by debris lying on top of the detectors. Paint flakes from the optical baffles are one possible source. Currently about 150-200 of these bad pixels have been measured in NIC1 and NIC2, and similar numbers are expected for NIC3. The bad pixels are often clustered in groups of up to 20-40, and appear as dark spots in flatfield frames. The position of the flakes varies.

Pixel Characteristic

NIC1

NIC2

NIC3

COLD

68

94

17

HOT

10

11

3

GROT

~ 200

~ 150

...

Statistics on Cold and Hot Pixels and Grot

Pixel Characteristic	NIC1	NIC2	NIC3
COLD	68	94	17
HOT	10	11	3
GROT	~ 200	~ 150	...

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