In this section, we give a brief description of the basic operations of each NICMOS detector (see
Chapter 7 for more details), and compare the infrared arrays to CCDs. We then discuss the target acquisition modes for coronagraphy (see
Chapter 5 for a more extensive description of coronagraphy), as well as the simultaneous use of NIC1 and NIC2.
NICMOS employs three low-noise, high QE, 256в256 pixel HgCdTe arrays. Active cooling provided by the NCS keeps the detectors’ temperature at ~77.15 K. The detector design is based on the NICMOS3-type design, described in more detail in
Chapter 7. Here we summarize the basic properties of the NICMOS
detectors most relevant to the planning of observations.
The NICMOS detectors have dark current of about 0.1–0.2 electrons per second and the effective readout noise for a single exposure is approximately 30 electrons.
The NICMOS detectors are capable of very high dynamic range observations and have no count rate limitations in terms of detector safety. The dynamic range, for a single exposure, is limited by the depth of the full well, or more correctly by the onset of strong non-linearity, which limits the total number of electrons which can be accumulated in any individual pixel during an exposure. Unlike CCDs, NICMOS detectors do not have a linear regime for the accumulated signal; the low- and intermediate-count regime can be described by a quadratic curve and deviations from this quadratic behavior is what we define as ‘strong non-linearity’. Current estimates under NCS operations give a value of ~120,000 electrons (NIC1 and NIC2) or 155,000 electrons (NIC3) for the 5% deviation from quadratic non-linearity. This non-linearity is corrected in the standard NICMOS pipeline reduction. See
Section 4.2.4 for further discussion of the topic.
NICMOS has three detector read-out modes that may be used to take data (see
Chapter 8) plus a target acquisition mode (
ACCUM,
MULTIACCUM,
BRIGHTOBJ, and
ACQ).
The simplest read-out mode is ACCUM which provides a single integration on a source. A second mode, called
MULTIACCUM, provides intermediate read-outs during an integration that subsequently can be analyzed on the ground. A third mode,
BRIGHTOBJ, has been designed to observe very bright targets that would otherwise saturate the detector.
BRIGHTOBJ mode reads-out a single pixel at a time. Due to the many resets and reads required to map the array there are substantial time penalties involved.
BRIGHTOBJ mode may not be used in parallel with the other NICMOS detectors.
BRIGHTOBJ mode appears to have significant linearity problems and has not been tested, characterized, or calibrated on-orbit.
Users who require time-resolved images will have to use MULTIACCUM where the shortest spacing between non-destructive exposures is 0.203 seconds.
MULTIACCUM mode should be used for most observations. It provides the best dynamic range and correction for cosmic rays, since post-observation processing of the data can make full use of the multiple readouts of the accumulating image on the detector. Exposures longer than about 10 minutes should always opt for the
MULTIACCUM read-out mode, because of the potentially large impact of cosmic rays. To enhance the utility of
MULTIACCUM mode and to simplify the implementation, execution, and calibration of
MULTIACCUM observations, a set of
MULTIACCUM sequences has been pre-defined (see
Chapter 8). The observer, when filling out the Phase II proposal, needs only to specify the name of the sequence and the number of samples which should be obtained (which defines the total duration of the exposure).
These arrays, while they share some of the same properties as CCDs, are not CCDs and offer their own set of advantages and difficulties. Users unfamiliar with IR arrays should therefore not fall into the trap of treating them like CCDs. For convenience we summarize the main points of comparison:
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As with CCDs, there is read-noise (time-independent) and dark current noise (time-dependent) associated with the process of reading out the detector. The dark current associated with NICMOS arrays is quite substantial compared to that produced by the current generation of CCDs. In addition, there is an effect called shading which is a time-variable bias from the last read affecting the readout amplifiers.
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Unlike a CCD, the individual pixels of the NICMOS arrays are strictly independent and can be read non-destructively. Read-out modes have been designed which take advantage of the non-destructive read capabilities of the detectors to yield the optimum signal-to-noise for science observations (see Chapter 7, 8). Because the array elements are independently addressed, the NICMOS arrays do not suffer from some of the artifacts which afflict CCDs, such as charge transfer smearing and bleeding due to filling the wells. If, however, they are illuminated to saturation for sustained periods they retain a memory (persistence) of the object in the saturated pixels. This is only a concern for the photometric integrity of back to back exposures of very bright targets, as the ghost images take many minutes, up to one hour, to be flushed from the detectors.
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Most target acquisitions can be accomplished by direct pointing of the telescope. The user should use the Guide Star Catalog-II to ensure accurate target coordinates. Particular care must be exercised with targets in NIC1 due to its small field of view.
However, direct pointing will not be sufficient for coronagraphic observations since the achieved precision (
) is comparable to the size of the coronagraphic spot (0.3").
Note that this is the HST pointing error only. Possible uncertainties in the target coordinates need to be added to the total uncertainty.
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On-board acquisition (Mode-2 Acquisition). This commands NICMOS to obtain an image of the target and rapidly position the brightest source in a restricted field of view behind the coronagraphic hole. This is one of the pre-defined acquisition modes in the Phase II proposals ( ACQ mode).
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The re-use target offset special requirement can be used to accomplish a positioning relative to an early acquisition image.
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While ACQ mode is restricted to coronagraphic observations in Camera 2, the last two target acquisition modes may be useful for positioning targets where higher than normal (1–2 arcsec) accuracy is required (e.g., crowded field grism exposures).
While the three NICMOS cameras are no longer at a common focus, under many circumstances it is desirable to obtain data simultaneously in multiple cameras.
Although some programs by their nature do not require more than one camera (e.g., studies of isolated compact objects), observers may nonetheless add exposures from the other camera to their proposals in order to obtain the maximum amount of NICMOS data consistent with efficiently accomplishing their primary science program. Internal NICMOS parallel observations obtained together with primary science observations will be known as
coordinated parallels and will be delivered to the prime program’s observer and will have the usual proprietary period.
