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, 256x256 pixel HgCdTe arrays. Active cooling provided by the NCS keeps the detectors' temperature at 77.1 K. The detector design is based on the NICMOS 3 design; however, there are differences between the two (see 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 usefully 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.
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There are no bright object limitations for the NICMOS detectors. However, one must consider the persistence effect. See Section 4.6 Photon and Cosmic Ray Persistence for details. |
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
).
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Only ACCUM, MULTIACCUM, and ACQ are supported in cycle 13 and ACCUM mode observations are strongly discouraged. |
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:
Most target acquisitions can be accomplished by direct pointing of the telescope. The user should use target coordinates which have been measured with the Guide Star Astrometric Support Package (GASP)
to ensure the best accuracy with respect to the HST Guide Star Catalog. 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.
There are three target acquisition options for coronagraphic observations, which are extensively discussed in Chapter 5:
ACQ
mode).
re-use target offset
special requirement can be used to accomplish a positioning relative to an early acquisition image.
INT-ACQ
) can be obtained although this is costly in spacecraft time and is a limited resource.
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.
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The foci of Cameras 1 and 2 are close enough that they can be used simultaneously, whereas Camera 3 should be used by itself. |
The rest of this section applies only to Phase II proposals-there is no need to worry about this for Phase I proposals.
Although some programs by their nature do not require more than one camera (e.g., studies of isolated compact objects), observers are still encouraged to add exposures from the other camera(s) 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 attached parallels and will be delivered to the prime program's observer and will have the usual proprietary period.
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |