ACCUM is the simplest readout mode available on NICMOS. One or more non-destructive readouts occur at the beginning and at the end of the exposure.
ACCUM Mode will be appropriate for many targets, particularly for short integrations of relatively bright targets.
The Wavelength parameter is not required for NICMOS observations and should be left blank.
Specifies the NICMOS focus position to be used. If DEF is specified or this parameter is omitted, NICMOS will be focussed in the best position for the selected camera. A value of 1-2 selects a compromise focus position between the optimum foci for cameras 1 and 2. This will mainly be useful with coordinated parallels, but
CAMERA-FOCUS may be specified only on exposures using the primary NICMOS detector in the sequence as defined in
Section 6.3, “Coordinated Parallel Containers,” on page 105.
CAMERA-FOCUS = DEF or 1-2 is allowed only with
NIC1,
NIC1-FIX,
NIC2, or
NIC2-FIX as the chosen aperture.
NREAD
=1 (default), 9Each
ACCUM exposure is preceded and followed by a set of detector readouts, which are used to determine the initial and final pixel values.
NREAD specifies the number of readouts. Multiple readouts may be used to reduce read noise at the cost of extra overhead.
If the default value of SAM is used, offsets from the target that cannot be supported by small angle maneuvers (
SAMs) while locked onto the target guide stars will require new guide star acquisitions.
If Number_Of_Iterations > 1, the specified number of iterations will be taken at a single pointing. For a pattern (see
Chapter 8: Pointings and Patterns) the specified number of iterations will be taken at each point in the pattern.
Exposure times in ACCUM Mode are quantized. The exposure consists of a set of
NREAD initial readouts, followed by a period of data accumulation, followed by a set of
NREAD final readouts. The
Time_Per_Exposure (in seconds) refers to the total integration time, which begins at the start of the first initial readout and ends at the start of the first final readout. The NICMOS Timing Pattern Generator (TPG) uses a list of discrete values for the time between the last initial readout and the first final readout, known as TPG expose time (
TPG_TIME), which is given in
Table 12.3. The specified exposure time will be one of a set of possible values allowed by the following table:
For example, suppose the desired exposure time is 10 seconds. With NREAD=1, the ideal TPG expose time is 9.402. The closest TPG expose time to that, from
Table 12.3, is 9.117. So the corresponding exposure time is 9.117 + 0.598 = 9.715 seconds.
With NREAD=9, the ideal TPG expose time is 4.842. The closest TPG expose time to that is 4.781. So the corresponding exposure time is 4.781+ 5.158 = 9.939 seconds.
If the exposure time entered by the user is not one of the values allowed by the above formula, it will be reduced to the next lowest legal value. It is illegal to specify an exposure time which corresponds to a
TPG_TIME below the minimum value of 0.0. In other words, do not specify an exposure time shorter than 0.598 sec, which is the fastest
ACCUM read time for
NREAD=1. If the brightness of the source requires a shorter exposure time,
MULTIACCUM Mode or
BRIGHTOBJ Mode is suggested instead.
