Space Telescope Science Institute  Chapter 8:  Detector Readout Modes  8.2 Multiple-Accumulate Mode

8.1 Introduction


NICMOS has four detector readout modes that may be used to take data. After the observing time has been approved, the readout mode will be selected by the observer when completing the Phase II proposal entry. However, potential observers may want to understand the characteristics of the NICMOS readout modes to help design their Phase I proposal.

There are three supported readout options:

  1. Multiple-accumulate Mode.
  2. Accumulate Mode.
  3. Acquisition Mode.

The basic scientific rationale behind each of these modes, and a summary of their capabilities is outlined in Table 8.1, along with a recommendation regarding their use. The Phase II proposal instructions needed to identify the readout modes are given in brackets under the mode name.

Table 8.1: Readout Modes and their Functions
Mode
Use
Functionality
Recommendation
Multiple-Accumulate
(MULTIACCUM)

Faint targets.
Large dynamic range.
Optimal image construction.
Ground processing of
cosmic rays and saturation.

Multiple non-destructive readouts at specific times during
an integration.
0.215 < t < 8590 seconds.
Number of readouts 25.

Recommended for most programs.
Ensures the highest dynamic range.
Most effective for correction of cosmic ray hits and saturation.

Accumulate
(ACCUM)

Simplest observing mode.
Produces a single image.

t > 0.57 seconds.

MULTIACCUM mode is preferred.

Onboard Acquisition
(ACQ)
For coronagraphy only.
Locate brightest source in a subarray and reposition telescope to place source in coronagraphic hole.
ACCUM exposures are obtained, combined with cosmic ray rejection, hole located, sources located and centered.
Reasonably bright sources in uncrowded fields.
See Chapter 5 for more details.
Bright Object
(BRIGHTOBJ)
For coronagraphic acquisition of bright targets which would saturate the arrays in the other modes with the shortest integration time allowed.
reset/read/wait/read each pixel sequentially in a quadrant.
0.001024 < t < 0.2 seconds.
When possible use a narrow filter with MULTIACCUM instead.

The BRIGHTOBJ mode is not supported for Cycle 13; it is, however, an available mode for the special case of acquisition of very bright targets under the coronagraphic hole. See Appendix C:Bright Object Mode for a more detailed description of this mode.

Detector Resetting as a Shutter

NICMOS does not have a physical shutter mechanism. Instead, the following sequence of operations are performed to obtain an exposure:


Chapter 8:  Detector Readout Modes  8.2 Multiple-Accumulate Mode
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