8.1 Introduction

Detector Resetting as a Shutter

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 modes:

1.	Multiple-accumulate Mode (MULTIACCUM).

2.	Accumulate Mode (ACCUM).

3.	Acquisition Mode (ACQ).

The ACCUM mode is, however, supported only for NREAD=1. For other configurations the ACCUM mode is available but not supported, i.e. the observer must request the use through a Contact Scientist during preparation of the Phase II proposal. 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 parentheses 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 is recommended instead of ACCUM for almost all programs; ACCUM might be useful with large numbers of very short exposures (see Section 8.6).
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 and later cycles; 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:

•	Array reset: All pixels are set to zero—the bias level.

•	Array read: The charge in each pixel is measured and stored in the on-board computer’s memory. This happens as soon as practical after the array reset. In effect, a very short exposure image is stored in memory.

•	Integration: NICMOS exposes for the period specified for the integration.

•	Array read: The charge in each pixel is measured and stored in the on-board computer’s memory.