NICMOS Instrument Handbook for Cycle 11

Introduction

---

NICMOS has four detector readout modes that may be used to take data. After observing time has been approved, the readout mode can be selected by the observer when completing the Phase II proposal entry. However, potential observers must understand the advantages and limitations of each of the readout modes in order to properly design their Phase I proposal. Since BRIGHTOBJ is an available mode (i.e., not supported by STScI), the necessity for its use should be spelled out in the Phase I proposal. Efficiency is not an acceptable reason to use an available mode.

There are three supported readout options within this general framework:

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
Accumulate (ACCUM)	Simplest observing mode Produces a single image May help ease data volume constraints	t > 0.57 seconds Limited to 173 tabular integration times	MULTIACCUM mode is preferred.
Multiple-Accumulate (MULITACCUM)	Faint targets Large dynamic range Optimal image construction. Ground processing of cosmic rays and saturation.	Multiple readouts at specific times during an integration 8590 > t > 0.215 seconds Number of readouts 25	Suitable for most programs. Use whenever high dynamic range needed e.g., source with bright core and faint extended emission or long integrations times.
Onboard Acquisition (ACQ)	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 ACCUM or MULTIACCUM instead

 

The BRIGHTOBJ mode is not supported for Cycle 11; it is, however, an available mode for the special case of acquisition of very bright targets under the coronagraphic hole. See Appendix 3 for a more detailed description of this mode.

 

Detector Resetting as a Shutter

It is important to remember that 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.