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Introduction

NICMOS has four detector readout modes that may be used to take data. After observing time has been approved, the choices of readout mode can be selected by the observer when completing the Phase II proposal entry. However, a potential observer must understand the advantages and limitations of each of the readout modes in order to properly design their Phase I proposal. Since BRIGHTOBJ and RAMP are "available" modes (i.e., not fully supported by STScI), the necessity for their use should be spelled out in the Phase 1 proposal. Efficiency is not a reason to use an "available" mode.

There are three supported readout options within this general framework:

Multiple-accumulate Mode.
Accumulate Mode.

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

Can reduce noise by performing and averaging multiple initial and final readouts
t > 0.57 seconds
Limited to 173 tabular integration times, 32 of which may have matched dark current calibrations.

MULTIACCUM mode is preferred.

Multiple-Accumulate
(MULITACCUM)

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

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 need
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 behind coronographic spot

Two ACCUM exposures are obtained, combined with cosmic ray rejection, sources located, and centered.

Reasonably bright sources on uncrowded fields.

Bright Object
(BRIGHTOBJ)

For 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
t < 0.2 seconds

When possible use a narrow filter with ACCUM instead

Ramp
(RAMP)

Faint targets
Large dynamic range
Uncertain target flux
On-board cosmic ray removal
On-board saturation detection

Slope computation
Reduces Data Volume
Provides Variance and valid samples array
t > 50 seconds
number of readouts £ 50

MULTIACCUM superior. Only use this if data volume becomes an issue

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	Can reduce noise by performing and averaging multiple initial and final readouts t > 0.57 seconds Limited to 173 tabular integration times, 32 of which may have matched dark current calibrations.	MULTIACCUM mode is preferred.
Multiple-Accumulate (MULITACCUM)	Faint targets Large dynamic range Optimal image construction. Ground processing of cosmic rays and saturation. Long wavelength integrations	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 need 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 behind coronographic spot	Two ACCUM exposures are obtained, combined with cosmic ray rejection, sources located, and centered.	Reasonably bright sources on uncrowded fields.
Bright Object (BRIGHTOBJ)	For 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 t < 0.2 seconds	When possible use a narrow filter with ACCUM instead
Ramp (RAMP)	Faint targets Large dynamic range Uncertain target flux On-board cosmic ray removal On-board saturation detection	Slope computation Reduces Data Volume Provides Variance and valid samples array t > 50 seconds number of readouts £ 50	MULTIACCUM superior. Only use this if data volume becomes an issue

RAMP and BRIGHTOBJ modes are not supported for Cycle 7-NICMOS. See Appendix for a more detailed description of these modes.

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.

Fast and Slow Readout Modes

The NICMOS detectors have two readout speeds: FAST and SLOW. The use of SLOW mode imposes a 3 second readout overhead which increases the minimum possible integration time (to slightly more than 3 seconds). When multiple readouts are obtained in ACCUM mode, the readout overhead becomes rather large (e.g., 30 seconds at the beginning and end of an exposure for NREAD=10 making 60 seconds the minimum exposure time).

At this time we advise observers to use the FAST readout mode. No significant advantages results from using SLOW readout mode, and calibrations for that mode are not planned.

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stevens@stsci.edu