Phase 2 Proposal Instructions for Cycle 10

13.2 Mode = ACCUM
Configuration = NIC1 or NIC2 or NIC3

---

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.

13.2.1 Aperture or FOV

See Table13.4: NICMOS Apertures.

13.2.2 Spectral Elements

See Table13.5: Spectral Elements for the NICMOS.

13.2.3 Wavelength

The Wavelength parameter is not required for NICMOS observations and should be left blank.

13.2.4 Optional Parameters

CAMERA-FOCUS
=DEF (default); 1-2

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 6.3Coordinated Parallels.

CAMERA-FOCUS 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.

OFFSET
=SAM (default)Specifies the method of FOV offset to be used in a predefined pattern of offsets.

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 be executed under gyro control. After this happens, any subsequent pointings in the pattern within range of the target guide stars (such as returns to the initial target position in a chop pattern) will reacquire the guide stars.

13.2.5 Time Per Exposure

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 13.3. The specified exposure time will be one of a set of possible values allowed by the following table:

NREAD	Exposure Time (seconds)
1	TPG_TIME + 0.598
9	TPG_TIME + 5.158

 

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 13.3, 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.

13.2.6 Special Requirements

The three NICMOS detectors may be operated in parallel. Coordinated parallels are not possible with certain special requirements; see Chapter6:Parallel Science Exposures for details.