NICMOS Instrument Handbook for Cycle 11

Chapter 3: 
Designing NICMOS
Observations

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In this chapter . . .

The Visual Target Tuner (VTT)

 

In the preceding chapters, we provided an overview of the scientific capabilities of NICMOS and the basic layout and operation of the instrument. Subsequent chapters will provide detailed information about the performance and operation of the instrument. In this Chapter, we briefly describe the conceptual steps which need to be taken when designing a NICMOS observing proposal. The scope of this description is to refer proposers to the relevant Chapters across the Handbook. The basic sequence of steps in defining a NICMOS observation are shown in a flow diagram in Figure 3.1, and are:

• Identify the science requirements and select the basic NICMOS configuration to support those requirements (e.g., imaging, polarimetry, coronagraphy). Refer to the detailed accounts given in Chapter 4 and Chapter 5.
• Select the wavelength region of interest and hence determine if the observations will be Background or Read-Noise limited using the Exposure Time Calculator available on the STScI NICMOS WWW page (see also Chapter 9 and Appendix 1).
• Establish which MULTIACCUM sequence to use. Detailed descriptions of these are provided in Chapter 8. This does not need to be specified in a Phase I proposal. However, if a readout mode other than MULTIACCUM is required, this should be justified in the Phase I proposal.

Estimate the exposure time to achieve the required signal to noise ratio and check feasibility (i.e., saturation limits). To determine exposure time requirements and assess whether the exposure is close to the brightness and dynamic range limitations of the detectors, the Exposure Time Calculator on the WWW or the sensitivity plots in Appendix 1 should be used.

Proposers should always use the NICMOS ETC to calculate exposure times. The default settings of the ETC assume the presence of a dark current anomaly that was observed during the NICMOS warmup (see Chapter 7a). This corresponds to a dark current of ~2 e-/s/pixel at a detector temperature of 78 K.

 

• If necessary a chop and dithering pattern should be chosen either to measure the background or to enable mapping. See Chapter 11.
• If coronagraphic observations are proposed, additional target acquisition exposures will be required to center the target in the aperture to the accuracy required for the scientific goal (e.g., the proposer may wish to center the nucleus of a galaxy in a crowded field behind the coronagraphic spot). The target acquisition overheads must be included in the accounting of orbits.
• Calculate the total number of orbits required, taking into account the overheads. In this, the final step, all the exposures (science and non-science, alike) are combined into orbits, using tabulated overheads, and the total number of orbits required are computed. Chapter 10 should be used for performing this step.