In this section we describe some instrument-specific behavior which must be taken into account when estimating required exposure times. The WWW NICMOS Exposure Time Calculator (ETC) provides the most convenient means of estimating count rates and signal-to-noise ratios (SNRs) for imaging observations. The ETC handles either point sources or extended objects and can be accessed at
http://stsdas.stsci.edu/ETC/NIC/nic_img_etc.html.
The ETC currently works for imaging observations only. Grism observers are referred to Chapter 5 for a discussion of estimating exposure times.
The NICMOS APT-ETC is going to be the future official version of the ETC. Because it is under development at the time of preparing this handbook, we do not include information about it here. Once this new tool has been completed, tested, and delivered, it will be accessible from the NICMOS web site
. Although the user interface may look slightly different from the current ETC (i.e. the one described in this chapter), it will have a similar format. A help file will explain its characteristics
The current NICMOS WWW ETC is documented in detail in an Instrument science report NICMOS ISR 2000_01
and 05
(Sivaramakrishnan et al.). Recent updates are also explained in its help file (accessible from the user interface). The ETC generates either the exposure time required for a specified SNR or the expected SNR for a user-defined exposure time. It will also indicate the read noise and the source and background count rate, as well as the saturation-limited exposure time for the object in question.
A few comments about the limitations of making exposure time and SNR estimates are in order here.
NICMOS performance with the NCS has changed (relative to Cycles 7 and 7N) because the instrument now operates at a different temperature. This has improved the Detector Quantum Efficiency (DQE), but has also increased the dark current. The ETC has been updated to use parameters for Cycle 11 and beyond, with a temperature of 77.1K.
The instrumental characteristics used by the ETC are average values across the field of view of the camera. The actual sensitivity will vary across the field because of DQE variations (See Chapter 7 for details.). A second source of spatial variation not included in the ETC occurs near the corners of the chip because of amplifier glow (see Section 7.3.2).
The ETC will calculate SNR or exposure time, where the signal is the number of electrons in the peak pixel (i.e. brightest pixel) and noise is the standard deviation in a pixel due to photon statistics and instrumental noise.
For photon-limited observations (limited either by the target count rate or the background), SNR increases as the square root of the total observation time, regardless of how the observation is subdivided into individual exposures. However, some NICMOS observations may be significantly affected by read noise, and the net SNR from the sum of several exposures will depend on the relative contributions of read noise and photon noise. Read noise variations that depend on different read sequences are not accounted for in the ETC.
Phenomena such as cosmic ray persistence ( Chapter 4) can degrade sensitivity for faint object imaging by increasing the level of background noise. The impact of cosmic ray persistence is not easily quantified because it is non-Gaussian, correlated noise. It is not possible to predict the extent of this effect at the time the observations are planned, so it is not included in the ETC calculations. Additional information on the ETC and its structure can be found in the Instrument Science Reports (ISRs) posted on the STScI NICMOS Instrument web page.
http://www.stsci.edu/hst/nicmos/
The detector properties which will affect the sensitivity are simply those familiar to ground-based optical and IR observers, namely dark current and read noise, and the detector quantum efficiency (DQE). The dark current and DQEs measured in Cycle 11 are included in this ETC (see Chapter 7 for more details). The variation of the DQE as a function of wavelength and temperature is also taken into account.
NICMOS contains a fairly small number of elements which affect the sensitivity. These elements are the filter transmission, the pixel field of view (determined by the NICMOS optics external to the dewar, in combination with the HST mirrors), the reflectivities and emissivities of the various mirrors and the transmission of the dewar window.
Filter transmissions as a function of wavelength were measured in the laboratory, and the resulting curves are presented in Appendix A, convolved with OTA, NICMOS fore-optics and detector response.
NICMOS contains a total of seven mirrors external to the dewar, each of which reduces the signal received at the detector. The mirrors are silver coated (except for the field divider assembly which is gold coated) for a reflectivity of 98.5%. The dewar window has a transmission of roughly 93%. Therefore, the combination of optical elements is expected to transmit ~84% of the incoming signal from the OTA.
The sensitivity will obviously be affected by the pixel field of view. The smaller the angular size of a pixel, the smaller the fraction of a given source that will illuminate the pixel, but compensating will be a lower sky background. Finally, the optical efficiency will be degraded further by the reflectivities of the aluminum with MgF2 overcoated HST primary and secondary mirrors.
At long wavelengths (> 1.7 microns) the dominant effect limiting the NICMOS sensitivity is the thermal background emission from the telescope. The magnitude of this background mainly depends on the temperatures of the primary and secondary mirrors and their emissivities. At shorter NICMOS wavelengths, sensitivities are affected by the zodiacal background. Both sources of background are described in Chapter 4.
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