Instrument Design

Figure 3.1 is an overview of the NICMOS instrument; Figure 3.2 shows details of the dewar.

Figure 3.1: Instrument Overview

After NICMOS was installed in HST, the dewar expanded considerably further than it had on the ground for reasons still not understood. A maximum motion of ~11 mm was observed in March 1997 and a slow contraction since then has occurred. The motion history of NICMOS and the resulting image quality are discussed in Chapter 4.

This unexpectedly large deformation has several undesirable effects:

• The NIC3 camera's focus has moved outside of the focus range of the PAM. While it is presently moving back toward being within the focus range, the rate of motion is slow and unpredictable. The present extrapolation would result in an acceptable focus becoming available in late 1997.
• The three cameras now have significantly different foci, hence parallel observations are degraded (although the difference between the NIC1 and NIC2 foci is sufficiently small that an intermediate focus yields good -quality images in both cameras).
• Due to the off-axis optical design, the relative positions of the fields of view change with the position of the PAM mirror as it is moved to adjust the focus. This increases the calibration burden somewhat.
• Lateral motion of the detectors relative to the field divider assembly (located outside the dewar) has resulted in a slight (~20 rows) vignetting in all three cameras.
• NIC3 has an additional vignetting which effects ~25% of its field of view. It is presently expected that adjustment of the FOM mirror will reduce or remove this vignetting, and an experiment is being implemented to test this.
• Lateral motion of the detectors in the focal plane results in a variable position for the coronographic hole in NIC2. New flight software is being developed to rapidly locate this hole as part of the target acquisition process. This software should be available for Cycle 7-NICMOS.
• Finally, the motion of the inner shell of the dewar was sufficiently large that it made physical contract with the vapor cooled shield. This resulted in a thermal short which increased the heat flux into the inner shell (and therefore the solid nitrogen) by a factor of 2.5 and has thereby reduced the expected lifetime of NICMOS from 4.5 to < 2 years. The expected and present thermal states of NICMOS are shown in Table 3.2.

  Table 3.2: Dewar Thermal State (° K)

  	Expected	Present
  Cold Well	58	60
  VCS	155	101
  TEC1	199	190
  TEC0	218	217
  Dewar Shell	270	270

Figure 3.3: Ray Diagrams of the NICMOS Optical Train. The left panel shows the fore-optics. The right panel shows the field divider and re-imaging optics for the three cameras.

After the FOM, the Field Divider Assembly provides three separate but closely-spaced imaging fields, one for each camera (right hand panel of Figure 3.3). The dewar itself contains a series of cold masks to eliminate stray IR emission from peripheral warm surfaces.

A series of relay mirrors generate different focal lengths and magnifications for the three cameras, each of which contains a dedicated 256 x 256 pixel HgCdTe detector array that is developed from the NICMOS 3 design. NICMOS achieves diffraction limited performance in the high resolution Camera 1, NIC1, longward of 1.0 microns, and in Camera 2, NIC2, longward of 1.75 microns.

The operation of each camera is separate from the others which means that filters, integration times, readout times and readout modes can be different in each, even when two or three are used simultaneously. The basic imaging properties of each of the cameras is summarized in Table 3.3.

Table 3.3: Basic Imaging Parameters

Parameter	Camera 1	Camera 2	Camera 3
Pixel Size (arcsec)	0.043	0.075	0.2
Field of View (arcsec x arcsec)	11 x 11	19.2 x 19.2	51.2 x 51.2
Diffraction Limited Wavelength (µm)	1.0	1.75	...

Camera 1

Camera 1 (NIC1) provides the highest available spatial resolution with an 11 x 11 arcsec field of view and 43 milliarcsec sized pixels (equivalent to the WFPC2 PC pixel scale). The filter complement includes broad and medium band filters covering the spectral range from 0.8 to 1.8 microns and narrow band filters for Paschen , He I, [Fe II] 1.64µm, and [S III] 0.953 µm, both on and off band. It is equipped with the short wavelength polarizers (0.8 to 1.3 microns).

Camera 2

Camera 2 (NIC2) provides an intermediate spatial resolution with a 19.2 x 19.2 arcsec field of view and 75 mas pixels. The filters include broad and medium band filters covering the spectral range from 0.8 to 2.45 microns. The filter set also includes filters for CO, Brackett , H2 S2 (1-0) 2.122 µm, Paschen , HCO2 + C2, and the long wavelength polarizers (1.9-2.1 microns). Camera 2 also provides a coronographic mask with a 300 milliarcsec radius.

Camera 3

Camera 3 (NIC3) provides the lowest spatial resolution with a large 51.2 x 51.2 arcsec field of view and 200 milliarcsec pixels. It includes broad filters covering the spectral range 0.8 to 2.3 microns, medium band filters for the CO band (and an adjacent shorter wavelength continuum region), and narrow band filters for H2 S2 (1-0), [Si VI] 1.962 µm, Paschen-, [Fe II] 1.64 µm, and He I 1.083 µm. Camera 3 also contains the multi-object spectroscopic capability of NICMOS with grisms covering the wavelength ranges 0.8-1.2 microns, 1.1-1.9 microns, and 1.4-2.5 microns.

Placement and Orientation of Cameras

Note that the position of the NICMOS cameras relative to the HST focal plane (i.e., the FGS frame) depends strongly on the focus position of the PAM. Since independent foci and their associated astrometric solutions are supported for each camera, this is transparent to the observer.

Figure 3.4: NICMOS Field Arrangement

Comparison to WFPC2 and STIS

A unique feature of the WFPC2 is the presence of ramp filters which permit observations in any narrow bandpass up to 9800Å. These filters are limited to a ~10 x 10 arcsec field of view. The WFPC2 has a narrow band methane filter at 8920Å. Both WFPC2 and NICMOS have [S III] filters. NICMOS offers an adjacent off-band (or slightly redshifted) complementary continuum filter.

In Table 3.4, we summarize the sensitivities of WFPC2 and NICMOS in their region of overlap. In this wavelength region WFPC2's sensitivity is dropping rapidly with increasing wavelength, while NICMOS's is rising. The signal to noise achievable in one hour on a V=20 A0 star is seen to be comparable in the overlap region, and so the choice of which instrument to use is likely to be driven by the field of view desired (WFPC2's is much larger) and whether any further observations are required at either shorter (WFPC2) or longer (NICMOS) wavelengths. NIC3 observations assume that it is in focus.

The Space Telescope Imaging Spectrograph (STIS) also offers CCD based imaging to 1.1 µm with 0.05" pixels and higher quantum efficiency than WFPC2 (although without a useful filter set). Additionally, STIS offers a large complement of slit and slitless spectroscopic capabilities that could complement near-infrared NICMOS science.