NICMOS is an axial bay instrument which replaced the Faint Object Spectrograph (FOS) in the HST aft shroud during the Second HST Servicing Mission in February 1997. Its enclosure contains four major elements: a graphite epoxy bench, the dewar, the fore-optics bench, and the electronics boxes. The large bench serves to establish the alignment and dimensional stability between the HST optics (via the latches or fittings), the room temperature fore optics bench, and the cryogenic optics and detectors mounted inside the dewar. The NICMOS dewar was designed to use solid nitrogen as a cryogen for a design lifetime of approximately 4.5
± 0.5 years. Cold gas vented from the dewar was used to cool the vapor cooled shield (VCS) which provides a cold environment for both the dewar and the transmissive optical elements (i.e., the filters, polarizers, and grisms). The VCS is itself enclosed within two layers of thermal-electrically cooled shells (TECs).
Figure 2.1 is an overview of the NICMOS instrument;
Figure 2.2 shows details of the dewar. The external plumbing at the dewar aft end, which was used for the periodical recooling of the solid nitrogen during ground testing, now forms the interface to the NCS. During SM3B, the NCS was connected to the bayonet fittings of the NICMOS interface plate. This allows the NCS to circulate cryogenic Neon gas through the cooling coils in the dewar, thus providing the cooling power to bring the instrument into the temperature range required for operation. The concept and working principles of the NCS are discussed in
Appendix E:The NICMOS Cooling System.
The NICMOS fore-optics assembly is designed to correct the spherically aberrated HST input beam. As shown in the left hand panel of
Figure 2.3 it comprises a number of distinct elements. The Pupil Alignment Mechanism (PAM) directs light from the telescope onto a re-imaging mirror, which focuses an image of the Optical Telescope Assembly (OTA) pupil onto an internal Field-Offset Mechanism
(FOM) with a pupil mirror that provides a small offset capability (26 arcsec). An internal flat field source is also included in the FOM assembly. In addition, the FOM provides correction for conic error in the OTA pupil.
After the FOM, the Field Divider Assembly (FDA) provides three separate but closely-spaced imaging fields, one for each camera (right hand panel of
Figure 2.3). 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 в 256 pixel HgCdTe chip that is developed from the NICMOS 3 detector design. NICMOS achieves diffraction limited performance in the high resolution NIC1 longward of 1.0 microns, and in 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 are summarized in
Table 2.3.
NIC1 offers the highest available spatial resolution with an 11 в 11 arcsec field of view and 43 milliarcsec sized pixels (similar to the WFC3 UVIS 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
α, He I, [Fe II]
λ 1.64
μm, and [S III]
λ 0.953
μm, both on and off band. It is equipped with the short wavelength polarizers (0.8 to 1.3 microns).
NIC2 provides an intermediate spatial resolution with a 19.2 в 19.2 arcsec field of view and 75 milliarcsec 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
γ, H
2 S2 (1-0)
λ 2.122
μm, Paschen
α, HCO
2 + C
2, and the long wavelength polarizers (1.9–2.1
microns). Camera 2 also provides a coronagraphic hole with a 0.3 arcsec radius.
NIC3 has the lowest spatial resolution with a large 51.2 в 51.2 arcsec field of view and 200 milliarcsec pixels. It includes broad band 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 H
2 S2 (1-0), [Si VI]
λ 1.962
μm, Paschen-
α, [Fe II]
λ 1.64
μm, and He I
λ 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.
The placement and orientation of the NICMOS cameras in the HST focal plane is shown in
Figure 2.4. Notice that the cameras are in a straight line pointing radially outward from the center of the telescope focal plane. From the observer’s point of view the layout of NICMOS is most relevant when trying to plan an observation of an extended source with all three cameras simultaneously. The user must then bear in mind the relative positions and orientations of the three cameras. The gaps between the cameras are large, and therefore getting good positioning for all cameras may be rather difficult.
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. However, the relative positions of the NICMOS cameras in the focal plane could affect the planning of coordinated parallels with other instruments.
