In total, there are 32 different filters, three grisms, and two sets of three polarizers available for NICMOS. Each camera has 20 filter positions on a single filter wheel: 19 filters and one blank. As a result, not all filters are available in all cameras. Moreover, the specialized optical elements, such as the polarizers and grisms, cannot be crossed with other filters, and can only be used in fixed bands. In general, the filters have been located in a way which best utilizes the characteristics of NICMOS. Therefore at shorter wavelengths, the most important narrow band filters are located in NIC1 so that the diffraction limited performance can be maintained wherever possible, while those in NIC2 have been selected to work primarily in the longer wavelength range where it will also deliver diffraction limited imaging.
Following the traditional HST naming convention, the name of each optical element starts with a letter or group of letters identifying what kind of element it is: filters start with an “
F”, grisms with a “
G”, and polarizers with “
POL”. Following the initial letter(s) is a number which, in the case of filters, identifies its approximate central wavelength in microns, e.g.,
F095N implies a central wavelength of
0.95 microns. A trailing letter identifies the filter width, with “
W” for wide, “
M” for medium and “
N” for narrow. In the case of grisms, the initial “
G” is followed by a number which gives the center of the free-spectral range of the element, e.g.,
G206. For the polarizers, a somewhat different notation is used, with the initial “
POL” being followed by a number which gives the PA of the principal axis of the polarizer in degrees, and a trailing letter identifying the wavelength range it can be used in, which is either “
S” for short (
0.
8-1.3 microns) or “
L” for long (1.9-2.1 microns).
Tables 4.1 through
4.3 list the available filters and provide an initial general description of each, starting with NIC1 and working down in spatial resolution to NIC3.
Figures 4.1 through
4.3 show the effective throughput curves of all of the NICMOS filters for cameras NIC1, NIC2, and NIC3, respectively, which include the filter transmission convolved with the OTA, NICMOS foreoptics, and detector response. provides further details and the individual filter throughput curves.
In order to make use of the high spatial resolution of HST, many observers expect to use NICMOS to observe very red objects (e.g., protostars) at relatively short wavelengths. These objects have very low effective color temperatures. Thus, the flux of such objects at 2.5 microns is expected to be orders of magnitude larger than their flux at desired wavelengths. In such a case, exceptionally good out-of-band blocking is required from the filter since out-of-band filter leaks could potentially have a detrimental impact on photometry. We have, therefore, investigated whether any of the NICMOS filters show evidence for out-of-band levels. The results indicate that actual red leaks were
insignificant or non-existent.
