NICMOS Filters

The NICMOS instrument is equipped with filters ranging from 0.8-2.5 microns. Each camera has 20 filter positions on a single filter wheel: 19 filters and one blank (view filter wheel mechanism schematic). 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. The filters are located in such a way to best utilize the characteristics of the NICMOS detectors. That is, at shorter wavelengths, the most important narrow band filters are located in NIC1 so that the diffraction limited performance can be maintained wherever possible. The filters in NIC2 have been selected to work primarily in the longer wavelength range, where diffraction limited imaging is also possible.(View nic1, nic2, and nic3 filters and bandwidths.)

The name of each optical element starts with a letter or letters that identifies the type of optical element. Filters start with "F", grisms with "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 is centered about 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 indicates the center of the free-spectral range of the element. For the polarizers, the initial "POL" is followed by a number, which gives the PA of the principle axis in degrees, and a trailing letter identifying the wavelength range it can be used in (either "S" for short, or "L" for long).

For observations of objects with extreme colors (such as highly reddened sources, protostars etc.), out-of-band filter leaks could potentially have a detrimental impact on photometry. A limited number of in-flight tests have been made. Actual red leaks were found to be insignificant or non-existent.