Optical and UV Astronomy in the Internet Age

5.3. Near Infrared Camera and Multi-Object Spectrometer (NICMOS)

NICMOS ⁽³⁾ can obtain images and spectra between 0.8 to 2.5 µm. It has three independent cameras (NIC1, NIC2 and NIC3) with different magnifications imaging on 256 x 256 pixel HgCdTe arrays. Each camera has its own collection of broad, medium and narrow-band filters. NIC1 and NIC2 have a set of polarizers while NIC3 has grisms, in addition NIC2 has a coronographic hole. Some characteristics of the cameras are detailed below:

**Table 7.** NICMOS characteristics

	NIC1	NIC2	NIC3

Field of View	11" x 11"	19.2" x 19.2"	51.2" x 51.2"
Pixel Size	0.043"	0.075"	0.2"
Spectral Range	0.8-1.8µm	0.8-2.45µm	0.8-2.3µm
Special Filters	[SII], He I, [Fe II],	Pa Br, H₂	Pa, [Si VI], H₂
	Pa H₂O	HCO₂, C₂, CO	CO, HeI, [FeII]
Diffraction limi ted wavelength	1.0µm	1.75µm	-

Most observations are performed in the MULTIACCUM mode. In this mode, the images are read at intermediate stages of the integration and at the end. All these intermediate images are then stored and transmitted to the ground. There are some predefined sequence for this sampling, each with 25 readout times as detailed in the Instrument Handbook. The selection of a particular sequence depends on the type of observation and the target.

To eliminate detector non-uniformities, removing the thermal background and to map areas larger than the field of view of NICMOS 3 techniques can be used:

dithering small movements of the telescope between the target and an adjacent sky region (< 40'')
chopping large movements (up to 1800") that allow for a proper background subtraction
mosaicing to image an extended source

The Instrument handbook describes several chopping and dithering sequences (linear (along the x- or y-directions of the detector), spiral and square, for example). Each can have between 2 and 40 positions.

5.3.1 What constitutes a NICMOS observation - Associations

Due to the nature of the data acquisition, it is necessary to combine several exposures to create a useful science observation. These associations are then a collection of exposures that are related and processed (read, calibrated, stored) as a single unit. This new format also implies that NICMOS files need to remain in FITS format through their reduction and analysis.

These FITS files have a three-letter suffix that identifies its contents, some of the most important are shown in Table 8.

**Table 8.** Suffixes for NICMOS data files.

Suffix	File Contents

_raw	raw science data
_cal	calibrated science data
_ima	intermediate MULTIACCUM calibrated data
_mos	Mosaiced data

Each NICMOS science data file is in fact composed of a set of five image extensions:

the image
image variance
data quality map
number of valid samples at each pixel
integration time at each pixel

These arrays are stored in five individual FITS extensions within the file (see the "File Organization" chapter in the NICMOS section of the HST Data Handbook for more details).

5.3.2 NICMOS Calibration

Due to the nature of its data the calibration of NICMOS images is done in two parts. The calnica program performs the steps that can be done to a single exposure using the header information in a way similar to calwp2. In MULTIACCUM data each readout is partially calibrated. The calnicb program calibrates and merges the associations previously calibrated with calnica generating a unique final image from this combination. A third program calnicc is used for grism data. Manuals for this program are available from ST-ECF.

³ Please refer to the NICMOS Intrument Handbook for complete details of the instrument. It can be found at http://www.stsci.edu/ftp/instrument_news/NICMOS/topnicmos.html Back.