Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

---

NICMOS: Performance and Capabilities

Rodger I. Thompson
Steward Observatory, University of Arizona, Tucson, AZ 85721 USA

Abstract:

Keywords: NICMOS, infrared, instrumentation

Introduction

The scheduled launch date for NICMOS is February 13, 1997, on STS-82, the space shuttle Discovery. This instrument, now under final integration at Ball Aerospace under contract to the University of Arizona, will finally fulfill the initial requirement of infrared observations with HST. NICMOS operates in the 0.8--2.5 micron spectral range utilizing the 256x256 NICMOS 3 HgCdTe detector arrays developed by Rockwell International and the University of Arizona for the NICMOS project. NICMOS utilizes three of these detector arrays in three separate cameras to provide all of the functions of the instrument. A brief description of their characteristics is as follows.

Camera 1

Pixel size = 0.043 arc seconds, field of view = 1111 arc seconds

Camera 2

Pixel size = 0.075 arc seconds, field of view = 19.219.2 arc seconds

Camera 3

Pixel size = 0.2 arc seconds, field of view = 51.251.2 arc seconds

Imaging

Figure 1 indicates the NICMOS field of view and camera placement relative to the HST pickles and the current instruments. NICMOS will replace the FOS while STIS will be placed in the current GHRS position.

  
Figure: The NICMOS field of view.

The key to the imaging program is the NICMOS 3 detector characteristics. The measured flight detectors have rather similar characteristics. All NICMOS 3 detectors have 4040 micron pixel areas in a 256256 format. The pixel to pixel crosstalk is less than 0.2% and the pixels are individually read out which eliminates row or column smearing due to charge transfer. The average read noise is 35 electrons and the average dark current is 0.1 electrons per second at the operating temperature of 58 K. The quantum efficiency varies from a high of about 80% at 2.4 microns to a low of about 15% at 0.8 microns. The detector readouts are non-destructive which means that the detector may be read out at any time without destroying the image. This feature offers attractive alternatives such as examining the image at various time during the integration to eliminate cosmic ray effects or multiple reads at the beginning and end of the integrations to reduce the read noise.

Figure 2 gives an indication of the sensitivity of the NICMOS instrument. We have picked the F160W filter in Camera 3 which operates near the minimum of the background in NICMOS and roughly is equivalent to the H filter in ground-based imaging. The figure contains four panels. The first shows the transmission of the filter. The second panel indicates the time required to reach a signal to noise of 1, 3, and 10 for a point source of strength given in either Janskys or H Magnitudes. In this calculation only the central pixel is utilized, so some gain is expected through the proper weighted summing of other pixels. The third panel is the same calculation only using the flux in a line if it is the only source of illumination of the pixel. Finally the last panel indicates how long a source of a given strength can be integrated before non-linear effects start to appear. This limit is on the order of 200,000 electrons.

  
Figure: The NICMOS Sensitivity

One of the advantages of the NICMOS cameras is their ability to operate independently of each other. For example, during a long exposure in one camera the other cameras can take a series of short exposures in a set of other filters. This can be useful when one camera is perhaps on the nucleus of a galaxy and the others are observing parts of the outer regions.

Spectroscopy

Grisms in Camera 3 provide the NICMOS spectroscopic capability. The grisms operate in a slitless mode to provide spectra of all of the objects in the field. The absence of strong telluric OH emission makes this mode attractive for NICMOS. In crowded fields, spectra will be required at more than one orientation angle to eliminate cross talk between spectra.

There are three grisms in the filter wheel of Camera 3, centered on 0.964, 1.410, and 2.058 microns with average resolutions per pixel of 200. The first two grisms are quite sensitive, however, the long wavelength grism suffers from the thermal emission from HST. It is provided mainly to ensure complete spectroscopic coverage of the NICMOS spectral region. Figure 3 shows the grism performance in the same manner as the imaging performance in the Figure 2. This particular case looks at the performance at the [Fe II] line at 1.644 microns.

  
Figure: The NICMOS Grism Sensitivity

Coronographic Imaging

NICMOS is the first instrument to contain a true coronograph in HST with the proper correct optics to utilize it. Camera 2 is the coronographic camera in that it has a 0.63 arc second diameter occulting spot in the middle of one quadrant of the field. This spot is a hole in the imaging mirror so it is a permanent part of the camera field. The second part of a coronographic system is an apodizing stop at a pupil. All of the NICMOS cameras have a cold pupil stop inside of the dewar with apodizing masks. These masks are required for thermal suppression in addition to their coronographic properties. The mask for Camera 1 masks the secondary hole, primary edge, and spiders. The masks for Cameras 1 and 3 also mask the hold-down pads on the primary. Camera 2, however, is the only camera with an occulting spot. Use of the coronographic mode will require special acquisition techniques to place the central object on the occulting spot.

Polarization

NICMOS provides short wavelength polarization capability centered on 1 micron in Camera 1 and medium wavelength polarization centered on 2 microns in Camera 2. Each of the two cameras contains three polarizers separated in angle by 120 degrees. With these polarizers, the direction and strength of linear polarization at any angle may be obtained without rolling the telescope. Since there are angular reflections in NICMOS there is an instrumental polarization of about 5%. The polarization capabilities of NICMOS are oriented toward regions of high polarization such as is found in many star formation regions.

Operation

There are four basic modes of operating the NICMOS cameras. Each mode is designed for a specific purpose such as noise reduction or cosmic ray detection. In addition, there are subsets of these modes which are currently being implemented by the team and STScI which will be detailed in later reports on the instrument. The basic modes are as follows:

ACCUM

N non-destructive reads at the start and N non-destructive reads at the end of the integration, where N is a user-specified number. The image is the difference between the average of the initial and the average of the final reads.

MULTI-ACCUM

A non-destructive read at the beginning, end, and at user-specified times during the integration. All readouts are returned to the observer. This mode monitors the progress of the integration during the observation.

RAMP

N evenly spaced non-destructive reads during the integration fit by a linear function. This mode includes options of cosmic ray correction and the detection of saturation. N is specified by the proposer.

BRIGHT OBJECT

Pixel by pixel integration for sources that would saturate the detector in the normal cycle time through the whole array.

Observing Strategies

The following offers some short tips on expected observing strategy with NICMOS. It is very probable that an orbit experience will alter some of these strategies.

For low read noise:

Use the ACCUM mode with N at least 10 or employ the RAMP mode with N at least 20.

For good cosmic ray rejection:

Perform at least three independent observations in the ACCUM mode, use MULTI-ACCUM with a least three observations or use the RAMP mode with cosmic ray rejection turned on.

To reach the natural background limit:

Integrate long enough to be on the square-root slope of the source strength versus time curve. This time will vary greatly among the filters. For short wavelength narrow band filters, the detector noise will be the limit. For objects with source fluxes greater than the background, you will be source-noise limited rather than background limited.

For background subtraction:

In some cases it will be prudent to arrange your observing to provide for background subtraction. At the longer wavelengths, this will be generally a requirement. For point sources take at least nine independent integrations spaced by small increments of the prime camera field of view. The background is then determined by median filtering. Use the spacecraft small angle maneuvers unless you are in parallel with another instrument. Note the built-in patterns provided in the proposal instructions. For extended sources, take at least nine pairs of observation with the source in and out of the field of view. Again, the built-in patterns are very useful. Note that small angle maneuvers or field offset mirror motions affect all of the cameras. It is generally not possible to have effective background subtraction in all cameras simultaneously. Also note that the long wavelength filter and grism observations are strongly affected by the thermal background and will require background subtraction. Use them only if the science requires it.

Grism spectroscopy:

Take a continuum image in the filter closest to the grism band before and after the grism observation to provide the base point for the spectrum. If possible, take observations with different roll angles to identify and remove overlapping spectra. Also note that since the grisms are slitless, extended objects will degrade the spectral resolution.

Coronographic imaging:

Coronographic imaging requires an acquisition procedure that places the central bright object behind the occulting spot. Also note that it is very difficult to do background subtraction in the coronographic mode.

Acknowledgments:

This work is a result of work by the entire NICMOS Instrument Definition Team. NICMOS is being built by the University of Arizona under contract from the National Aeronautics and Space Administration.