The highest sensitivity gain relative to ground-based observations is at wavelengths shorter than 1.8 microns. The background at J and H seen by HST is a few hundred times smaller than at ground-based observatories. The background at K is only marginally better on HST, due to the telescope's thermal emission. However, observations in the thermal regime (longward of 1.8 microns) may be more advantageous with NICMOS if high angular resolution is a requirement for the science goal.
At long wavelengths the dominant effect limiting the NICMOS sensitivity is the thermal background emission from the telescope. The magnitude of this background depends on the temperatures of the primary and secondary mirrors and their emissivities. At shorter NICMOS wavelengths, sensitivities are affected by the zodiacal background. Both sources of background are described in Chapter 4 of the Instrument Handbook.
Background radiation will be a greater concern for grisms than for imaging observations. Every pixel on the array will receive background radiation over the spectral bandpass of the particular grism, while the source spectrum will be dispersed over many pixels. Therefore, the ratio of the source to background flux will be much lower for the grisms than for the regular imaging mode filters.
Observations of extended sources in the thermal regime (longward of 1.8 microns) may need to obtain background observations as well (chopping off the target). Given the stability of the thermal background, however, it will not be necessary to get background measurements more frequently than once per orbit. For point sources or extended sources which do not fill the camera field-of-view, images of the thermal background can be obtained with dithering. Techniques for Dithering, Background Measurement, & Mosaicing are discussed in Appendix D of the Instrument Handbook.
For more specific details on the NICMOS background, follow the links to the documents listed below.
NICMOS Background Documentation
Important updates, discoveries and developments that could potentially affect NICMOS observations, calibration, or data analysis.
Status reports reflecting the current understanding of instrument characteristics, performance and calibration.
The NICMOS Instrument Handbook is the primary guide regarding the characteristics and use of the instrument. The HST Data Handbook is the primary guide for calibration, reduction and analysis of NICMOS data.
Instrument Science Reports
ISRs are technical reports written by members of the NICMOS Group about various aspects of the instrument and data. They usually contain in-depth information about specific topics.
This report describes the results from the SMOV3b and Cycle 11 thermal background calibration programs for NICMOS. The HST aft shroud is warmer due to ongoing degradation of the external MLI and increased power of the instruments installed during SM3B (ACS and the Nicmos Cooling System (NCS)). This warms the NICMOS fore-optics, affecting the thermal background in long wavelength camera 2 and camera 3 filters by approximately 20% beyond the increase due to DQE alone. This should be taken into account when planning observations with these cameras in filters longward of 1.9 microns, or in especially broad filters such as F175W in camera 3. Since camera 1 has no filters with a central wavelength longward of 1.9 microns, thermal emission effects in this camera are not an issue, and have not been studied.
M. Sosey 14 May 2003 (pdf)
An unexpected increase in measured thermal background during the Cycle 11 early calibration program caused speculation that the cold mask position could have shifted since Cycle 7. To address this concern, a single orbit NICMOS program was executed (Program ID: 9704) to obtain deep PSF images of the star LHS1846 in all three cameras. Analysis of this data using the Phase Retrieval software package revealed a minimal amount of cold mask shift since Cycle 7 and provided new, more accurate cold mask values for the Tiny Tim PSF modeling software. It was concluded that the cold mask position was not the cause of increased thermal background observed during the Cycle 11 early calibration program. Increased thermal background has since been determined to be the result of increased thermal load on the HST aft shroud due to the addition of ACS and NCS during SM3b.
E. Roye 23 Apr 2003 (pdf)
This ISR presents the results of the analysis of NICMOS parallel thermal background data obtained as part of both SMOV and the cycle 7 calibration plan.
D. Daou 18 May 1998 (pdf)
This ISR presents the results of the analysis of NICMOS pointed thermal background data taken as part of the Servicing Mission Orbital Verification (SMOV).
D. Daou 14 Oct 1997 (pdf)
Papers and Proceedings:
Selected NICMOS related published papers and workshop proceedings.
A. B. Schultz
Cookbook style instructions, prescribed procedures, and helpful tips.
Examples of how to use various NICMOS-related tools, calibration and analysis techniques.
Space Telescope Analysis Newsletters
STANs contain useful information regarding calibration and data reduction.