NICMOS

Accordion

• Instrument Design

  NICMOS is an axial bay instrument which replaced the Faint Object Spectrograph (FOS) in the HST aft shroud during the Second HST Servicing Mission in February 1997. Its enclosure contains four major elements: a graphite epoxy bench, the dewar, the fore-optics bench, and the electronics boxes. The large bench serves to establish the alignment and dimensional stability between the HST optics (via the latches or fittings), the room temperature fore optics bench, and the cryogenic optics and detectors mounted inside the dewar. The instrument has three adjacent but not contiguous cameras, designed to operate independently and simultaneousely. The NICMOS dewar was designed to use solid nitrogen as a cryogen. Cold gas vented from the dewar was used to cool the vapor cooled shield (VCS) which provides a cold environment for both the dewar and the transmissive optical elements (i.e., the filters, polarizers, and grisms). The VCS is itself enclosed within two layers of thermal-electrically cooled shells (TECs). The dewar was built with a design lifetime of approximately 4.5 ± 0.5 years. However, the thermal short developed by NICMOS after installation on HST caused Camera 3 to be no longer parfocal with the other two cameras (Camera 1 and 2 are still close to parfocality); thus, operating Camera 3 simultaneousely with the other two cameras is not scientifically viable as a re-focussing of the instrument is involved. View a schematic of the NICMOS instrument.

  The external plumbing at the dewar aft end, which was used for the periodical recooling of the solid nitrogen during ground testing, will now form the interface to the NICMOS Cooling System (NCS). During SM3B, the NCS was connected to the bayonet fittings of the NICMOS interface plate. This allows the NCS to circulate cryogenic Neon gas through the cooling coils in the dewar, thus providing the cooling power to bring the instrument into the temperature range required for operation.

• Performance

  NICMOS is a multifaceted instrument that is capable of imaging between 0.8 and 2.5 microns. Furthermore, within this range the instrument has polarimetric imaging capabilities. A coronagraph built into NIC2 provides for coronagraphic imaging capabilites, and a grism in NIC3 allows for multi-object spectroscopy.

  The NICMOS instrument produces very high image quality. Minimal amounts of geometric distortion are present in the NICMOS cameras, and must be accounted for. Additionally, NICMOS data have many unique image anomalies which must be considered and compensated for during data reduction. These issues are discussed more closely in the NICMOS Data Handbook.

• Observing Strategies

  A summary of general recommendations for both Phase I and Phase II proposal preparation is available in Chapter 1 of the NICMOS Instrument Handbook; observers are strongly urged to review the material therein.

  APT Phase II examples for coronagraphy, polarimetry, and grism spectroscopy are available for download in pdf format.

  A new syntax for dither patterns has been developed which now allows multiple exposures (e.g. in different filters) to be taken at each dither position (for details, refer to Appendix D of the NICMOS Instrument Handbook for Cycle 14).

  In order to reduce the degrading impact of cosmic ray persistence (see Chapter 4 of the NICMOS Instrument Handbook for details) after passage through the South-Atlantic Anomaly (SAA), a pair of ACCUM dark exposures will be obtained immediately after each HST orbit through the SAA. The scheduling of these dark exposures is automatic and transparent to the user. The darks yield a map of the persistence pattern, and can be used to subtract a significant amount of the unwanted persistence signal. Software for implementing this correction and are being tested and distributed since the start of Cycle 11 observations.

  Observers are strongly advised to dither their observations as much as possible. This general advice regarding dithering is generally not applicable to observations of faint sources around/near bright ones. Dithering generally aids in the processes of removing cosmic rays, photon and cosmic-ray persistence, detector artifacts, and in averaging out flat-field sensitivity variations. HST absolute pointing is only good to about 1 arcsecond; dithering patterns should be designed to place science targets away from the edges of the cameras by at least this amount.

  In the case of crowded fields, observers are advised to dither their observations with sub-pixel sampling.

  Observers proposing to use the NICMOS coronagraphic hole may want to consider back-to-back visits, possibly within one orbit, of their targets with an in-between roll of the spacecraft for optimal PSF subtraction. Contemporary flat-field observations should also be considered for coronagraphic programs. Coronagraphic hole movements and HST focus changes (breathing) will result in residual noise during PSF subtraction. PSF stars should be observed close in time to the primary target.

  The bottom 10-15 rows of the three cameras' field of views are somewhat vignetted and interesting targets should not be placed there.

• Data Analysis

  The calibration of NICMOS data can be a particularly intricate process due to the many unique data anomalies inherent in the NICMOS cameras and electronics.

  When the data arrive at STScI, they pass through the OPUS pipeline which processes and calibrates them. The calibration software used by the pipeline is exactly the same as that provided within STSDAS. The calibration reference files and tables used are taken from the Calibration Data Base System (CDBS) at STScI and are the most up-to-date versions available at the time of the observation. However, a software tool called the dark generator now exists, which generates synthetic temperature-dependant darks. A similar, color dependent, synthetic flat-making tool called the flat generator is also available. Because of this, you may wish to re-calibrate your data using synthetic darks and/or flats, based on the nature and needs of your data. Starting in Cycle 11, when NICMOS comes back on line, the dark generator will be built into the standard calibration pipeline. This will not be relevant, however, for pre-Cycle 11 data. Additionally, users may often opt to reprocess their data regardless due to other data anomalies. Yet it is important to note, that as the understanding of the NICMOS data anomalies increases, more advanced and effective calibration tools are being developed to compensate for their affects. Slowly, these software tools are making their way into the calibration pipeline. Any developments regarding NICMOS data calibration will be published in newsletters and reports as they occur, and you can stay informed by routinely checking this website for new updates. You can find more detailed information in Chapter 3 of the NICMOS Data Handbook

• Software Tools

  Units Conversion Form

  A quick tool for flux conversion between diverse unit systems.

  We warn the users that this tool is outdated and the results of the calculations when using the VEGAMAG system might be off by ~ 1-1.5% due to the Vega spectrum being updated in 2020 (Bohlin et al. 2020). A Jupyter Notebook will replace this tool soon. 

  Units Conversion Form