Accurately transforming pixel positions in NICMOS images to relative or absolute astronomical coordinates involves several considerations.
The distortion of the NICMOS dewar due to the cryogen expansion affected the optical path, pushing the cold well and cameras closer to the field divider assembly and cold mask. This caused the effective focal length, and hence the pixel scale, to change somewhat as a function of time during the lifetime of the instrument. The pixel scale was monitered regularly throughout the entire time.
The NICMOS arrays are slightly tilted relative to the focal plane, and therefore the NICMOS pixel scales along the X and Y axes of each camera are slightly different. I.e., projected on the sky, the pixels are actually slightly rectangular, not square. This is a small effect.
Additionally, the distortion corrections for the NICMOS cameras are small, but for precision astrometry or when registering and coadding images (especially taken with a wide dither pattern, or when assembling mosaics covering a large field) it should be taken into account. See Chapter 5 of the NICMOS Data Handbook for more detailed information on the NICMOS platescale and how to appropriately address geometric distortion.
The documents listed below provide plate scale and geometric distortion information for data taken in the post-NCS era (Cycle 11 and beyond).
- This pdf document shows the calculations for plate scale and geometric distortion from SMOV 3b.
- The field observed and analysis methods used to calculate the SMOV 3b plate scale and geometric distortion parameters are identical to those used for the analogous Cycle 7 measurements, described fully in OSG-CAL-97-007 (pdf).
- Simple table of plate scales calculated for all three cameras during SMOV 3b.
Below are plate scale resources from Cycle 7:
For more specific details on the NICMOS plate scale, follow the links to the documents listed below.
NICMOS Plate Scale Documentation
Important updates, discoveries and developments that could potentially affect NICMOS observations, calibration, or data analysis.
Frequently Asked Questions.
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.
S. Arribas 10 Jul 2002
J. Najita 25 Feb 1998
C. Cox 15 Dec 1997
Papers and Proceedings:
Selected NICMOS related published papers and workshop proceedings.
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.
NICMOS STScI Analysis Newsletter 19
Temperature dependent synthetic dark image generator
Polarizer Data Analysis Software
NICMOS Histroy Tool
NICMOS Group 30 Sep 1999 (html)