NICMOS Plate Scale

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

Advisories:
Important updates, discoveries and developments that could potentially affect NICMOS observations, calibration, or data analysis.

FAQs:
Frequently Asked Questions.

Performance Summaries:
Status reports reflecting the current understanding of instrument characteristics, performance and calibration.

Complete Table of Plate Scale Measurements
The relative plate scale measures are just that: a measure of the *change* in plate scale between 2 different epochs. The dewar anomaly pushed the cold-well and the cameras closer to the field-divider assembly and cold-mask, thus changing their effective focal-lenghts. This change in focal length is reflected as a change in the plate scale at each camera. The amplitude of the change is inversely proportional to each camera's focal length. As the dewar slowly relaxed, the cameras in the cold-well slowly (asymptotically) moved back towards their nominal positions.
E. Bergeron 04 Jan 1999 (html)

Table of Absolute Plate Scale Measurements
We have taken the relative plate scale changes at each of the 3 epochs where there are absolute measures from the 7039 proposal. We then back out the value of the absolute plate scale at the reference epoch to which all the relative plate scale measurements are referenced to (day 63). We get X and Y numbers for each of the three 7039 epochs, extrapolated to day 63, which are presented here.
E. Bergeron 04 Jan 1999 (html)

Plot of Relative Plate Scale Measurements
This plot shows the X and Y plate scales in each of the 3 cameras vs. days since Jan 1, 1997. The lines show the relative plate-scale mesurements made from the focus monitor data, approximagely every 2 weeks. The symbols show the absolute measures of the plate scale made from the 7039 NICMOS to FGS Alignment proposal which ran during SMOV.
E. Bergeron 04 Jan 1999 (postscript)

NICMOS Plate Scale Measures Plot
This plot shows the X and Y plate scales in each of the 3 cameras vs. days since Jan 1, 1997. The lines show the relative plate-scale mesurements made from the focus monitor data, approximagely every 2 weeks. The symbols show the absolute measures of the plate scale made from the 7039 NICMOS to FGS Alignment proposal which ran during SMOV.
E. Bergeron 04 Jan 1999 (gif)

Handbooks:
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.

NICMOS ISR 2002-003: NICMOS Cycle 11 Calibration Plan
This document describes the NICMOS calibration activities for Cycle 11. It complements the SMOV3b and the NICMOS Cycle 10 interim calibration programs. These activities cover a period of 13 months and pursue the following major objectives: i) to monitor the main instrument properties, ii) to test the model for generating darks, iii) to analyze in detail the intra-pixel sensitivity effects for cameras 2 and 3, iv) to calibrate the polarimetry mode, and v) to establish NICMOS stability when tracking differential (spectrophotometric) changes over time. In addition, a contingency program (‘Pupil Transfer Function’) will be activated if differential photometry over the chips obtained from SMOV data does not reach the required levels of accuracy.
S. Arribas 10 Jul 2002 (pdf)

NICMOS ISR-98-001: Cosmic Ray Persistence in NICMOS Data
In this report, we discuss the implications of cosmic ray persistence for the overall quality of NICMOS data, quantify the tradeoff between data quality and timing of observations relative to SAA passages, and discuss possible strategies to minimize the problem.
J. Najita 25 Feb 1998 (pdf)

OSG-CAL-97-007: NICMOS Distortion Correction
The NICMOS distortion was measured by a series of images of the star cluster NGC1850. The degree of distortion was very small, amounting to about a one pixel shift at the corners. The pixel axes depart measurably from orthogonality being at most 0.2 degrees acute on camera 1.
C. Cox 15 Dec 1997 (pdf)

Papers and Proceedings:
Selected NICMOS related published papers and workshop proceedings.

Tutorials:
Cookbook style instructions, prescribed procedures, and helpful tips.

Examples:
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)