Grism Wavelength Solutions

Wavelength Solution

Wavelength calibration for the NICMOS grisms is carried out by observations of Planetary Nebulae. Up to January 1998, Vy 2-2 has been used for that purpose. Since then, HB12 has been observed in during 5 different periods. The wavelength calibration is carried out by extracting spectra (flux versus distance from the position of the object on the direct images) with NICMOSlook. These spectra are then cross-correlated with ground-based spectra and a the parameters l0 and b of the functional form: lambda = l0+b*p are fitted, where p is the separation from the position of the object on the direct image.

The numbers for individual periods are give below. The parameters for the first order spectra as a function of time are shown in the following plots. For HB12, the number from individual observations are given. For the older Vy 2-2 observations, the average number in each period are given. No systematic change has been detected so far.

G096 parameters as function of time G096       G141 parameters as function of time G141       G206 parameters as function of time G206


For second order spectra, the parameters were determined only once. The median of the numbers for the first order and the determination for the second order spectra lead to the following relations for the three grisms:


G096

G141

G206

1st Orders

l0

0.94871.4012.045

b

-0.005369-0.007992-0.01152

2nd Orders

l0

0.52060000.69N/A

b

-0.0020-0.004220N/A






While the parameters remain fairly constant during the in-orbit observations, they are significantly different from the pre-flight measurements:
Comparison of wavelength solution with pre-flight measurements. Click here to see a plot comparing in-orbit wavelength calibrations with pre-flight numbers.

The latest version of the ST-ECF spectrum extraction programs NICMOSlook/ Calnic-C include the latest calibration data. IF you are have already installed the software, the new calibration data can be downloaded here. This file should replace the grismspec.dat file in the calib directory which comes with the distribution of the software.

Numbers from individual periods. are given in this table.

This page was last updated Nov 5, 1998