Wavelength Calibration of the NIRC2 Grisms

Wavelength Calibration of the NIRC2 Grisms

The NIRC2 grisms combined with the different camera scales and slit widths provide a variety of different spectral coverages and resolutions. They are used in a way that may not be familiar to some observers, providing selection of the central wavelength by moving the slit and science target within the NIRC2 field of view. Observers are encouraged to familiarize themselves with the spectroscopy setup procedures in the manual.

This page covers:

• Wavelength calibration of spectra
• Calibration of the central wavelength vs. slit position in the field

Spectra of emission line lamps were taken with different combinations of grism, camera, filter, and slit position, the latter parameterized by SLITMM, a keyword specifying the position of the slit in the NIRC2 focal plane.

One caveat is that the thermal IR filters, Lp and Ms, do not have enough lines to properly calibrate.

Wavelength calibration of spectra

Spectra are generally well fit by third-order polynomials, sometimes requiring fourth-order or even fifth-order terms. The table below is representative of some wavelength fits performed in the lab. Note that the polynomial fit (most obviously the central wavelength) depends on the slit position. The column labeled SLITMM below represents the slit position in millimeters in the AO focal plane.

The wavelength at a given pixel, p, is given by:

WL(p) = c0 + c1*(p-512) + c2*(p-512)^2 + c3*(p-512)^3

If no value for c2 or c3 is given it was found to be consistent with zero.

lowres Grism

The medium resolution grism was calibrated with night sky lines. Unfortunately the night sky was too faint for the narrow camera.

medres Grism

Central Wavelength vs. Slit Position and Camera Column

The central wavelength was calculated (previous section) and found to be linear in the SLITMM keyword, corresponding to slit position in the focal plane. The inverse function is used to map a user's requested central wavelength into SLITMM, and later to .

Each camera has a separate translation from focal plane position (SLITMM) to column number. This is used to indicate to the observers at what column they should expect to find the slit image, hence where they should place the target.

The following table shows the coefficients that allow an observer to convert from SLITMM to central wavelength. The formula is:

WLc = b1 * SLITMM + b0

Note that WLc corresponds to the wavelength at pixel 512. Also note that combining the following table with the previous table you can estimate a wavelength scale for any combination of slit position, grism, and filter listed. However, this is not as accurate as doing your own wavelength calibration for whatever combination you end up choosing. This is because the higher-order coefficients in the first table also have some dependence on SLITMM, which we have not explored.

lowres Grism

medres Grism

* estimated by extrapolating from other orders calibrated with the same camera.

Estimating Parameters for Missing Camera/Grism/Filter Combinations

We have not yet accurately calibrated all combinations of camera, grism, and filter. However, we have provided estimates so that observers will still be able to approximately place their spectra properly. In particular, the thermal IR bands have few arc lines, and could not be calibrated in the same way that the JHK bands were.

Thermal IR

To estimate the wavelength calibration in the L and M bands, we used the half-transmission points of the band passes. Measured at different values of SLITMM, these provided estimates of the dispersion and central wavelength as a function of SLITMM.

JHK

Missing parameters were generally copied from similar, known combinations.

NIRC2 Master
2 April 2001