LASCO C1 Wavelength Calibration
Questions? call (202) 767 2263 ask for the LASCO C1 Calibration Person

Definition of the Plate Scale

The Fabry-Perot (FP) interferometer and the blocking filters in the C1 instrument transmit only light of a particular wavelength. The wavelength of the light transmitted depends upon the separation of the FP plates and the angle of the light ray with respect to an axis normal to the FP plates. This axis is called the optical axis of the FP, and the point at which it intercepts the detector is the optical axis intercept (FPAI). The relationship between the transmitted wavelength of a normal ray and that of a ray passing through the FP at some other angle is given by:

        w = w0 cos(ap)         Eq(1)

where w₀ is the wavelength passed by a normal ray, p is the distance (in pixel units) from the point where the ray strikes the detector and the point on the detector at which the optical axis intercepts the detector, and a is the "plate scale" in angular units per pixel unit.

The plate scale for the LASCO C1 instrument has been measured to be a=5.62 arc seconds per pixel unit in any direction. The position of the FPAI has been measured to be at (577.9,378.46) pixel units. Note that for even the most extreme case, the product of a and p is so small that the Taylor expansion of Equation 1 may be used quite accurately:

	w ~ w0(1-(ap)^2/2)     Eq(2)

The Command Wavelength

The Fabry-Perot interferometer is "commanded" to a particular wavelength by an algorithm which converts a "command wavelength" (in Angstroms) to a particular separation of the Fabry-Perot plates. This algorithm is somewhat imprecise and a method of determining exactly the relationship between the command wavelength and the actual wavelength at a particular pixel of the image is needed. Measurements have been performed, and this wavelength function has been determined.

An IDL program named fpc1_wcw.pro is now available which converts a command wavelength to vacuum or air wavelength at a given pixel on the detector. Also available is an IDL program named fpc1_wwc.pro which reverses the procedure, determining the command wavelength for an image, given the vacuum or air wavelength at a particular pixel. NOTE: it seems that the relationship between the command wavelength and the true wavelength varies in time. This means that the above programs may be in error by as much as a half of an Angstrom. They do work well for data taken around late 1997, which is when the calibration images were taken.

Note that the command wavelength is not accurately specified in the FITS header under the field 'FP_WL_CM'. This field is actually the same as the "uplink wavelength" in the field 'FP_WL_UP', which is equal to the command wavelength to within about .015 Angstroms

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PR Sep 26, 1998
PR Nov 01, 1999