5.3.3 Relationship Between Wavelength and Pixel

Table 5.4 gives the dispersion relationship in the form:,

where wavelength is in microns and the 0 pixel is at the central wavelength defined by the position of the object in the direct image. The relationship is plotted in Figure 5.10. The actual location of the positive and negative pixels will be dependent on the grism orientation and the location of the source in the image. The grisms were aligned as accurately as possible along a row or column of the array. Distortion and curvature in the spectrum are negligible.

The orientation and position of the spectra relative to the direct object has been measured in-orbit and was found to be similar to the Thermal Vacuum measurements except for a small 0.5^o rotation. The dispersion parameters have remained fairly constant during the in-orbit observations. They are significantly different from the pre-flight measurements, and the current best estimates of the dispersion relations are those determined from on-orbit observations.

Table 5.4: Grism wavelength to pixels relationship

Grism	m	b
G096	-0.00536	0.9487
G141	-0.007992	1.401
G206	-0.01152	2.045

Figure 5.10: Wavelength Versus Pixel Number for each Grism. Note that the actual location of the central wavelength on the detector depends on the position of the source.