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Preliminary results from SMOV indicate that variations in the large scale structure of the on-orbit flatfields relative to the thermal vacuum flatfields are of the order of 1-2% in Camera 2, and up to 5% in Camera 1. Figure 17.1 shows two examples of the ratio of the on-orbit to thermal vacuum flatfields for Camera 2 and Camera 1.
Figure 17.1: Ratio of On-Orbit to Thermal Vacuum Flatfields for Camera 2 in the F110W (left) and Camera 1 in the F140W (right). The ratios are averages across 20 columns and are given as a function of row number.
17.1.2 Characteristics and Uncertainties of Flatfields
Our current knowledge of the NICMOS flatfields is based on tests carried out using a flight spare detector array. The general characteristics are the same for the thermal vacuum flatfields, and we expect them to remain the same for on-orbit flatfields.
The amplitude of pixel-to-pixel variations in response is displayed in several ways in Figure 17.2, for a wavelength of 1.5 µm. The figure shows that the variations are essentially random with position on the array, and have a typical 1 amplitude ~8% and that the pixel-to-pixel variations are independent of the global response.
Figure 17.2: Flat Field Response Images Using 10% Bandwidth Filters on a Flight Spare Array. Wavelengths used include (a) 0.8µm, (b) 1.5µm, (c) 2.1µm and (d) 2.5µm. The images have been normalized to the mean response for each wavelength. The contours and greyscale are linearly spaced in each image between normalized responses of 0.4 and 2.2. Significant areas of the array span this whole range at 0.8µm, while at 2.5µm the array is almost flat.
Figure 17.3: High Spatial Frequency Noise at 1.5µm. This was measured by dividing the image in Figure 17.2. (b) by a smoothed version of itself. The greyscale version in (a) is scaled between 0.9 and 1.1. Slices through the image are plotted in (b) along row 100 and in (c) along column 100. The distribution of data is plotted as a histogram in (d).
The size of the pixel-to-pixel sensitivity variations with wavelength is similar to that measured for spatial variations in the global flatfield response. At 0.8 µm the standard deviation of the pixel-to-pixel sensitivity variations is ~11%, at 1.5 µm it is ~7%, at 2.1 µm it is ~6%, and at 2.5 µm it is lower than the measurement uncertainties. Figure 17.4 shows the general behaviour of the flat-field response with wavelength, which is similar for low and high frequency variations.
The response of individual pixels relative to the mean of the array as a function of wavelength is given in Figure 17.5, for both low sensitivity and high sensitivity spots. The relative response is a slowly changing function of wavelength between 1.0 and 2.2 µm, while it changes dramatically beyond 2.25 µm, to become a linear function of wavelength.
In summary, NICMOS flatfields indicate: