17.1 Flatfield Errors

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.

NICMOS flatfields show significant large-scale non-uniformity as well as pixel-to-pixel fluctuations. In addition, the non-uniformity is a strong function of the wavelength. Figure 17.3 shows the measured flatfield response for a flight spare array at a number of wavelengths. At 0.8 µm, the most sensitive areas on the array are more than twice as sensitive as the mean, and the least sensitive areas less than half as sensitive (i.e., there is variation by a factor of ~5 in the relative response across the array). This declines to a factor of ~3 at a wavelength of 2.2µm, and at 2.5 µm the array is almost flat. We estimate that the mean uncertainties of the flat field response measurements are ~4%; the accuracy is however largely non-uniform, due to large variations in the response across the detector.

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 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:

• The flatfield response variations are large and wavelength dependent. The difference in response between the most and least sensitive areas is almost a factor of five at the shortest wavelengths and a factor of 1.1 at the longest wavelengths.
• The variation with wavelength is not linear, the largest variations occurring in small wavebands shortward of 1.1 microns and longward of 2.2 microns. The variations in response longward of about 2.2 microns are much more extreme than those shortward of 1.1 microns.
• The arrays exhibit wavelength dependent pixel-to-pixel response variations, ranging from an amplitude of order 10% at the shorter wavelengths to less than our measurement uncertainties at the longest wavelengths. The variation with wavelength of the pixel-to-pixel response variations is almost identical to the behavior of the global flat field response variations. First indications from on-orbit flatfields are that all these characteristics are stable in time, and therefore calibratable with high accuracy. However, the change in temperature from the thermal vacuum experiments to the on-orbit operating conditions requires on-orbit flatfields to be used to calibrate NICMOS data.

Figure 17.5: Responses of Selected Pixels Relative to the Mean of the Array as a Function of Wavelength. Both regions of low sensitivity (top panel) and high sensitivity (bottom panel) are considered. These figures show that the response flattens rapidly longward of 2.2 microns.