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J. Eur. Opt. Society-Rapid Publ. 21, 20( 2025)
Figure
6. Waterfall plot of the MTF curves of repeated focus scans displaying the shift of the LSF maximum after changing the ambient temperature. Left: tangential slit object orientation, right: sagittal slit object orientation. Coordinates given refer to the measurement setup coordinate system( cf. Fig. 1).
Figure
7. Maximum MTF value of the repeated focus scans obtained from polynomial fit of the measurement data. Top: tangential slit object orientation, bottom: sagittal slit object orientation. Coordinates given refer to the measurement setup coordinate system( cf. Fig. 1).
Also in the z direction( i. e. along the optical axis), a shift of the maximum position can be observed. It reaches a maximum deflection of about 3.5 lm approximately 2 h after changing the ambient temperature. Owing to the vanishing contribution of the x component shift, the MTF maximum deflection decreases slightly and reaches equilibrium after about 10 h.
3.3 Effect on the MTF value at the focus point
To assess the effect of the focus point shift on the MTF value two different cases are taken into account: The absolute MTF maximum value determined from the polynomial fit of the measurement data does not change significantly during the repeated focus scans( Fig. 7).
However, the repeated focus scans are started using the previously determined focus point position as the scan center. Thus, at the center position of the initial focus scan the maximum MTF value is obtained only in the case the focus point position does not change significantly. As the focus point moves owing to the temperature change, so does the MTF versus z curve. Thus, the MTF value detected at the initially set center position regarded as focus point decreases, as shown in Figure 8. In comparison to the absolute MTF value for the evaluation frequency obtained from polynomial fit, the changes due to the focal point shift are an order of magnitude greater than the changes in the absolute value due to the slightly altered beam path. Although relatively small, this still requires the ambient temperature to be kept stable for reliable measurement of the MTF.