Vignetting of the Planewave 14" CDK

The setup is composed of the Planewave 14" CDK, a protective T-ring adapter with a 2" UV/IR blocking filter and a Canon 500d (mod.). The flat-field frame analysed is a stack of 64 individual frames (ISO200; calibrated with bias frames; august 2015). In figure 1, beside several 'dust-donuts', vignetting can be seen as it reduces the light flux especially at the corners of the sensor.

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Figure 1 Top: Flat-field frame (stack of 64 frames) enhanced using STF (screen transfer function / Pixinsight).
Bottom: 3D-rendering of the flat-field frame.

This flat-field image is analysed using the script 2DPlot v3.58 [1] (Pixinsight). The pixel values along the diagonal line (bottom-left to top-right) are extracted from the image and saved as a .csv-file. As an acceptable approximation, a polynomial of the 2nd degree is chosen to fit the pixel values (Scidavis). The sensor of the Canon 500d has a size of 3177 px to 4769 px. The diagonal of 5730 px corresponds to a length of 26.93 mm (with a pixel-size of 4.7 μm).

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Figure 2: Normalized pixel values (blue dots) across the diagonal of the flat-field frame. A 'dust-donut' is responsible for the dip near the maximum. The horizontal axis is expressed in mm, being zero at the centre of the sensor. The red curve is the fitting function (polynomial of 2nd degree).

The vignetting is calculated by the following formula: $$ \frac{|{Y-Y_{MAX}}|}{Y_{MAX}} * 100 $$ with YMAX = 0.017047 being the value of the maximum of the fitting function. The resulting vignetting from the bottom-left to the top-right border can be seen in figure 3. The vignetting rises to about 11 % at the corners of the sensor, corresponding to a distance of 13.5 mm from the centre. Furthermore, the minimum is not exactly at the centre of the sensor. This is maybe a hint, that I should check the collimation of the telescope.

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Figure 3: Vignetting of the Planewave 14" CDK in combination with a Canon 500d (mod.) and a protective T-ring adapter holding a 2" UV/IR blocking filter.