The corrector is optimized for F4.5 so with my 13" F4.0 Newtonian it was expected that its optimal position would not be at the designed spacing of 55mm. (The T2 focus distance). As an experiment I mounted a variable length T2 ring between camera and corrector and made a series of exposures. (at about 1mm increments).
The images were dark and flat field calibrated but not further processed. Stars in the center and perimeter of the field were identified and marked in all images. Cropped areas with the stars were taken from each image and pasted together.
Here are 3 images covering the extreme positions and one near the best
The left image shows that coma is not fully corrected at this distance. The center is pretty good. The right image shows that stars are still somewhat round at this overcorrected position, but are very fuzzy.
I also tried a quantitative approach of analyzing the data. I looked at the fwhm in X and Y direction and calculated some sort of diagonal extend of the stars.
Z= (fwhm_X^2 + fwhm_Y^2) ^ 0.5
Additionally I plotted the brightness change of the star relative to a reference position.
Here are the plots. The X-axis is a measured length of the T2 extension. A dimension of 23.1mm has to be added for the optical path in the CCD camera.
There is lots of noise in the data. The extend of the stars seems to go through s weak minimum at around 30mm to 31mm and then to take off at about 34.5mm. The maximum counts plot is very noisy. It seems as if there are three maxima at 26.5mm, 29mm-30mm and 33mm-34mm. Beyond 34mm the curve finally turns downward.
After this analysis I set the length of the T2 extension to 31mm.
Copyright images and processing Gert Gottschalk.
Copyright Bilder und Verarbeitung Gert Gottschalk.
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