Log Book Entry No.16

Days, Calendar Created: 24 Sep 2013 17:09:04 Days, Calendar Last Revised: 27 Sep 2013 13:31:23 Person PedroPaiva Text Focus Study (Pt II)

Focus Theory
Basic FWHM and MaxPixel techniques
Diffraction aids

Using a Bahtinov mask to find the focus:

Based on Laurie's idea and this article a set of 144 images were taken.
3 images for each of the 48 moves made by the focuser. All the movements were 'in' moves of 250 steps.
The images had a 5s exposure time and a master dark (made of 10 dark) was made and reduced from the images.

On each image, a gaussian was fitted to the brightest star (142/144 success) and the amplitude and sigma median were taken for each set of 3 images per focuser position. The data was then plotted and assumed to be in a quadratic form. The results can be seen on the images below.




For the Sigma X values, the minimum can be found at: 4841 steps
While Sigma Y minimum is at: 5411 steps
And the Amplitude maximum at: 6804 steps

According to LogBookAstronomyEntry0015, the 2000 step (aprox 600Ám) difference would be more than double the depth of field for the telescope (287Ám on a f/10).
A visual analysis of the graphs would suggest a minimum and a maximum for Sigma Y and the Amplitude close to 6000 steps and, visually, the image at step 5750 (image set 23) is the sharpest presenting a nice 'X' diffraction pattern produced by the Bahtinov mask.

Back to: LogBookAstronomy

  • Animation showing star going into and out of focus (with Bahtinov mask):
Subject Focus Study (Pt II)
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GIFgif focus_stern.gif r1 manage 5064.8 K 27 Sep 2013 - 13:31 PedroPaiva Animation showing star going into and out of focus (with Bahtinov mask)
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Topic revision: r4 - 27 Sep 2013 - PedroPaiva

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