PHD and PHD2 will accept same dither size levels from various imaging programs. Each of 5 Levels of dither aggressiveness corresponds to a number of pixels as shown in the table below. Additionally, this number of pixels will get multiplied by the number specified in dither scale in PHD's brain settings.

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Dither Level   |   Pixels

---------------       ---------

1       -             +/-  0.5  x  Dither Scale   

2       -             +/-  1.0  x  Dither Scale

3       -             +/-  2.0  x  Dither Scale  

4       -             +/-  3.0  x  Dither Scale

5       -             +/-  5.0  x  Dither Scale

 

Since dithering happens on the Guider and not on the Imager, we need to calculate the difference between both of the imaging scales. For that we simply divide our Guider Scale by the Imager scale. In my example it would be 0.85 / 0.44 = 2. Simply means, that if star moves 3 pixels on the Guider it will move 3 x 2 = 6 pixels on the Imager.

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Let's say I decided that I want to dither 30 pixels on the Imager, so I need that on the Guider it will move for 30 / 2 = 15 pixels. Now, according to the table above, in my imaging software I would select Aggressiveness Level 5, that would correspond to 5 pixels and in PHD's Dither Scale I would put 3, so the actual amount of pixels on the Guider would be 5 x 3 = 15 pixels.