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Tutorial on how to calculate Minimum Move parameter in PHD.


In this tutorial I want to explain how to calculate the Minimum Move parameter in PHD for your specific setup. The default is set to 0.15 pixel which in some cases maybe too tight and in the others - too relaxed.


(I have to mention that in PHD2 you can also change the algorithm for Declination axis and have different parameter there if your guider have not square pixels.)


So, instead of guessing by trial and error, this number can be calculated. First thing we would need to know is what our Imager Image Scale and Guider Image Scale are.


For this we would use this formula:



Image Scale = ( 206 x Pixel Size (µm) ) / Focal Length (mm)

Let's do an example. My Imaging scope is 2000 mm Focal Length and since I'm guiding with Off Axis Guider my Guiding Scope is also 2000 mm. My Imaging camera is Canon T3i which have pixel size of 4.3 µm. My Guider camera is Lodestar Express which doesn't have square pixels and they corresponds to 8.3 µm height and 8.6 µm width.


Since I want more sensisitivty in RA axis I place my Lodestar so, that the smaller pixel side of 8.3 µ will be aligned with RA axis and for calculations I'm going to use this number:


Imager Scale = ( 206 x 4.3 ) / 2000 = 0.44 arcsex/pixel


Guider Scale = ( 206 x 8.3 ) / 2000 = 0.85 arcsec/pixel


Next, we need to calculate the Total Allowable Error (TAE), which is the safe amount of movement of the star on the Guider, before it will be noticable in the Imager. Personally, I want my total guiding error to be 80% of the Imager Scale,  doing so I ensure that my guiding error would be just below the imaging resolution. For this we going to do:


TAE = Imager Scale x % / 100


( Note: If you would like to use 100% of the Imaging Scale as TAE, then the simple formula is TAE = Imager Scale  / Guider Scale )


So, with 80% of the Imaging Scale my TAE would be:


TAE = 0.44 x 80 / 100 = 0.35 arcsec/pixel


After we got our TAE in arcseconds to pixel, we need to know how much that would be in terms of pixels on the Guider Scale, since PHD's Minimum Move parameter in Pixels.

TAE (pixels) = TAE arcsec/pixels / Guider Scale arcsec/pixels


From that I have :


TAE (pixels) = 0.35 / 0.85 =~ 0.4 pixels


My Total Allowable error would be 0.41 pixels on the Guider's sensor.

After we got our TAE in pixels, we must remember, that the Minimum Move setting in PHD is the radius.



Minimum Move = TAE / 2


In my case , Minimum Move = 0.4 / 2 = 0.2 pixels.

Couple of words about this setting.

This calculation is for perfect world which we are not living in. This will get affected by the seeing from your imaging site. So, after you set your calculated number, you have to check your guiding graph and if seeing is really bad, you'll have to relax this setting accordingly, to compensate for more smoother guiding.

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