# Dither scaling between SGP and PHD

#1

I note that SGP has dither scales as medium; high; very high but what do these settings represent in respect to PHD dither scales??
I calculated the dither scales in PHD so that each of my set-ups has a different setting (say 1.0; 1.5; 3 etc) and this gives me a fairly consistent amount of dithering for all my set ups. (circa 12-15 pixels max)
But what is the equivalent in SGP and can I still use the PHD scaling as well as the SGP one and if so what would the combined scaling be??

#2

DaveB,

The dither sizes (in pixels) are: 0.5, 1.0. 2.0, 3.0, 5.0.

The dither scaling parameter in PHD/PHD2 multiplies the value, and the final dither amount in each direction is a random value between zero and the scaled max value.

Andy

#3

That is guide camera pixels BTW (not imaging camera pixels.) You’ll need to use your pixel scales for the imager and guider to come up with an appropriate dither amount.

Example: guide camera scale = 3.0 arc-sec/pixel
imaging camera scale = 1.0 arc-sec per pixel

If desired dither amount on imaging camera is 4.0 pixels, that is 4.0 px x 1.0 arcsec/px = 4.0 arcsec. 4.0 arcsec on the guide chip is 4.0 arcsec / 3.0 arcsec/px = 1.3 px. So the closest larger dither size would be 2.0 px (“Very High Dither”).

Andy

#4

Ok thanks Andy
Yes I have worked out my imaging/guiding ratio in pixels based on PHD dither of +/- 0.5 & +/-1.5 pixels so I can now cross reference the SGP scales with my own calculated ones so thanks for the Info

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