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Over on the Yahoo Stark labs group, there has been some interesting discussions on guide focal lengths v's imaging focal lengths.
With PHD it comes down to arc sec/ pixel for both systems, and the general suggestion is that the ratio between them should be "optimised" around 5...this means if your guider camera is seeing 2 arc sec per pixel then this would be good to guide your imager at 0.4 arc sec per pixel.
You can use the CCDCalc freeware etc. to get the various numbers.
HTH
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