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I found an interesting post in Cloudy Nights which basically sums ups PHD2 , EQMOD , guide speed settings and their affect on guiding with Ascom mounts
“Guide rate is the fraction of the sidereal rate that is used for guiding moves. For DEC, this is simple. If your guide rate is 0.5x, then the guide pulses will be at half of the sidereal rate. A rate of 0.9x would guide at 90% of the sidereal rate. For RA it is a bit more complicated since the RA axis is already moving at sidereal speed. Here, if your guide rate is 0.5x, it means that the RA guide rate will change by half of the sidereal rate. So a pulse to the west will move the mount at 1.5x the sidereal rate, and a pulse east will move it at 0.5x the sidereal rate. Similarly, if your guide rate is 0.9x, a west pulse would be at 1.9x sidereal, and a pulse east would be at 0.1x sidereal.
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There is no relationship between guide rate and guiding aggressiveness. The aggressiveness setting determines the amount of correction to send to the mount. The guide rate lets PHD2 translate that amount of correction into a pulse length. Guide rate will not affect the amount of correction needed, only the amount of time the correction takes. For example, if PHD2 determines that a 0.5 arc-second move is needed to the north, it calculates the pulse length needed based on the guide rate. If you are guiding at 0.5x, that translates into 7.5 arc-seconds per second. To make a 0.5 arc-second correction, PHD2 would need to send a pulse that is 7.5/0.5 = 67ms in duration. If your guide rate is 0.9x, PHD2 would calculate a pulse length of 37ms*to make the same correction.
If you're using ASCOM guiding (if you're not, you should be for this very reason) don't worry to much about guide rate. You typically can set it as high as you want up to the point of inducing mechanical bounce. When people ask me about this, I typically tell them to start at 0.5x and work your was up until you notice a degradation in guiding, or until you reach your mounts guide rate range limit - whichever comes first. If you don't want to do that, just start at 0.5x and leave it there. You are not likely to see mechanical bounce at that rate.
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Finally, when using ASCOM guiding, the guide rate should have little to no effect on guiding performance provided you aren't seeing bounce. Even long pulses will almost always be completed before the next guide image is taken since even long pulses are still only a fraction of a second long. As a result, the correction is almost always in place before the next guide image is taken. So, no harm, no foul regarding guiding performance. However, a higher guide rate can produce better images even when the guiding is unchanged. That's because the corrections happen faster so the imaging sensor is collecting less time with the tracking deviated. However, this effect will likely be tiny and not noticeable. If you are having guiding problems, there are at least half a dozen other genuine gremlins you should be chasing before you even consider changing the guide rate”
Sort of got my head around the guide speed settings issue now, so in the end not super critical for guiding performance, there are many other factors more important relating to good or bad guiding
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