SWG Mounts and MaxMove Parameter

[Ross Salinger]

Do you recommend setting the max move parameter to a value lower than the guiding cadence? Given a guiding cadence of 1 second and a max move of 1000 ms, doesn't that equate to a move of 7.5 arc seconds? Even so, would PHD oscillate if that was the case - cadence faster than max move? 

Rgrds-Ross 

[Bruce Waddington]

Before the arrival of SWG mounts, this was never really a question.  PHD2 only generates max-move guide pulses when the tracking error is abnormally large, when large dithers require it, or when Dec backlash compensation is being used.  Reducing the max-move values has no effect unless one of these conditions arises. AFAICT this has become a question with SWG mounts because the RA tracking accuracy of some of these mounts can be quite poor which is also the reason for using relatively short guide exposures.  So a long guide pulse, particularly in Dec, can allow the RA tracking error to become large because the RA tracking is being "left unattended" for exposure_time + max_move_time seconds.  If you reduce the max-move parameters, you will induce under-correction in the guiding for the situations I mentioned  - I can't picture why it would cause oscillation.  Remember, all of this is serialized, you can't start a guide camera exposure until all the guiding commands have completed.  You have to decide if under-correction is a price you're willing to pay in terms of recovery times for things like dithering or large excursions caused by something in the mount payload.

Regards,

Bruce

[Brian Valente]

I would also add that while many prefer to guide swg mounts with shorter guide exposures/faster cadence, the aggressiveness (therefore actual movements) is usually dialed down so the guide pulse durations are considerably shortened.  

Brian

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Brian 

Brian Valente

astronomy portfolio https://www.astrobin.com/users/bvalente/ 

[Bruce Waddington]

Good point.  Reducing aggressiveness basically continues in the direction of under-correcting.  So I think it comes down to figuring how poor the native RA tracking really is on a particular SWG mount, which means it's hard to do any automatic tuning of these parameters.  It's unfortunately an exercise for the user.

Bruce

[Ross Salinger]

I don't see why you would ever need a 3 second pulse other than to recover from a problem in a well aligned correctly adjusted mount. Slowing down that recovery just means that it will take time to correct, given that corrections are serialized. I can see that if there is a need for such a pulse, then the other axis is not being "attended" to. It's really irrelevant since the error is several arc seconds in the first place and the resulting images are useless. Maybe if you are imaging at 4 arc"/pixel, that sort of correction makes sense. I just don't see why there are so many reports of improved guiding (but I have not yet seen a guide log to document this). I have a very strong (sigh) bias that people are not considering target altitude and/or seeing changes. They are getting mediocre results and keep tinkering until some "tink" shows reduced RMS. 

Rgds-Ross

[Brian Valente]

Ross

Are you looking for feedback on your comments? 

You mentioned some new details there, 4"/pix, 3 second corrections, etc. It kind of sounds like you are talking about a separate discussion you were having somewhere else?

Brian

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