Hi Peter,
I think you like you, I cannot see it as being a problem, I would have thought it better to use PEC than not. Here is an excerpt form the EQMOD guiding section.
Do PEC and AutoGuidng fight?
Running PEC and autoguiding simultaneously should, in theory, deliver the best of both worlds. The PEC
keeps the mount on track. The only errors observed are non periodic ones which the autoguiding makes
corrections for. Many folks are successfully using PEC and autoguiding.
So where does the widely held opinion that PEC and Autoguiding fight come from? The answer lies in how
ST-4 and/or PulseGuiding has actually been implemented for your particular mount – here's why:
Consider the case where PEC is doing its job and keeping the guide star centred. To do this
PEC is overriding the mounts tracking rate. The Autoguiding software however is totally
unaware of any rate corrections made by PEC it assumes that the mount is tracking perfectly by
itself.
Now consider what happens if a “non periodic” error occurs. The Autoguiding software see the
error and when it deems it significant will calculate a pulse duration needed to correct it.
Remember this correction is being made in response to an error observed with PEC already
active and potentially already making a significant correction of its own. The Autoguiding
software issues its correction by setting the appropriate ST-4 signal. This is where it all goes
wrong. As soon as the mount sees the ST-4 override it moves at a fixed “preset guiding rate”.
Any existing speed correction made by PEC is immediately lost and, because the autoguiding
software has no knowledge of PEC, the ST-4 correction made will not be sufficient to hold the
guide star on track. Effectively we have introduced a step change into the closed loop
autoguiding system. In due course the autoguiding software should be able to recover the
situation by making further corrections but now it is having to correct for periodic error as well.
Eventually the guidestar is brought back on target at which point control passes back to PEC.
Some folks assume that all that is happening is that PEC is lost for a short period whilst autoguiding takes
over and that having PEC operating 80% of the time must be better than not having it at all. What they fail to
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The EQMOD Project
EQASCOM & AutoGuiding
take into account is the level of disturbance to the control system that occurs whenever ST-4 corrections are
made. This can result in a greater error than would have occurred if simply using autoguiding alone.
Although ST-4 is used in the example above the same problem could also apply to ASCOM Pulse Guiding.
The key is in the implementation. In order for Guiding and PEC to operate together the guiding rate
corrections must be made as relative corrections to the PEC tracking rate rather than as absolute rate
overrides.
If your mount/driver cannot provide relative corrections then PEC+Guiding is not a useful combination (those
selling PEC applications may try to convince you otherwise, but then they would wouldn't they!).
For those using the EQMOD EASCOM driver the good news is that the Pulse Guiding implementation
has been carefully designed to work alongside PEC. I would not however advise the use of PEC with
ST-4 guiding for the Synta mounts.
Thanks for dropping in and having a say,
Bart. 