Re: Help with guiding the Mach1GTO
Howdy,toggle quoted messageShow quoted text
Howard and I had a chance to log in to a customer's Mach1 mount last night to help him with some guiding issues using PHD. He had sent his calibration graph and it looked quite good, but he was having some problems guiding. After doing some mechanical tests we set up parameters in PHD to get the mount to track and respond accurately. Since PHD is somewhat different from MaximDL, which I use, we had to approach the settings a bit differently.
The first step is to do a quick 2 - 3 minute Unguided run using 1 second guide exposures, and look at the guider graph to see what the maximum excursions are in Declination. The guide star will bounce around a certain amount and this peak error will be the seeing that you cannot guide out with normal guide software. This P-V value, which in our case was between +-0.5 and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying to correct for that with guide moves to the mount is impossible (only a fast acting AO system can chase that seeing error).
So, knowing that the minimum seeing error is +-0.8 arc seconds, you will want to set the initial Min Move setting in PHD to be approximately this value as a starting value. We set the Min Move to 0.8 arc sec, the guide rate at 1x and the aggressiveness to 100%. This means that no correction pulses are sent to the mount while the guide star is bobbling within that envelope, but once it exceeds even slightly, the mount gets a full correction command (-0.8 arc sec) to bring it back toward the zero position. We turned guiding on and the result was that the mount responded quickly whenever the error exceeded the bobble limits and overall guiding was tight, accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An exposure with the main camera showed tight round stars.
So, as a starting point:
Determine the amount of guide star bobble above and below the axis
Set the Min Move to that value
Set Guide rate to 1x sidereal
Set the guide star exposure rate to 1 second
Set Aggressiveness to 100%
Begin guiding and note the RMS value for both axes.
You can then change any of the parameters to see if you can tune the RMS to a lower value. We tried longer guide star exposures, but found that the response to tracking errors became more and more sluggish and less accurate. So for these seeing conditions a faster guide rate of 1 per second resulted in the lowest RMS error on both axes. In pristine seeing it might allow longer guide exposures, at least that is what we found using PHD Guiding.
From: Ron Kramer ronkramer1957@... [ap-gto]
Sent: Wed, Jan 31, 2018 8:25 am
Subject: Re: [ap-gto] Help with guiding the Mach1GTO
I had those long alternating pulses - no backlash eliminated them. Do not bother with backlash test in guiding assistant. I kinda don't have much faith in any of that assistance with this mount.
Mine just started withing great with a clean PHD install. ODDLY - the default for both was resist switch and not hysteresis as was the case with my previous version install. Once I'm sure of consistent results I might do some comparisons with hysteresis again, but really I don't want to mess with whats working at this point. total rms .3 acrc sec is pretty pleasing. Actually under .5 is adequate for me.
On Fri, Jan 26, 2018 at 3:24 PM, chris1011@... [ap-gto] <ap-gto@...> wrote: