Re: Help with guiding the Mach1GTO

Roland Christen


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.


-----Original Message-----
From: Ron Kramer ronkramer1957@... [ap-gto]
To: 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:

Here is a link with screen shots from the PHD lab program
I suspect the backlash comp is to blame. 

I'm not sure what error your screen shot is showing. It looks like normal guiding during relatively poor seeing. The RMS values of tracking are 0.3 to 0.4 arc seconds.

The only question I have is what is the guider rate set at? Is it set at 1x or .5x or .25x? If the guide rate is set to less than 1x, you will have sluggish response, especially in Dec.


-----Original Message-----
From: dandadrumman dandadrumman@... [ap-gto] <ap-gto@...>
To: ap-gto <ap-gto@...>
Sent: Fri, Jan 26, 2018 1:30 pm
Subject: Re: [ap-gto] Help with guiding the Mach1GTO

Here is a link with screen shots from the PHD lab program
I suspect the backlash comp is to blame. 

On Friday, January 26, 2018, 1:37:58 PM EST, Bill Long bill@... [ap-gto] gto@...> wrote:

The method Roland mentions here is good. 

Roland, does it matter if folks are using Resist Switch vs Hysteresis?  

From: ap-gto@... gto@...> on behalf of Dan Pelzel [ap-gto] gto@...>

Sent: Friday, January 26, 2018 10:07 AM
To: ap-gto@...
Subject: Re: [ap-gto] Help with guiding the Mach1GTO
Great advice. Thank you very much! That sounds like exactly what was happening. 
When I get home I’ll upload a screen shot. 

If I can bother another quick question, what are the thoughts of PHDs predictive PEC vs more traditional settings? Specifically if you have PEC enabled on the mount. Would they work together in harmony or fight each other? 

Dan Pelzel

On Jan 26, 2018, at 11:45 AM, chris1011@... [ap-gto] <ap-gto@...> wrote:

First of all, no one is going to wade thru a lot of log files. It would be easier to just look at screen shots of a guider graph to see what is happening.

Second, PHD has a lot of features to help mounts that have a lot of backlash, but in the case of the Mach1, you will have to turn off any and all backlash settings. Because if you don't, I guarantee you will get oscillations on the Dec axis. It will bobble back and forth and never settle down.

Start with a real good calibration run. Make sure that you follow the PHD instructions to the letter when calibrating. Next, turn aggressiveness down all the way and slowly bring it up and watch your guider graph. Somewhere between 50% and 70% aggressiveness the guiding in Dec should settle down and become smooth and accurate. Do the same on RA.


-----Original Message-----
From: dandadrumman@... [ap-gto] <ap-gto@...>
To: ap-gto <>
Sent: Fri, Jan 26, 2018 9:55 am
Subject: [ap-gto] Help with guiding the Mach1GTO

So I have a used (GTOCP4 w/ auto adjusting gearbox) 
I have had it out now 4 or 5 nights and it's worked really well, but I am getting this issue in Dec. It seems like it's just bouncing back and forth. I was seeing big corrections alternating between N and S. At first I thought it was just the area of the sky, but it happened a few times in different parts of the sky. I know normally you want to balance a little camera heavy. I always find the balance shifts a lot depending on where the scope is pointing. So I balance it essentially perfectly with the counterweight down and the OTA pointing East/West. 
Here is a link to the PHD

< /p>
I would be grateful for any insight anyone could give. I have heard people say you should be well balanced no matter where the scope is pointing, but I have never found a way to do that. Should I just go a bit more camera heavy? I used the guiding assistant in PHD to set the backlash, someone mentioned that it might be wise to turn off backlash comp in PHD. 
This is using an 8in RC w/ the ASI1600 camera package w/ an added counterweight upfront since the RC is already backend heavy, especially with the MoonLite and camera. 

Thanks in advance for any advice. 

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