Re: Guiding AP mounts with PHD

Roland Christen

It's my understanding that the mount is going to track well, assuming that
PEM is programmed correctly and enabled, and that guiding should basically
be correcting any drift due to slight polar misalignment, refraction,
The problem is not in RA where the axis does not have to reverse direction.


-----Original Message-----
From: 'Wade Hilmo' y.groups@... [ap-gto]
To: ap-gto
Sent: Thu, Feb 1, 2018 5:44 pm
Subject: RE: [ap-gto] Guiding AP mounts with PHD

I am curious about this result, especially the part at the end where you say
that 1 second exposures worked well, but longer exposures "became more and
more sluggish and less accurate".

It's my understanding that the mount is going to track well, assuming that
PEM is programmed correctly and enabled, and that guiding should basically
be correcting any drift due to slight polar misalignment, refraction,
flexure, etc. If my understanding is correct, it would seem that longer
exposures (within reason) would be just as effective as shorter exposures -
perhaps more effective, since longer exposures would tend to average out
seeing distortion in the individual guider subs. If you were getting better
results at 1 second, that would suggest that you were chasing seeing with
some success.

Do you understand why it worked the way that it did for you? Was it
correcting for slower seeing effects? Is there some other reason?



From: ap-gto@... [mailto:ap-gto@...]
Sent: Thursday, February 1, 2018 9:36 AM
To: ap-gto@...
Subject: Re: [ap-gto] Guiding AP mounts with PHD

Great info. A few questions:

- You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in the
PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would
have been 0.8 pixels. Did you guys just do the math to figure out the
correct pixel value?

- Were these changes made on both axis in PHD2? MinMo and Aggressiveness can
be configured per axis.

The rest of this looks very interesting and as soon as the weather gets
better I plan to test this and see how my guiding performs.


From: ap-gto@... gto@...> on behalf of
chris1011@... [ap-gto] gto@...>
Sent: Wednesday, January 31, 2018 4:51 PM
To: ap-gto@...; ap-ug@...
Subject: [ap-gto] Guiding AP mounts with PHD


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.


Posted by: "Wade Hilmo" <wade@...>

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