Re: Spikes in Dec

Sébastien Doré

Very interesting. Posted that over the openPHD forum as well. 
Will let know how that turns out.


De : <> de la part de Roland Christen via <chris1011@...>
Envoyé : 6 mai 2021 18:19
À : <>
Objet : Re: [ap-gto] Spikes in Dec
Basically the way RA guiding happens is the same as a non-encoder mount, except that the guide software doesn't have to fight a periodic error. So the guide pulses can be sent at a slower cadence. (cadence is not equal to guide star exposure).

Declination is definitely different in an Astro-Physics encoder mount. Reversal backlash is gone, so that a 1 arc sec command in the reverse direction will cause the axis to move 1 arc sec in that reverse direction. A non-encoder mount sometimes needs many 1 arc sec move commands before the Dec axis actually reverses direction. That's because the motor shaft must move a significant rotational distance (number of arc seconds) in reverse before the worm wheel begins to move. It could be 5 arc seconds, or 10 or even more on some mounts. It all depends how tight the gearing is and the clearances in the reduction gears or the amount a belt stretches in a belt drive mount. It's never zero in any mount.

There is a caveat however for the 1 arc sec reversal in an encoder mount. The encoder does not eliminate the actual mechanical delay, or the distance it takes for the reversal to happen. The motor shaft still needs to move 5 or 10 arc seconds at the 1x rate, but the encoder makes that happen automatically after only one move command. It means that any move command in Dec might take a second of time if it is a reversal move, rather than being instant. Therefore the best way to guide in Dec is to let the axis settle for a second or two before taking another guide exposure and sending another guide pulse. Too fast cadence will cause the tracking to be less accurate than it could be. Adding a 2 second delay between guide exposures is a good way to to keep the Dec axis stable and prevent hunting and chasing the seeing.

It would even be better if PHD would wait 1 second after a move command before actually taking the exposure of the guide star so that the axis has a chance to reverse and fully settle. But perhaps that is not in the cards at this time. Maxim DL does insert a small delay of that size after issuing a guide command, and so I have found that it will guide very accurately even at the longest focal lengths.


-----Original Message-----
From: Brian Valente <bvalente@...>
Sent: Thu, May 6, 2021 4:58 pm
Subject: Re: [ap-gto] Spikes in Dec

You want to enable DEC compensation here

This adjusts the guiding based on your sky position, which doesn't change (unless you are going to say you have a space telescope)

>>>I realize this is more a question for the PHD forum, (sorry about that) but I thought it could be of general interest here as well if you have the answer at hand.

that's fine - happy to help. Maybe you can shoot me some of your logs direct via email and I can review them. 

I'm not sure I saw what Howard mentioned, but guiding with high quality encoders is a relatively new thing, so anything is possible. lowpass2 was introduced only a few years ago iirc


On Thu, May 6, 2021 at 1:45 PM Seb@stro <sebastiendore1@...> wrote:
Hi Brian,

For the DEC, yes, you recalled right, Lowpass2. For RA, I have Hysterersis selected. 

I see there's also the possibility to select lowpass2 for RA, but I believe I read somewhere not to use it for that axis (possibly in the PHD2 manual). Anyway, I didn't seem to have any guiding problems in RA, so I'd leave it there for now.

Just looking around at other settings, I realize that I have the "Use DEC compensation" checked in the Guiding tab,  Calibration section. Help file says that it usually should stay checked unless in "unsual" cases, like when a mount's controller would apply a compensation automatically. Should I assume that a mount with abs encoders correspond to that definition and uncheck the box? Do you think, this could explain the issue reported by Howard/Roland ?

I realize this is more a question for the PHD forum, (sorry about that) but I thought it could be of general interest here as well if you have the answer at hand.


De : <> de la part de Brian Valente <bvalente@...>
Envoyé : 6 mai 2021 17:01
À : <>
Objet : Re: [ap-gto] Spikes in Dec
Seb i seem to recall you using lowpass2 in PHD which is the correct algorithm for high res encoders


Brian Valente

Roland Christen

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