Apparent loss of tracking after dither command


Peter Bresler
 

I was shooting with SGP and PHD2; things seemed to be going well for about 18 4 minute exposures. I had selected "dither" for PHD2. On the 19th shot after the dither signal the mount started drifting badly and I had to stop the sequence. Same thing happened again when I restarted, but after I recentered and unchecked dither things went well again with good guiding. What is going on?


Dale Ghent
 

It’s hard to say what happened with such a superficial description. Did PHD2 start complaining about the loss of lock on the guide star? It would have been flashing its UI and displaying a warning to that effect. Did prior dither operations succeed? Usually one dithers every 1 to 5, at most, exposures so there should have been a few dithers happening somewhere in between those prior shots... dithers that I presume went off without a hitch?

The key and basic thing to know here is whether PHD2 lost its guide star or not, an event that it gripes loudly about. 

On Feb 24, 2020, at 02:58, Peter Bresler via Groups.Io <PABresler@...> wrote:

I was shooting with SGP and PHD2; things seemed to be going well for about 18 4 minute exposures. I had selected "dither" for PHD2. On the 19th shot after the dither signal the mount started drifting badly and I had to stop the sequence. Same thing happened again when I restarted, but after I recentered and unchecked dither things went well again with good guiding. What is going on?


 

Hi Peter

your best bet to track this down is to head over to the PHD forums and post your guidelog there with the above explanation. That can be hunted down pretty quickly. My guess is it's unrelated to the dither, but the guidelogs will tell (FYI guidelogs are automatically captured for you)


On Sun, Feb 23, 2020 at 11:58 PM Peter Bresler via Groups.Io <PABresler=aol.com@groups.io> wrote:
I was shooting with SGP and PHD2; things seemed to be going well for about 18 4 minute exposures. I had selected "dither" for PHD2. On the 19th shot after the dither signal the mount started drifting badly and I had to stop the sequence. Same thing happened again when I restarted, but after I recentered and unchecked dither things went well again with good guiding. What is going on?



--
Brian 



Brian Valente


Peter Bresler
 

There were problems with retaining the guide scar, although guiding had been working for quite a while. In any case I expect that the mount would keep tracking, but that did fail. When I turned off dither and recentered with a plate solve things were ok.


 

if you slew, guiding will stop. that's what it looks like happened here from your screen shot, but again you can get more details at PHD forums if you want to investigate further


On Mon, Feb 24, 2020 at 9:35 AM Peter Bresler via Groups.Io <PABresler=aol.com@groups.io> wrote:
There were problems with retaining the guide scar, although guiding had been working for quite a while. In any case I expect that the mount would keep tracking, but that did fail. When I turned off dither and recentered with a plate solve things were ok.



--
Brian 



Brian Valente


Dale Ghent
 

Assuming that the screenshot you provided was taken right after the erroneous event, the reason why guiding stopped in plainly-stated: Something external to PHD2 commanded the mount to slew while PHD2 was still guiding. In such a case, PHD2 will stop guiding and post a complaint.

You mentioned that you turned dithering on in SGP. Dithering is an operation that's normally handled by PHD2 - the sequencing application pauses between exposures, and through its direct connection to PHD2, orders PHD2 to begin a dither operation using either the provided parameters or PHD2's defaults. PHD2 executes this by sending pulse guide commands to the mount to move the field of view a few pixels in a random direction, then reacquires the guide star and resumes guiding. The sequence application observes through its direct connection to PHD2 that the dither op is completed, then starts the next exposure.

I don't know if this option is in SGP, but there is a "manual dither" feature that's common in sequencing apps, typically employed when there is no guide camera and no guiding app such as PHD2 operating, such as with an unguided setup. In such a case, it is the sequence application that tells the mount to move a little to effect the dither. Could it be that you have this feature on in SGP, and thus SGP is not coordinating the dithers through PHD2 at all? If this were the case, PHD2 would then see the guide star and mount move unexpectedly and produce the error you see in the screenshot you provided. This could be one possible explanation.

You didn't explain if you have had previous dither operations execute and succeed during your sequence. Doing your first dither 18 shots into a sequence is pretty late in the game to start doing it as, like I said, one normally sets up dithering to happen every 1-5 shots and no less frequent than that.

I would take Brian's advice and take this up on either the PHD2 or SGP forums.

On Feb 24, 2020, at 12:35 PM, Peter Bresler via Groups.Io <PABresler=aol.com@groups.io> wrote:

There were problems with retaining the guide scar, although guiding had been working for quite a while. In any case I expect that the mount would keep tracking, but that did fail. When I turned off dither and recentered with a plate solve things were ok.

<guiding 2_23.jpg>


Roland Christen
 

It appears that RA is oscillating quite a bit. Possible reasons:

1) wrong calibration values - highly likely
2) mount is not balanced in RA - somewhat likely

Unfortunately your graph doesn't show the guide corrections which might help determining the cause. I would run Guide Assistant, there is a lot of information generated which will help you in your settings. The RA axis should not oscillate, unless the calibration values are way wrong for that part of the sky. It's happened to me when I fed the wrong focal length and pixel scale info to the guide program and had wild oscillations that did not go away until i reduced the aggressiveness to 10%. That was the clue that I had entered something wrong.

Rolando


-----Original Message-----
From: Peter Bresler via Groups.Io <PABresler@...>
To: main <main@ap-gto.groups.io>
Sent: Mon, Feb 24, 2020 11:35 am
Subject: Re: [ap-gto] Apparent loss of tracking after dither command

There were problems with retaining the guide scar, although guiding had been working for quite a while. In any case I expect that the mount would keep tracking, but that did fail. When I turned off dither and recentered with a plate solve things were ok.


Suresh Mohan
 

Roland !!,
        Nice to know you use phd 2 for guiding , it’s a vastly improved one now . The only other I’ve used is maxim dl however the former is more user friendly 


On 25-Feb-2020, at 12:48 AM, uncarollo2 <chris1011@...> via Groups.Io <chris1011@...> wrote:


It appears that RA is oscillating quite a bit. Possible reasons:

1) wrong calibration values - highly likely
2) mount is not balanced in RA - somewhat likely

Unfortunately your graph doesn't show the guide corrections which might help determining the cause. I would run Guide Assistant, there is a lot of information generated which will help you in your settings. The RA axis should not oscillate, unless the calibration values are way wrong for that part of the sky. It's happened to me when I fed the wrong focal length and pixel scale info to the guide program and had wild oscillations that did not go away until i reduced the aggressiveness to 10%. That was the clue that I had entered something wrong.

Rolando


-----Original Message-----
From: Peter Bresler via Groups.Io <PABresler@...>
To: main <main@ap-gto.groups.io>
Sent: Mon, Feb 24, 2020 11:35 am
Subject: Re: [ap-gto] Apparent loss of tracking after dither command

There were problems with retaining the guide scar, although guiding had been working for quite a while. In any case I expect that the mount would keep tracking, but that did fail. When I turned off dither and recentered with a plate solve things were ok.