Repeated Pulse Guide failed Message
Philip Keyser
bw_msgboard
Sorry you’re having problems Philip. Judging from the log data, it looks like your mount controller or the associated ASCOM driver is having some problems. Here’s a typical example:
20:24:19:034 PHD2 issues a pulse guide command to move west for 69ms
19:094 PHD2 gets a mount response acknowledging receipt of the command. We now wait an additional 20 ms before checking on the status
19:114 It’s now 80 ms after the guide command was issued but the mount is still moving (guiding)
At this point, we start polling every 20ms, expecting the mount will stop moving
20:24:21.116 It is now 2 seconds since we asked for a 69ms guide pulse – but the ASCOM driver still reports the mount is moving. So we time out and issue the alert message
This is not a fatal event but we don’t know what happened to this guide pulse. Guide commands that follow this one work ok but then we will hit the problem again, typically in less than a minute. So you are still guiding, but a significant number of commands are encountering this problem. You might want to check in with people who are knowledgeable about the mount and its ASCOM driver. Just to be more specific, PHD2 is checking the ‘IsGuiding’ property in the ASCOM driver to determine that the mount is still moving. The PHD2 logic here is applied to all ASCOM mounts, nothing specific about your mount.
Good luck.
Bruce
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Andy Galasso
Bruce Waddington
Bruce
Philip Keyser
bw_msgboard
Yes, switching to ST-4 would eliminate the ASCOM driver use and thus PHD2’s checking of the IsGuiding property. I don’t know how it would affect the mount’s backlash comp behavior. But I will bet you a jelly doughnut that the backlash comp setting in the mount is the source of your trouble… <g> You might want to start by letting the PHD2 guiding assistant measure the backlash to see how big a correction is needed. Let us know how it works out…
Bruce
From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Philip Keyser
Sent: Friday, September 25, 2015
10:44 AM
To: Open PHD Guiding
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Colm Brazel
bw_msgboard
Hi Colm. This is kind of all over the map, so yes, it would be good to do some reading and become more familiar with the app and guiding in general. That said, I’ll try to give short answers to some of your questions.
- The PulseGuide command failed message means your ASCOM scope driver didn’t perform the requested guide command. That, combined with your other problems with the Orion camera, suggest you may have underlying problems with your USB system. This is a common source of problems, often becoming worse in cold weather because of cable problems and low-power.
- The guide speed settings in your mount don’t have the same meaning as the calibration step-size in PHD2 if that’s what you’re talking about. But if you change the guide speed settings, you will always need to recalibrate and may need to choose a different calibration step-size. Look in the help docs to learn how to use the calculator for this.
- Lost star messages need to be tracked down on a case-by-case basis. There can be lots of different reasons including dew, but you should assume the problem is real and probably not a bug in PHD2.
- There is no reason for PHD2 to know the image scale of your main imaging scope. That’s why we ask people to think in terms of arc-secs – an arc-sec is an arc-sec, regardless of the scope.
- You don’t need to re-do a drift alignment based on where your target is. The drift alignment is going to get your physical RA axis pointing close to the celestial pole. One it’s there, you’re done.
Good luck,
Bruce
From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Colm Brazel
Sent: Monday, January 04, 2016
3:14 AM
To: Open PHD Guiding
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Colm Brazel
If you are using an OAG or a self-guiding camera, even though you are guiding and imaging through one scope, it is still worth understanding whether your imaging camera, guide camera and scope work well together or not. (It is unlikely that you will have a problem unless you have a ridiculous difference in pixel sizes between the two, but knowledge is power!)
PHD Guiding can track the centroid (centre point) of a guide star to ‘sub pixel accuracy’. Basically it is able to calculate the position of the guide star down to a fraction of a camera pixel. This leads people to the erroneous conclusion that they don’t need to worry about matching pixel scales of guider and imager because PHD can “track to a 10th of a pixel” or even “a 50th of a pixel” (depending on who you ask).
In theory that is true, but in practice you are not likely to get PHD guiding reliably to more than one quarter of a camera pixel. So a good rule of thumb is your imaging pixel scale should be no more than four times your guiding pixel scale. You might be able to exceed 4x by some margin, but sticking to less than that will make your life much easier.
So how do you calculate your pixel scale?
pixel scale in arcseconds per pixel = (camera pixel size in µm / scope focal length in mm ) x 206.3
So I image with a Canon EOS 500D with pixels that are 4.7µm square and a Skywatcher Evostar 80ED and 0.85x Reducer with an effective focal length of 510mm:
(4.7µm / 510mm) x 206.3 = 1.9 arcseconds per pixel
I guide with a QHY5 with pixels that are 5.2µm square and an Orion ST80 with an effective focal length of 400mm:
(5.6µm / 400mm) x 206.3 = 2.67 arcseconds per pixel
If you don’t feel like doing the maths, use my Imaging Toolbox to do the hard work for you. The upshot is that I my imaging resolution is about one and a half times my guiding resolution (2.67 / 1.9 = 1.41). That is well within the 4 x rule we established above.
Don’t forget, you can use a Barlow lens or a focal reducer to adjust the relative pixel scales of imager and guider. For example, the quality of the image through the guider is not critical so a cheap Barlow can be used cut the number of arcseconds per pixel if needed.
Now there is a lot more that could be said about the optimal pixel size of imaging cameras vs different scopes, but that is another topic. The best advice I have read is that your first priority is to have a scope and camera combination that can fit your target in the frame, and to worry about everything else later! Again using the Imaging Toolbox you can try different combinations of scope and camera to see what works."
bw_msgboard
Hi Colm. I really don’t want to start critiquing other people’s editorials, tutorials, and opinions on guiding. There’s a vast amount of this stuff out there and you need to decide for yourself how much you want to rely on it. And of course, much of it is outdated, such as the one you referred to. In this case, I don’t see any experimental or theoretical data to support this person’s assertions about centroid accuracy. I recommend reading the attached document, which is based on something more solid.
Obviously, there is some practical upper limit to the difference between the two image scales, and imagers have their own opinions about how far they want to push it. The PHD2 step-size calculator can be used to calculate your image scales – just enter whatever values you want for focal length and pixel size. In many cases, a large difference between image scales is the result of imaging at long focal lengths and for those situations people often go to off-axis-guiding to eliminate differential flexure. In the process, of course, the difference in image scales is also eliminated. Plus, most users end up battling mechanical and seeing-related problems that are much larger than any centroid-related errors. Personally, I think these two things explain why this whole topic isn’t a burning issue, at least not for the PHD2 community
Hi Bruce,
1. The PulseGuide command failed message means your ASCOM scope driver didn’t perform the requested guide command. That, combined with your other problems with the Orion camera, suggest you may have underlying problems with your USB system. This is a common source of problems, often becoming worse in cold weather because of cable problems and low-power.
2. The guide speed settings in your mount don’t have the same meaning as the calibration step-size in PHD2 if that’s what you’re talking about. But if you change the guide speed settings, you will always need to recalibrate and may need to choose a different calibration step-size. Look in the help docs to learn how to use the calculator for this.
3. Lost star messages need to be tracked down on a case-by-case basis. There can be lots of different reasons including dew, but you should assume the problem is real and probably not a bug in PHD2.
4. There is no reason for PHD2 to know the image scale of your main imaging scope. That’s why we ask people to think in terms of arc-secs – an arc-sec is an arc-sec, regardless of the scope.
5. You don’t need to re-do a drift alignment based on where your target is. The drift alignment is going to get your physical RA axis pointing close to the celestial pole. One it’s there, you’re done.
Colm Brazel
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Colm Brazel
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bw_msgboard
Hi Colm. I think you will have to make a decision about how you want to do your guiding. First of all, you aren’t talking about ASCOM, you are talking about EQASCOM, which is a completely different thing. If you want to do guiding with PHD2, you need to set your guide speeds in the mount once AND THEN LEAVE THEM ALONE. If you do change them, you will have to re-calibrate in PHD2 and THEN LEAVE THEM ALONE. You can’t use this so-called PulseGuide adjustment feature with PHD2, or at least not if you want to get any help from us. This is a classic example of “too many cooks in the kitchen” – only one app can handle guiding, there’s no way to have some sort of collaboration. The EQASCOM PulseGuide adjustment feature will defeat the guiding algorithms in PHD2, and we can’t support it – you’d just have to figure things out for yourself, I guess.
The guide speed setting we ask for in PHD2 is used primarily to calculate a reasonable calibration step-size. So it doesn’t need to be completely accurate. In your example, just specify a guide speed of 0.5X sidereal.
Bruce
Colm Brazel
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