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Bad tracking with great polar alignment?

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#1 stobiewankenobi

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Posted 10 August 2020 - 01:04 AM

After a very frustrating night, I turn to my friends here in hopes of guidance. 

 

I had (according to sharpcap) "good" polar alignment (not sure if they rate higher than that in Sharpcap). However with good polar alignment, I could not image for more than 4s without star streaks. When opening up PHD2, I see massive movement on the graph. Some images below. 

 

PHD Guiding Image

PHD2 Guiding Image 2

Sharpcap showing good polar alignment

 

 

At first I assumed I must be super off balance in terms of weight on my mount, but after triple checking my equipment was perfectly balanced. 

 

Equipment in question

Celestron CGX

Williams Optic Zenithstar 61

QHY 247c

Atik EFW 2.2 Filter Wheel

 

 

So the total weight is very very low, additionally I have used this same equipment many nights without issue. 

 

Any thoughts as to what is going on here?

 

Thanks!



#2 Stelios

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Posted 10 August 2020 - 01:49 AM

Did you calibrate?

 

You are either not calibrated, or your mount is not tracking. I suspect the former. 

 

Polar alignment is one thing (I get mine to excellent, under 10 arcseconds--then do a 2nd pass for good measure). But regardless, tracking in RA has nothing to do with polar alignment. Most of tracking error is periodic error, and additionally you have wind, seeing, etc. 

 

In the first PHD picture you have selected *too bright* a star to guide on. Always use "Auto-select star." Good stars have single pointed profile and good SNR. Any saturated star (wide flat top) is a total no-no.



#3 stobiewankenobi

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Posted 10 August 2020 - 01:52 AM

Thanks for the reply!

So PHD was stating that things were calibrated but I'm guessing maybe a recalibration is in order? I hadn't messed with it since a few days ago and it was working fine then.

Seems odd Id need a sudden recalibration?

#4 sg6

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Posted 10 August 2020 - 02:06 AM

Read a few where Sharpcap was saying Good, Excellent whatever and the person had poor tracking.

Will ask the same question: Did you ever just look up the polar scope to check the polar alignment?

 

I know that all apps are perfect to the last bit of code, but strange therefore that many apps seem to have problems.

The god of software, JavaC, may not be smiling on you.

 

Whatever it is but with a ZS61 something must be way out, they are literally a pocket sized scope. You should be able to image for quite a bit over 4s with a fixed tripod and have no streaks with one of those.


Edited by sg6, 10 August 2020 - 02:09 AM.


#5 Alex McConahay

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Posted 10 August 2020 - 08:36 AM

Poor polar alignment takes a bit of time to show up. If you are showing streaks and such in four seconds, you have to look elsewhere. 

 

I would suggest you start by turning off PHD2. After polar alignment, point your scope at a bright star near meridian/celestial equator, and start a long exposure (maybe two minutes). Look at the image......

 

Is it streaked? Which direction--North/South, or East-West? Something in between?

 

Is the streak a streak, or a jagged line?

 

 

What do you see?

 

If it is north-south, you do not have good polar alignment.

 

If it is east-west, you may (or may not) have polar alignment, but you definitely have drive speed problems, including the mount not running, gears slipping, (or even the wrong North-South switch setting). 

 

If it is in between, you probably have both problems, or slippage in both axes. 

 

If it is a jagged line, or if the star is brighter in one part of the image compared to another, then you have dirt in the gears, slippage, power problems, or other weird things. 

 

If it is not streaked (but a relatively pinpoint star), then turn on PHD2, recalibrate,  and try it all again. If it is now streaked, you can figure it is in the way PHD2 runs the mount. And start to trace the problem there.  

 

Alex


Edited by Alex McConahay, 10 August 2020 - 08:38 AM.

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#6 stobiewankenobi

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Posted 10 August 2020 - 01:13 PM

Read a few where Sharpcap was saying Good, Excellent whatever and the person had poor tracking.

Will ask the same question: Did you ever just look up the polar scope to check the polar alignment?

 

I know that all apps are perfect to the last bit of code, but strange therefore that many apps seem to have problems.

The god of software, JavaC, may not be smiling on you.

 

Whatever it is but with a ZS61 something must be way out, they are literally a pocket sized scope. You should be able to image for quite a bit over 4s with a fixed tripod and have no streaks with one of those.

What? You mean all software isn't perfect? Lollol, as someone who primarily builds software, I am shocked. My code is always flawless and totally bug free on the first try. Definitely. (but this is good advice for sure). 

 

This particular mount does not have a polar scope, nor a way to use one without removing pieces of the mount (which is annoying as I also can't use my polemaster). 

 

Poor polar alignment takes a bit of time to show up. If you are showing streaks and such in four seconds, you have to look elsewhere. 

 

I would suggest you start by turning off PHD2. After polar alignment, point your scope at a bright star near meridian/celestial equator, and start a long exposure (maybe two minutes). Look at the image......

 

Is it streaked? Which direction--North/South, or East-West? Something in between?

 

Is the streak a streak, or a jagged line?

 

 

What do you see?

 

If it is north-south, you do not have good polar alignment.

 

If it is east-west, you may (or may not) have polar alignment, but you definitely have drive speed problems, including the mount not running, gears slipping, (or even the wrong North-South switch setting). 

 

If it is in between, you probably have both problems, or slippage in both axes. 

 

If it is a jagged line, or if the star is brighter in one part of the image compared to another, then you have dirt in the gears, slippage, power problems, or other weird things. 

 

If it is not streaked (but a relatively pinpoint star), then turn on PHD2, recalibrate,  and try it all again. If it is now streaked, you can figure it is in the way PHD2 runs the mount. And start to trace the problem there.  

 

Alex

This is super super helpful actually to narrow down what the issue is. I will run through this troubleshooting tonight, thank you for the reply. 



#7 OhmEye

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Posted 10 August 2020 - 02:26 PM

PHD2 itself can tell you how far off the polar alignment is, as well as help you drift align. It's useful for a second opinion after doing a PA with other software such as sharpcap or polemaster, and seems to be relatively accurate given it's based on calibrated measurement of actual guide star drift.


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#8 Madratter

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Posted 10 August 2020 - 02:41 PM

I agree with Stelios that it very much looks like a calibration problem. PHD2 is detecting an error, and driving the mount in the wrong direction. That shows up as an even bigger error on the next go around.

 

I'm going to assume the mount was slewing properly or you would have noticed that. I'm also going to assume the mount was tracking. That assumption is based on the PHD2 images you showed.



#9 sn2006gy

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Posted 10 August 2020 - 02:49 PM

Make sure in PHD2 that you have your focal length set to 360mm and not accidentally using your aperture 

 

I and many others made the same mistake of putting my aperture in there vs focal length of guide scope and well, it tracks ok but it says you're off by 1-2" ;)


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#10 kathyastro

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Posted 10 August 2020 - 03:18 PM

PHD2 is showing that the mount is not responding sensibly.  It is trying bigger and bigger corrections, and the mount keeps going farther and farther the wrong way.  In the first example, it is only RA that is messed up.  In the second example, both axes are messed up.

 

Are you using an ST-4 cable?  If so, it or one of the connectors into which it is plugged may be bad.  One more reason to use ASCOM pulse guiding.

 

Your calibration could be bad.  If you rotated the guide camera relative to the mount/OTA, that would cause it.  Any relative movement of guide camera and mount requires a recalibration.

 

The mount may be broken and not responding to guiding inputs.  If the clutches ware loose, that would produce similar symptoms.



#11 stobiewankenobi

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Posted 10 August 2020 - 03:58 PM

PHD2 is showing that the mount is not responding sensibly.  It is trying bigger and bigger corrections, and the mount keeps going farther and farther the wrong way.  In the first example, it is only RA that is messed up.  In the second example, both axes are messed up.

 

Are you using an ST-4 cable?  If so, it or one of the connectors into which it is plugged may be bad.  One more reason to use ASCOM pulse guiding.

 

Your calibration could be bad.  If you rotated the guide camera relative to the mount/OTA, that would cause it.  Any relative movement of guide camera and mount requires a recalibration.

 

The mount may be broken and not responding to guiding inputs.  If the clutches ware loose, that would produce similar symptoms.

This is interesting, you think it could be a bad cable? I know PHD2 says if it fails a pulse guide command. Also what is "ASCOM pulse guiding" ? Is there other software for that? I thought phd2 did ascom pulse guiding haha. Sorry for my ignorance here, thank you for your input! I definitely checked the clutches, both were locked in tight. 

 

Make sure in PHD2 that you have your focal length set to 360mm and not accidentally using your aperture 

 

I and many others made the same mistake of putting my aperture in there vs focal length of guide scope and well, it tracks ok but it says you're off by 1-2" wink.gif

Haha this is good to know. I will validate this as well. PHD2 was working a few days ago without issue, and I haven't changed anything since then. 

 

I agree with Stelios that it very much looks like a calibration problem. PHD2 is detecting an error, and driving the mount in the wrong direction. That shows up as an even bigger error on the next go around.

 

I'm going to assume the mount was slewing properly or you would have noticed that. I'm also going to assume the mount was tracking. That assumption is based on the PHD2 images you showed.

Yeah this is great feedback, I really appreciate everyones input here. I am going to do a force calibration tonight. 

 

PHD2 itself can tell you how far off the polar alignment is, as well as help you drift align. It's useful for a second opinion after doing a PA with other software such as sharpcap or polemaster, and seems to be relatively accurate given it's based on calibrated measurement of actual guide star drift.

This is good to know as a secondary method. 

 

 

 

 

 

-----------------------------------------------------------------------

 

In general sounds like I need to

1: Validate PHD2 calibration

2: Test tracking without PHD2 running to validate equipment issues or polar alignment issues

3: Check cables to mount

4: Ensure focal length is correct in PHD2

5: validate polar alignment with secondary software aside from SharpCap (PS: Can anyone recommend other software aside from SharpCap/PHD2?)



#12 kathyastro

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Posted 10 August 2020 - 06:58 PM

This is interesting, you think it could be a bad cable?

...

Also what is "ASCOM pulse guiding" ? Is there other software for that? I thought phd2 did ascom pulse guiding haha.

 

 

If you are using an ST-4 cable (from the guide camera to the mount), the cable and its connections are common points of failure.  The failure more would display the symptoms you showed: PHD2 issuing corrections; the mount not responding; PHD2 issuing bigger corrections; etc..

 

ASCOM pulse guiding is what PHD2 uses if you are not using an ST-4 cable.  If PHD2 shows your mount as "On Camera", you are using ST-4.  If it shows the mount as an actual named driver for your mount, then it is using ASCOM pulse guiding.
 



#13 stobiewankenobi

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Posted 10 August 2020 - 07:11 PM

If you are using an ST-4 cable (from the guide camera to the mount), the cable and its connections are common points of failure.  The failure more would display the symptoms you showed: PHD2 issuing corrections; the mount not responding; PHD2 issuing bigger corrections; etc..

 

ASCOM pulse guiding is what PHD2 uses if you are not using an ST-4 cable.  If PHD2 shows your mount as "On Camera", you are using ST-4.  If it shows the mount as an actual named driver for your mount, then it is using ASCOM pulse guiding.
 

Ah OK got it, yeah I connect my mount to my laptop with a usb type b mini cable directly to the hand controller. So it should be ascom, it's not guided through the camera, and not through an ST-4 cable. 

 

This is still good information though, thank you!



#14 stobiewankenobi

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Posted 11 August 2020 - 09:52 AM

UPDATE: So I do not know what the issue was. I went out tonight even though it was a bit cloudy and was able to get a polar alignment with sharpcap. After that I was able to do 2m subs unguided!?! 

 

I did not get to test PHD2 itself, so I am more inclined to agreeing with the folks above who suggested it was a PHD2 calibration issue or similar. 

 

I will test again tonight hopefully if it's not cloudy and will update again. 



#15 Alex McConahay

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Posted 11 August 2020 - 12:27 PM

I am assuming your two minute subs did not show significant rotation.

 

If so, what you have established is that whatever is wrong is not with Polar alignment. (although, really, even at two minutes, if you did not have some focal length on it, you may not have noticed field rotation)

 

It appears as if your mount by itself is tracking well. 

 

Now, it is up to PHD2 stuff. 

 

To start with in PHD2, get a star in the crosshairs, and then go to "manual" guiding. I forget what the actual command is called in the PHD menu, but basically, you get a little virtual keypad on the screen.....push one button and then another as you take short guide exposures. YOu should see the star move around on your screen according to which way you are pushing the buttons. If you are not getting appropriate reactions, then you need to get that fixed first. 

 

Then, do a calibration. If that succeeds, run the guiding assistant and do what it says to do (take the suggestions). 

 

And remember.....it does not have to be a great night for imaging. As long as you can see some stars, you can practice configuring and testing PHD2.

 

Alex



#16 stobiewankenobi

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Posted 11 August 2020 - 03:39 PM

I am assuming your two minute subs did not show significant rotation.

 

If so, what you have established is that whatever is wrong is not with Polar alignment. (although, really, even at two minutes, if you did not have some focal length on it, you may not have noticed field rotation)

 

It appears as if your mount by itself is tracking well. 

 

Now, it is up to PHD2 stuff. 

 

To start with in PHD2, get a star in the crosshairs, and then go to "manual" guiding. I forget what the actual command is called in the PHD menu, but basically, you get a little virtual keypad on the screen.....push one button and then another as you take short guide exposures. YOu should see the star move around on your screen according to which way you are pushing the buttons. If you are not getting appropriate reactions, then you need to get that fixed first. 

 

Then, do a calibration. If that succeeds, run the guiding assistant and do what it says to do (take the suggestions). 

 

And remember.....it does not have to be a great night for imaging. As long as you can see some stars, you can practice configuring and testing PHD2.

 

Alex

Hey @alex. Yeah so I changed out telescopes to my Explore Scientific ED102, I did the polar alignment, and am able to get 2m unguided subs right now. So that's great. 

 

I didn't get a chance to test PHD2 after that last night as the coastal fog rolled up to my place. If there are stars out tonight before the fog I will test PHD2 and update here as well. 

Really appreciate the guidance here. 



#17 stobiewankenobi

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Posted 11 August 2020 - 11:02 PM

Thanks everyone for the help. After calibrating tonight I am getting PERFECT tracking!!!

 

Calibration was the issue from what I can tell. I have no idea why PHD2 would randomly be different though. Very interesting.

 

 

I mean it doesn't get much better than this guiding. Times like this I really love the CGX.


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