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INDI Library v2.0.6 is Released (02 Feb 2024)

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Polar alignment module and CEM60 geometry

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See attached log file.

Thanks,

jmh
5 years 8 months ago #27139
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First time
[2018-07-04T22:30:37.772 CDT DEBG ][     org.kde.kstars.ekos.align] - P1 RA:  "21h 29m 25s" DE:  " 89° 27' 36\""
[2018-07-04T22:30:37.772 CDT DEBG ][     org.kde.kstars.ekos.align] - P2 RA:  "21h 09m 42s" DE:  " 89° 20' 27\""
[2018-07-04T22:30:37.772 CDT DEBG ][     org.kde.kstars.ekos.align] - P3 RA:  "20h 38m 12s" DE:  " 89° 15' 26\""
[2018-07-04T22:30:37.773 CDT DEBG ][     org.kde.kstars.ekos.align] - P1 X:  911.466 Y:  281.908
[2018-07-04T22:30:37.773 CDT DEBG ][     org.kde.kstars.ekos.align] - P2 X:  851.831 Y:  462.153
[2018-07-04T22:30:37.773 CDT DEBG ][     org.kde.kstars.ekos.align] - P3 X:  896 Y:  643.5
[2018-07-04T22:30:37.823 CDT DEBG ][     org.kde.kstars.ekos.align] - RA Axis Circle X:  1193.12  Y:  475.082  Radius:  341.532
[2018-07-04T22:30:37.823 CDT DEBG ][     org.kde.kstars.ekos.align] - RA Axis Location RA:  "19h 50m 44s" DE:  " 89° 26' 57\""
[2018-07-04T22:30:37.823 CDT DEBG ][     org.kde.kstars.ekos.align] - RA Axis Offset:  " 0° 33' 02\""

Second time:
[2018-07-04T22:49:56.817 CDT DEBG ][     org.kde.kstars.ekos.align] - P1 RA:  "23h 05m 45s" DE:  " 88° 57' 58\""
[2018-07-04T22:49:56.817 CDT DEBG ][     org.kde.kstars.ekos.align] - P2 RA:  "22h 45m 51s" DE:  " 88° 52' 53\""
[2018-07-04T22:49:56.817 CDT DEBG ][     org.kde.kstars.ekos.align] - P3 RA:  "22h 21m 45s" DE:  " 88° 51' 02\""
[2018-07-04T22:49:56.817 CDT DEBG ][     org.kde.kstars.ekos.align] - P1 X:  669.179 Y:  377.245
[2018-07-04T22:49:56.817 CDT DEBG ][     org.kde.kstars.ekos.align] - P2 X:  744.391 Y:  545.77
[2018-07-04T22:49:56.817 CDT DEBG ][     org.kde.kstars.ekos.align] - P3 X:  896 Y:  643.5
[2018-07-04T22:49:56.896 CDT DEBG ][     org.kde.kstars.ekos.align] - RA Axis Circle X:  986.474  Y:  336.685  Radius:  319.876
[2018-07-04T22:49:56.897 CDT DEBG ][     org.kde.kstars.ekos.align] - RA Axis Location RA:  "22h 20m 38s" DE:  " 89° 04' 08\""
[2018-07-04T22:49:56.897 CDT DEBG ][     org.kde.kstars.ekos.align] - RA Axis Offset:  " 0° 55' 51\""

So it appears the calculations are correct. What was the correction vector you got for the first go? I presume you centered the star by adjusting Alt & Az knobs on the mount?
5 years 8 months ago #27147

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I should have taken screen shots of the Alignment windows as i went - I'll try that tonight as I think it will illustrate what I think might be happening, And I do use the Alt/Az knobs, not the controller. (I know you have to ask ... ;-))

My first attempt resulted in a polar alignment error of 33arcmin. When I made the correction, and start the second attempt, the reported error was about 55arcmin. I never bothered trying to move the move on the second try and went back to using the polar scope that comes with the CEM60. My polar scope is slightly misaligned, but the interesting thing is that all of the mount movements required to get Polaris where it needed to be on my polar scope were exactly the opposite of what the Polar alignment module wanted me to do.

After I used the Polar scope, I went back to the alignment module, and it reported the Polar alignment error to be 5arcmin. This is where I should have taken a screenshot, because I could clearly see NCP, the circle of rotation for the current mount position, and the vector - the direction of the vector was suggesting I move the circle of rotation away from the NCP position, not towards it. Could it be that the direction is flipped somehow? Anyway, the clouds moved in, so I couldn't test it. I'll try again tonight and take screenshots as I go through the process.
The following user(s) said Thank You: Jasem Mutlaq
Last edit: 5 years 8 months ago by fmozza.
5 years 8 months ago #27159

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It's a possibility... looking forward to the screenshots + logs again.
5 years 8 months ago #27160

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As a test for this hypothesis instead of clicking on a star so the pink direction line starts there, click away from it so the line ends there and then make sure that the star ends up at the start of that line. If you are right then polar alignment should be perfect.
5 years 8 months ago #27161

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Exactly what I was planning.

jmh
5 years 8 months ago #27174

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Great minds think alike :whistle:
5 years 8 months ago #27175

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Keep in mind (if this isn't obvious to you already, in which case I apologize) that the adjustments you make to your mount, and the resulting motion of the star within the image will be diametrically opposite. To use a hypothetical example, say that you use the polar alignment utility, and it tells you that the NCP is 30 arcminutes up and to the left of your current center of rotation. That is, you need to adjust your mount's polar alt/az knobs to bring your RA axis upward and leftward by 30 arcminutes. When you do this, the camera moves up and to the left, so the star (which is stationary) will appear to move down and to the right within the frame. Once you've finished correcting for the polar alignment error, the star will be 30 arcminutes down and to the right of its original position in the image, the opposite direction of the adjustment you made.

Another observation that I can offer, as someone who likewise has struggled with getting accurate polar alignment, is that the adjustments you will need to make to the mount's alt/az knobs are very small. For reference, if you own a Telrad, 30 arcminutes is the angular diameter of the smallest circle at the center of the reticle. You may end up only turning the knob 1/16 of a turn, or something of that magnitude. It may be more than that depending on the thread pitch of your mount's adjustment knobs, but the point is that it's best to take small steps and check your progress regularly. I usually try to take it slow, check the refresh images, and make the smallest possible adjustments that I'm physically capable of making until eventually I get there.
The following user(s) said Thank You: Jasem Mutlaq
Last edit: 5 years 8 months ago by Ryan M.
5 years 8 months ago #27183

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Thanks for your comments, and no need to apologize for trying to help a fellow imager. I've been imaging for about 7 years, give or take, but the CEM60 is new to me, as is KSTARS/INDI/EKOS, so there is a learning curve. Using my Polar scope, I can get to about 5 to 8 arcmin of NCP, and will work reasonably well at my current focal length of 900mm, but I want a crack at imaging with my C11 @ 2800mm with exposures on the order of 10min or so. I know I'll have problems with mirror shift, but if I can get really close to NCP, that will help to limit rotation I see at long focal lengths.

It might be helpful for folks to know I am not using a tripod - my mount is on concrete pier sunk about 15 feet down, so its stable. (There's a story behind this, but where I live, the frost depth can hit 8ft, so you need to have sufficient depth to counteract frost heaves.)

I'm really happy with the CEM60 adjustments for Alt/Az - not as good as a really high-end mount I suppose, but it's a big step compared to my old CGEM. I am scratching my head with the Polar alignment module - when I set the vector tip on a star, I'm able to move the star to the crosshairs and hit the middle ease, but when I repeat the exercise to check my alignment, the result is far worse than when I started.

Stay tuned - I'll have another crack at this in another hour, provided the clouds stay away...

jmh
5 years 8 months ago #27185

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There was intermittent clouds tonight, but I decided to follow the process as per the instructions, collect the logs, and do screen captures so you can all see what I've done.

Here are the align module logs:

[2018-07-05T22:57:53.294 CDT DEBG ][ org.kde.kstars.ekos.align] - P1 RA: "02h 31m 33s" DE: " 89° 15' 50\""
[2018-07-05T22:57:53.294 CDT DEBG ][ org.kde.kstars.ekos.align] - P2 RA: "00h 38m 24s" DE: " 89° 13' 55\""
[2018-07-05T22:57:53.294 CDT DEBG ][ org.kde.kstars.ekos.align] - P3 RA: "22h 48m 24s" DE: " 89° 13' 14\""
[2018-07-05T22:57:53.294 CDT DEBG ][ org.kde.kstars.ekos.align] - P1 X: 119.525 Y: -48.4096
[2018-07-05T22:57:53.295 CDT DEBG ][ org.kde.kstars.ekos.align] - P2 X: 412.35 Y: 405.71
[2018-07-05T22:57:53.295 CDT DEBG ][ org.kde.kstars.ekos.align] - P3 X: 896 Y: 643.5
[2018-07-05T22:57:53.348 CDT DEBG ][ org.kde.kstars.ekos.align] - RA Axis Circle X: 1083.44 Y: -348.489 Radius: 1009.54
[2018-07-05T22:57:53.348 CDT DEBG ][ org.kde.kstars.ekos.align] - RA Axis Location RA: "22h 48m 49s" DE: " 89° 54' 35\""
[2018-07-05T22:57:53.348 CDT DEBG ][ org.kde.kstars.ekos.align] - RA Axis Offset: " 0° 05' 24\""



[2018-07-05T23:21:39.264 CDT DEBG ][ org.kde.kstars.ekos.align] - P1 RA: "02h 32m 11s" DE: " 89° 15' 44\""
[2018-07-05T23:21:39.264 CDT DEBG ][ org.kde.kstars.ekos.align] - P2 RA: "00h 52m 35s" DE: " 89° 12' 32\""
[2018-07-05T23:21:39.264 CDT DEBG ][ org.kde.kstars.ekos.align] - P3 RA: "23h 16m 30s" DE: " 89° 11' 42\""
[2018-07-05T23:21:39.264 CDT DEBG ][ org.kde.kstars.ekos.align] - P1 X: 181.254 Y: 33.2729
[2018-07-05T23:21:39.264 CDT DEBG ][ org.kde.kstars.ekos.align] - P2 X: 454.614 Y: 438.198
[2018-07-05T23:21:39.264 CDT DEBG ][ org.kde.kstars.ekos.align] - P3 X: 896 Y: 643.5
[2018-07-05T23:21:39.322 CDT DEBG ][ org.kde.kstars.ekos.align] - RA Axis Circle X: 1045.77 Y: -255.614 Radius: 911.502
[2018-07-05T23:21:39.322 CDT DEBG ][ org.kde.kstars.ekos.align] - RA Axis Location RA: "23h 31m 25s" DE: " 89° 49' 00\""
[2018-07-05T23:21:39.322 CDT DEBG ][ org.kde.kstars.ekos.align] - RA Axis Offset: " 0° 10' 59\""

And these indeed show that the alignment is worse. The attached zip contains the images that shows what I did. There was a couple of Capture and Slew to target steps that I didn't bother to capture - I wanted to start the second attempt from as close to the same position as the first attempt.

The cloud rolled in again, so I couldn't do the reverse process. The forecast is clear for tomorrow night, so I'll do that tomorrow night. But in the meantime, have a look and let me know if I'm doing the right things.

jmh
Last edit: 5 years 8 months ago by fmozza.
5 years 8 months ago #27186
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Interesting... the fact that the RA offset afterwards is almost exactly twice the original value, and almost exactly the same angle, does seem to strongly suggest that the correction guide is telling you to go the wrong way. I can't see how this could possibly have anything to do with your mount, but at the same time, it seems strange that no one else would have encountered this and brought it up already. I've had fairly inconsistent results with trying to do polar alignment corrections myself, but I never seem to get to the point where the star is perfectly centered in the crosshair, mainly due to camera/plate solving issues that show up during the "refresh" phase. Anyway, I think that what you're seeing here certainly merits a closer look, since your results are quite telling and I don't see any reason why you'd be experiencing this and others wouldn't.

Judging by the screenshots you posted, it appears that the correction vector is correct relative to the calculated center of rotation... the dashed green circle appears to be centered below and to the left of the NCP on the yellow grid, which would suggest that the mount needs to be raised and aimed to the right slightly. This would cause the star to move down and to the left in the image, which is what the correction guide is telling you to do. The question, then, is what is wrong? Is the center of rotation calculation incorrect, causing both the green circle and the correction guide to be mirrored 180 degrees? Very strange...
Last edit: 5 years 8 months ago by Ryan M.
5 years 8 months ago #27188

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What version of KStars are you running? Are you on 2.9.6 or are you running a nightly/bleeding-edge build? If the version you are using was built after June 3, perhaps it's possible that these changes are responsible? It seems likely to me that this is a regression of some sort, as I seriously doubt that the Polar Alignment Helper could have been broken ever since it was introduced a year ago and nobody would have noticed. It does look like some fairly major changes were made, so perhaps the bug was introduced there? These changes were made after 2.9.6 (the latest stable version) was released though, so unless you're running the bleeding-edge version, the problem would have to be older than that.
5 years 8 months ago #27190

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