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

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Inconsistent polar alignment results

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I actually rotate by 60 degrees twice because I thought it would be _more_ accurate.
Note that I am doing a manual slew so by no means I am accurately getting 60 degrees every time. I am assuming this does not matter that much ? Or is this a wrong assumption ?
Last edit: 2 years 8 months ago by Paul.
2 years 8 months ago #73856

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It could be that a manual slew would *not* work (although in principle the software *should* overcome this by not using the user rotation value for anything other than a mount command and then figuring out the "real" rotation by plate solve). If the SW isn't already doing this, your manual slew could be part of the problem. Again, although slower, I'd recommend a careful baseline PHD2 drift align (once), followed by another set of Ekos PA runs (back-to-back) to compare both precision and variability. In case it's not obvious, *don't* change the Alt or Az mount knobs after the PHD2 align for any of the Ekos repetitions! Then report your findings. Good luck and CS, Doug

Edit:  Also, to be fair to Ekos, please be careful with your PHD2 drift align!   You need a good/flat (stable) drift over at least a few minutes!   Also, if you don't have PEC sorted out for your mount, that could pose problems too.   It's worth spending some time on PEC too to beat that component of variability down....
Last edit: 2 years 8 months ago by Doug S.
2 years 8 months ago #73860

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I usually do 2-3 runs of the new PA routine with Ekos (no polaris view) and find it very consistent between runs, maybe some differences of max 1’ between iterations. I’ve run the routine close to the pole and in other regions using my guide scope with 120mm FL and the guide camera QHY5LIIM. I have to say though, that I need to set the FL to 128mm in order to match the FOV reported by Ekos, seems strange to me to have this 7% difference, however plate solving is working fine with this FL so I have been using that since the beginning.

However I then run PHD2 guiding assistant (setting FL as 128mm as well) and the error reported is much higher. In my last try after getting under 1’ error with Ekos i got 45’ of error in PHD2, which I think was correct as the drift was evident. I then run drift alignment in PHD2 and got under 3’ error so I was able to continue with my imaging session. Unfortunately I wasn’t able to run Ekos PA right after to get the error reported as it refused to plate solve any image (didn’t happen to me before)

Any ideas of what could be the issue? Maybe related to the FL not being set correctly?

In my next session I will definitely run PHD2 drift alignment first and then try Ekos PA, without touching the knobs but just trying different FLs to see if can get close results between both methods.

Thanks
2 years 8 months ago #73863

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I'd recommend setting the focal length to the actual system focal length and not changing it (neither for Ekos nor for PHD2). Changing the FL changes the image scale, so of course the error will change too. Just set the proper focal length values in both software, and let the chips fall where they may. Otherwise, you're just minimizing/normalizing the error to an adjusted/fake focal length.
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2 years 8 months ago #73867

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Here are the results:

 

Except for the first two, the rest were quite consistent within tolerance. The AZ/AL error would of course be different depending on where are you looking in the sky. These are all done with primary scope (Orion EON 120mm) on QHY268M Pro.
2 years 8 months ago #73872
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Hi Jaseem,

Thanks for running these experiments. OK, it's good to know the procedure itself works. Then I guess I am doing something wrong.

Paul
2 years 8 months ago #73875

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Well, I think it would be more accurate to say that from the data Jasem took, the procedure's variability isn't too bad (but 2 arcmins variability seems a bit higher than what is desired considering we're platesolving a solution IMO). Jasem didn't provide an output value comparison to PHD2's drift alignment output value. This or another vetted drift align output value is needed for comparison before you can say the software "works".   Consistency doesn't necessarily imply precision, and result precision is the goal so you know when good enough PA occurs.

Edit:   something just caught my eye in  Jasem's data.   All the tests were done starting on Polaris park position?  That's good for the OP test, but what about away from Polaris (say 60 degrees or lower DEC?).   This is where I experienced problems.   Jasem, could you possibly do another set away from Polaris and if possible include a drift align output value from PHD2 for comparison?  That would be very helpful....
Last edit: 2 years 8 months ago by Doug S.
2 years 8 months ago #73878

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Like magic! I set the FL to 120mm ignoring the warning message in Ekos and was able to get similar PA error both in Ekos PA and PHD2 drift alignment.

The routine I followed was to first run PHD2 drift alignment and guide assistant first, which reported an error of 6’5”.

Then I run Ekos PA and before adjusting the knobs it reported an error of 4’5”, a bit of a gap but much better than my previous tries.

Then I adjusted to 0’5” in Ekos and run a second iteration which confirmed the same error practically. I then run PHD2 guiding assistant and got 1’5” error.

So in my case there is something like 1-2 arcminutes of deviation between both, for me enough to use the much faster Ekos PA routine. I was able to confirm it by taking some 5mins pictures with perfect round stars.

So in my case it seems the wrong FL was the error, I hope this helps others.
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Last edit: 2 years 8 months ago by kobu.
2 years 8 months ago #73905

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Still struggling with this one.
I am using the skyguider pro. Should I turn tracking _off_ while I am doing the procedure ?

I want to try the PHD2 Drift alignment as suggested by others, but my visibility on the sky is limited by buildings. E.g. I cannot see the intersection of the meridian and the celestial equator on the south side. Does this matter ? Can I still try it out regardless ?
2 years 6 months ago #74988

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Keep it on.
2 years 6 months ago #75033

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@Kobu, That's an interesting hypothesis. I will try that out myself.
How did you come to this conclusion? And 120mm ?
Last edit: 2 years 6 months ago by Paul.
2 years 6 months ago #75034

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Just wanted to share I found the cause. The skyguider pro was not entirely stable when doing PAA. I fixed this and now the results are much better. however, still some minor variation between consecutive runs.
2 years 6 months ago #75338

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