Dear all,

Recently I moved from Spain to Chile and now I am trying to make use of the clear skies here. I have been setting up my astro photography gear for a few days in a row now for various tests and I noticed several times a problem with polar alignment. After the three images have been taken and the mount has rotated twice, the pink line is drawn to indicate how a star should be moved using the fine tuning knobs of the mount to improve the polar alignment. I notice that I need to aim the line such that the end point is on top of the star and the start where the star should end up. If I do this the other way around, the polar alignment will get worse. Has anyone else noticed this?

I have attached the logs of today's session for reference.


Clear skies, Wouter

There has been various reports about it being flipped before, and so I added a flip to reverse the effect. Maybe this is what caused it?

Frankly, I still don't understand why it is reversed on some occasions. Perhaps someone can figure this out? It's been an issue for some users even since the PAA was introduced.

Just a thought...
Moving to the Southern hemisphere has the effect of rotating the images by 180 degrees and maybe the direction vector is also rotated.
Another thing that rotates the image is a pier flip and maybe it would be different if the mount were on the other side of the pier when the images are collected.
That should be easy to test.

Having a diagonal (actually an odd number of reflections) in the image chain will flip the image and that could also have an effect.

I can't test because I can't see the pole from where I usually set up and the PAA assumes you are looking at the pole. I don't think that's essential, the three images could be any distance from the pole. It's on my list of thigs to try, but it's a long list and it's very cloudy in the UK.

Chris

Thanks Jasem and Chris for the quick replies.

Jasem: yes I remember that people had issues with it, but that never affected me before. I don't understand where this comes from either but I never inspected the code. Perhaps some checkbox in some advanced section of the configuration ("Warning: only tick this if you know what you are doing") that reverts the direction of the pink line could help until we get to the bottom of this?

As for Chris's reply: I don't have a diagonal in my optical train. I use a refractor with a field flattener and the camera directly attached to it without any reflecting devices, so if that were an issues then at least I am not affected by it. I can test what happens if I let the telescope move to the east side of the pier but I did test with going west first and then going back to the east ending in the park position via the polar alignment routine (i.e. autopark is disabled).


Wouter

I'm having the same issue and I thought I was doing something wrong! So glad it's not me.

I found last night after trying the Ekos PAA that my PA error got progressively worse (starting 5arcmin, ending at 9deg) each time I ran the routine.
In hindsight, when I first used Ekos a couple of years ago I gave up on the PAA after similar symptoms (it was a long cold frustrating night chasing alignment!) and I then switched to a polar drift alignment process using PHD2. I wanted to use the Ekos routine again and had a chance to do so last night.

After several attempts and it progressively getting worse, for an experiment I tried to adjust the star position in the opposite direction by imagining where the target would be if the vector was flipped. To my surprise the PA error reduced after each iteration - I got back down to 5 arcmin. I couldn't get under 3arcmin, it was well passed midnight and I gave up. Maybe it only needs to be flipped on one axis? It was too late to further test this idea.

So... here's the gear in use last night:
- KStars/Ekos/INDI running on UbuntuMate 18.04 LTS with all packages up-to-date as of 28th Jan 2020. Intel i3 4th gen PC, 8GB RAM, 256GB SSD. USB3 connection to Pegasus Astro Ultimate Powerbox v2.
- Skywatcher EQ6-R via EQMod (Bluetooth to PC, using XM-15B module).
- Saxon 200mm F5 (1000m focal length) newtonian, with corrector.
- Canon EOS 650D DSLR, via USB from Powerbox. DIY focus motor via Powerbox.
- QHY5-II-M guide camera on OAG via USB from Powerbox (not used for polar alignment).
- GPS via VK-172 USB GPS dongle (via USB on Powerbox).
- Offline astrometry.net solver
I'm in the southern hemisphere, Victoria, Australia. Latitude is 36.7570° S
The telescope/mount tab on the INDI Control Panel correctly shows location and the 'southern hemisphere' as being selected based on GPS data.
For the Ekos PAA routine, I'd selected 'West', with 30deg steps and a slew of 600x.

For now, I'm happy to know I'm not (a) crazy, or (b) dumb and will go back to doing a PA routine with PHD2 - though I'd really like to be able to manage it all in Ekos as I can't use my DSLR with PHD2, or at least I haven't worked out how to.

I'm happy to try any suggestions you have.

Cheers.

If you have logs with the alignment option checked so the PAA data is shown that may be useful.

I'm looking at the PAA routine, hoping that I can make it work for my situation where I can't see the pole. The code currently sets the declination for the solve hint to 90 so it doesn't work well at 50 or 60 deg. Using the scope position as the solve hint gives a good solve but the polar error increases as the size of the circle increases. I have a fix for that but it's slow going because there is so little documentation.

Currently I am traveling but I'll be back home on Sunday so hopefully I'll be able to provide new logs shortly after that. If you want me to compile and test any code of yours then let me know and I'll gladly do so.

Wouter

I have checked out the code from GitHub and reverted these two lines to what they were before according to the git log. I am recompiling KStars now and will test the polar alignment tonight. Then I will report back so we can see how to investigate this.


Wouter

Hi everyone.
Any news about this issue?
I live in Argentina and I realized that I have to reverse the correction vector to do PA but I did not improve below 4 minutes.
If anyone managed to solve it please share it.
Thanks

Sorry, no progress so far. It has been cloudy in La Serena, Chile, where I live so I have not been able to test my code changes.


Wouter

Oh, La Serena! I visited the place last year for Solar Eclipse. Great place and the sky is just unbelievable! Not mentioning Elqui Valley!
Great move from Spain to Chile!

Could the reason some, but not all southern observers experience this issue come down to different optics and the orientation they flip the image to?