Polar Alignment without Polaris (workflow)

Discussions on extending SharpCap using the built in Python scripting functionality
fly_n
Posts: 32
Joined: Wed Oct 02, 2019 7:54 pm

Re: Polar Alignment without Polaris (workflow)

Post by fly_n »

Hello Zerlolatitude,

you shoud look at this post:
Regards, Norman
fly_n wrote: Sun Jan 03, 2021 11:02 pm Ok here we go.
There are two versions.
* One without test files which weighs in at just under 1MB
https://drive.google.com/file/d/1ZqJMom ... sp=sharing

* the second version with 4 different sets of pictures to try out at about 50MB.
https://drive.google.com/file/d/1UDQVit ... sp=sharing

Let me know of successes or failures, suggestions etc.
I hope you enjoy, Norman
zerolatitude
Posts: 73
Joined: Mon Mar 01, 2021 5:24 am

Re: Polar Alignment without Polaris (workflow)

Post by zerolatitude »

Norman,

Thank you very much for this script.

I have got everything up and running with SkySimulator (including the RA error), and all seems well so far. I am waiting for a clear night to test it out in real life.

Two questions which may be very basic-
1. My FOV is restricted on east and west by buildings. I can probably do about 70 degrees range in AZ without a flip. Will that be enough to get robust data?
2. Since I am in a low latitude, best visibility is at DEC close to 0, eg +10 to -10. Does that create any problems in the maths?

Thanks
Jean-Francois
Posts: 126
Joined: Sun Oct 13, 2019 10:52 am
Location: Germany

Re: Polar Alignment without Polaris (workflow)

Post by Jean-Francois »

Hello,

I can answer some questions, but Norman does more mathematical analysis.
Important is the length of the arc in the sky, not really the delta RA value.
For Norman and I (at ~ 48° North) we can use the fields from meridian to East (or West) near the equator line.
Near the horizon, the air refraction perturbs the calculation (the script calculates the effect, but it depends from several parameters).
So from the meridian to the East, then with sufficient elevation above the horizon, we have 70° or less in RA.

Most of the mounts are German type ... the problem is the meridian flip. The script limits the region on only one side (first for simplicity in the script and second for safety).That is not necessary for a mount with a fork.
If you can program in Python, then you can maybe modify the script for a scan on both side or to start from one side without meridian flip.

DEC close +/- 10° ... I use the region at around +10° (if I remember, I can confirm tomorrow).
For you, yes, that is the best position.

Regards,
Jean-Francois
fly_n
Posts: 32
Joined: Wed Oct 02, 2019 7:54 pm

Re: Polar Alignment without Polaris (workflow)

Post by fly_n »

Hello,
yes 70 degrees range should serve you well for RA.
-------------------------------------------------------------

Anything relatively close to DEC = 0 works fine.
The reason for aiming there is that it spreads out most in the sky. The closer you get to DEC = 90, the less separation there is in the sky.
I haven't run any real world studies to see the impact, though.

If the segment length relates to a constant errror, I'd figure the following:
Error = Segment Length / const. value
Segment Length = 2 * PI * R
R = cos(DEC)

DEC SEG
0 100%
15 97%
30 87%
45 71%
60 50%
90 0%

In summary for this method don't point close to polaris ;-)
Norman
zerolatitude
Posts: 73
Joined: Mon Mar 01, 2021 5:24 am

Re: Polar Alignment without Polaris (workflow)

Post by zerolatitude »

Thank you.

And its easy not to point near Polaris - I can't see it :D
zerolatitude
Posts: 73
Joined: Mon Mar 01, 2021 5:24 am

Re: Polar Alignment without Polaris (workflow)

Post by zerolatitude »

One more question.

When the alignment error is calculated, which directions to the negative and positive values refer to?

Specifically,
1. If Az is negative, should mount be moved east or west?
2. if Alt is negative, should mount be moved up or down?

Thanks
zerolatitude
Posts: 73
Joined: Mon Mar 01, 2021 5:24 am

Re: Polar Alignment without Polaris (workflow)

Post by zerolatitude »

Norman,

Finally got some (semi) clear skies today and tried it out. It worked beautifully. I was polar aligned to less than 4 min (as measured by PHD drift) in about 6 min.

I could have got more with some more precision but that is enough for me with my setup and photo targets.

Many many thanks for this.
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