PA without view of celestial pole

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zerolatitude
Posts: 156
Joined: Mon Mar 01, 2021 5:24 am

PA without view of celestial pole

#1

Post by zerolatitude »

For those of us who cant see Polaris (and presumably SCP) :-(

I know this has been asked before, and also read your view on this. But still, no harm trying once again :)

Yes, it would involve 2 separate adjustments, but that's anyway the only option for someone who cant see the pole(s).

NINA has 2 adjustments and gives a degree offset readout. Since alt-az knob are notorious for their lack of fine adjustment, ...

Drift align with PHD2 is better - at least some visual feedback, but it takes more time.

A visual approach of moving a star to a "target" like the SC PA would be great, even if it meant 2 separate adjustments in alt and az. I found this paper - http://celestialwonders.com/articles/po ... oning.html which seems to do that manually - could that be programmed?

The other thing I got from that site is an eye-opener of how much leeway we actually have in polar alignment. Like the optimal sub time calculations. Maybe that could be part of a PA Brain?

http://celestialwonders.com/tools/polar ... rCalc.html
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admin
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Re: PA without view of celestial pole

#2

Post by admin »

Hi,

my favoured non-polaris approach is the measurement using RA movements (faster than drift align) and a plate solving engine to calculate the amount of declination change. That would then let you calculate the Alt/Az errors (providing you measure near the meridian and near E or W), from which an adjustment based on star movement could be calculated.

However, when I took some test data to try to validate the results from this method, the data did not give good results - I had deliberately set up a particular PA error to test and the results didn't even show the expected pattern :( I don't know what the cause of this was. Sometime I will have to revisit the experiment and see if I can do better.

cheers,

Robin
zerolatitude
Posts: 156
Joined: Mon Mar 01, 2021 5:24 am

Re: PA without view of celestial pole

#3

Post by zerolatitude »

That still needs off-line calculation, instead of a nice screen which says - "move the knobs so that the yellow star fits in the red circle" or equivalent.

Anyway, please do keep it in mind, am sure a lot of people would benefit.

Thanks
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Re: PA without view of celestial pole

#4

Post by admin »

Hi,

I think that if the software measures the dec drift at two points that are 90 degrees apart in RA then you know the two components of the PA error (E/W movement and up/down movement). Once you know that you can do the 'move star into target' thing from anywhere in the sky (with plate solving results and a little maths to handle a rotation of polar co-ordinates!)

cheers,

Robin
Jean-Francois
Posts: 360
Joined: Sun Oct 13, 2019 10:52 am
Location: Germany

Re: PA without view of celestial pole

#5

Post by Jean-Francois »

Hello,

That is what Norman's script is doing viewtopic.php?f=14&t=2331

It is not easy to set all the pieces of software together ... but it works.


Best regards,
Jean-Francois
zerolatitude
Posts: 156
Joined: Mon Mar 01, 2021 5:24 am

Re: PA without view of celestial pole

#6

Post by zerolatitude »

Thanks.

I'll take a look.
zerolatitude
Posts: 156
Joined: Mon Mar 01, 2021 5:24 am

Re: PA without view of celestial pole

#7

Post by zerolatitude »

Jean-Francois, thank you for the tip.

It is working well with the SkySimulator. Hopefully all will be well once I have a chance to try in the field also.
Jean-Francois
Posts: 360
Joined: Sun Oct 13, 2019 10:52 am
Location: Germany

Re: PA without view of celestial pole

#8

Post by Jean-Francois »

Hello,

Yes, it is nice to see the process with the SkySimulator.
I received my EQ8 mount yesterday, back after some tentative to correct the USB 3.0 problems.
I install it on my tripod and I performed the mount 2-stars and Polar-alignment methods.

I will check this evening the script of Norman again ... it is now a good period to do that with the full moon.

Concerning the script ...
I think that one point could be improved. The script uses the mount itself for the delta-displacement. The problem is if the mount has a significant backlash, then the polar alignment depends directly to this backlash. A proposal is that at the last telescope position, without movement of the telescope, the script shows on the live image the needed displacement (similar to other programs performing Polar alignment around the Polar star).

Regards,
Jean-Francois
zerolatitude
Posts: 156
Joined: Mon Mar 01, 2021 5:24 am

Re: PA without view of celestial pole

#9

Post by zerolatitude »

Yes, that would be nice. But this itself is a big improvement to drift align, even with the visual aids of PHD2.

Will know once it is tested in real life.

Luckily, with my setup, as per the calculator I listed earlier, PA less than 15 arc-min on each axis is acceptable.

http://celestialwonders.com/tools/polar ... rCalc.html
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