Polar aligning from ANY part of the sky

Using SharpCap's Polar Alignment feature
Juicy6
Posts: 4
Joined: Sat Apr 25, 2020 6:30 am

Polar aligning from ANY part of the sky

Post by Juicy6 » Sat Apr 25, 2020 6:35 am

Would it be possible to add a function for polar aligning after plate solving ANY part of the sky? https://staraid.ai/ claims to do this over ST4 port.

Christer, Sweden
Sky-Watcher Esprit 100 ED Super APO
ZWO ASI 294 MC Pro
ZWO ASI 120MM on a QHY Mini guider
iOptron CEM40

User avatar
admin
Site Admin
Posts: 5210
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Polar aligning from ANY part of the sky

Post by admin » Sat Apr 25, 2020 4:42 pm

Hi,

Maybe I'm being over sceptical, but I don't believe that it's possible for their claims to be true. You can't polar alignment from a single sky measurement unless you assume that there are no other errors in the whole of the system (polar misalignment is the only error). In the real world that's not the case – you have errors like the home position of the mount not being set quite correctly or periodic error or pointing errors or simply go to positions not being accurate. The key to SharpCap's polar alignment is that the rotation around the RA axis eliminates all those other errors from the problem to be solved and leave you are only adjusting the polar alignment.

Now of course you can do drift alignment, but that requires at least two measurements with the RA positions at 90° different from each other to allow you to correct for polar offset in both horizontal and vertical directions. Anyway, doing drift alignment requires reasonably long measurements to allow any drift to accumulate enough to be measured accurately.

Some tools perform a sort of accelerated drift alignment – instead of waiting for the Earth to rotate and reveal any misalignment between the telescope axis and the Earth axis, you rotate the mount in RA and perform plate solving measurements to see if the declination changes as the RA changes. The tools like that that work tend to require you to adjust the horizontal and vertical polar alignment separately, which is a pain because most mounts have poor altitude/azimuth adjusters and frequently adjusting one will inadvertently change the position of the other.

I did do some tests on whether a variant of this last approach could be included in SharpCap, but I got poor results. You can measure the variation in declination as the mount rotates in RA only using plate solving, but the need for a meridian flip makes it hard to get a good run of consistent data over a wide enough RA angle.

Cheers, Robin

User avatar
oopfan
Posts: 769
Joined: Sat Jul 08, 2017 2:37 pm
Location: New York
Contact:

Re: Polar aligning from ANY part of the sky

Post by oopfan » Sat Apr 25, 2020 10:41 pm

I visited the website.

Accuracy within 30 arc-seconds is not good enough for me. I can only speak for myself and one other person but we do not actively guide. We rely on the accuracy of our polar alignment and periodic error correction. My mount is 50 years old. I have no control over the declination axis except via manual control. On right ascension I have a self-designed stepper motor and Raspberry Pi combination with PPEC. Declination drift is quite significant if polar alignment is off by 30 arc-seconds. This is with a telescope having a short focal length of only 418mm. For people with FL of 1000mm or more the problem becomes even worse.

Brian
my-gear-from-afar.jpg
my-gear-from-afar.jpg (922.08 KiB) Viewed 1143 times

Juicy6
Posts: 4
Joined: Sat Apr 25, 2020 6:30 am

Re: Polar aligning from ANY part of the sky

Post by Juicy6 » Sun Apr 26, 2020 7:47 am

admin wrote:
Sat Apr 25, 2020 4:42 pm
"Maybe I'm being over sceptical, but I don't believe that it's possible for their claims to be true".

Maybe you are right. I think this gadget was introduced a year ago but I find no reviews of it online and absolutely no success stories. I will have to rely on PHD2 drift align routine from home where Polaris isnt visible and use my iOptron iPolar cam when I see NCP.

Christer, Sweden
Sky-Watcher Esprit 100 ED Super APO
ZWO ASI 294 MC Pro
ZWO ASI 120MM on a QHY Mini guider
iOptron CEM40

User avatar
oopfan
Posts: 769
Joined: Sat Jul 08, 2017 2:37 pm
Location: New York
Contact:

Re: Polar aligning from ANY part of the sky

Post by oopfan » Sun Apr 26, 2020 12:17 pm

This has got me thinking...

Let's suppose I was able to actively guide in both RA and DEC. If my polar alignment is off by 1 degree can I still get decent astrophotos using guiding software like PHD2? In other words, how critical is PA when you can guide?

Brian

chongo228
Posts: 134
Joined: Fri Nov 24, 2017 3:34 am

Re: Polar aligning from ANY part of the sky

Post by chongo228 » Sun Apr 26, 2020 1:51 pm

There is a program called Alignmaster that might do what you want.

You pick two stars from it's database, center the first one, slew to the second and center it and it calculates the error. I used it years ago and remember it worked decently for my situation.

Juicy6
Posts: 4
Joined: Sat Apr 25, 2020 6:30 am

Re: Polar aligning from ANY part of the sky

Post by Juicy6 » Sun Apr 26, 2020 2:13 pm

chongo228 wrote:
Sun Apr 26, 2020 1:51 pm
You pick two stars from it's database, center the first one, slew to the second and center it and it calculates the error. I used it years ago and remember it worked decently for my situation.
Yes I have used this software. But better yet would be plate solving ANY stars.

Christer, Sweden
Sky-Watcher Esprit 100 ED Super APO
ZWO ASI 294 MC Pro
ZWO ASI 120MM on a QHY Mini guider
iOptron CEM40

User avatar
admin
Site Admin
Posts: 5210
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Polar aligning from ANY part of the sky

Post by admin » Sun Apr 26, 2020 7:33 pm

Hi,

I also used a alignmaster sometime back - my experience with it was one of the things that prompted me to put polar alignment into SharpCap. I really wasn't getting good results and in the end I started testing it by doing the alignment once and then immediately afterwards restarting the alignment and then repeating a number of times. I found I just wasn't getting consistent results from their program - each time I tried to align it would make me make different adjustments in different directions. I got better results when I made it slew to each alignment star twice – EQMOD treats a slew to the same position as a request to move slightly away and then re-centre the target and my assumption is that this improve the accuracy of each slew, giving better overall results.

On the subject of needing accurate polar alignment if you can guide in both directions, there is certainly more flexibility available. You have to remember that guiding in RA is really correcting for periodic error – moderate errors in polar alignment will have practically no effect in RA. Guiding in dec is almost all correcting for polar alignment problems.

Cheers, Robin

BlackWikkett
Posts: 377
Joined: Sat Jun 23, 2018 8:48 pm
Contact:

Re: Polar aligning from ANY part of the sky

Post by BlackWikkett » Mon Apr 27, 2020 2:02 pm

oopfan wrote:
Sun Apr 26, 2020 12:17 pm
This has got me thinking...

Let's suppose I was able to actively guide in both RA and DEC. If my polar alignment is off by 1 degree can I still get decent astrophotos using guiding software like PHD2? In other words, how critical is PA when you can guide?

Brian
I can answer this with a resounding yes you can get decent results if you have bad PA and you guide with PHD2. If I'm forced to image from home where I have less than ideal conditions. My mount was eye balled for PA and drift aligned (which didn't help much.) I was able to get decent images using a small 70 mm refactor, shorter exposure and PHD2. Recently was able to get SC PA to work so this is no longer an issue. viewtopic.php?f=23&t=2524

-Chris

User avatar
oopfan
Posts: 769
Joined: Sat Jul 08, 2017 2:37 pm
Location: New York
Contact:

Re: Polar aligning from ANY part of the sky

Post by oopfan » Mon Apr 27, 2020 2:10 pm

Thanks, Chris, that's very helpful!

Post Reply