All-Sky Polar Align

w7ay

I just made an attempt to quantify the accuracy of the All-Sky Polar Align process in Experimental Feature of ASIAIR 1.8.3 (Firmware 9.12).

I used my MountSim program (http://www.w7ay.net/site/Applications/MountSim/) which is a virtual mount that has no imprecise mechanical parts. The simulated focal length is 158mm (ASI174mm mini pointed at an iPad Pro M1's that is used as an external display).

The measurement involves a perfectly aligned mount (for my Latitude of 45º 35' North). After a polar alignment (without adjusting altitude or azimuth bolts), a perfect algorithm should return a polar alignment of zero. Any non-zero result is an error from the algorithm, residual plate solve error, star centroid error, bad atmospheric refraction error, etc).

To confirm my older measurements, I reran the original Polar Alignment algorithm in Firmware 9.12, at 90 degree declination and obtained an error of 10" (10 arc seconds).

Like the original algorithm, the All-Sky Polar Align in ASIAIR uses only RA axis slewing. Unlike a GOTO command to the mount, an RA slew command ensures that the declination axis of the mount has no movement at all. Unlike the original method that rotates the RA axis westward by 4 hours (60º) to make two plate measurements, the All-sky method makes three plate solves about 1.5 hours (22.5º) apart, for a total of 45 degrees of slew.

And unlike the original method, the All-Sky method allows you to use declination angles that are more than 30 degrees from the pole.

Warning: because the all-sky method works with arbitrary declination angles, an RA slew that crosses the Meridian can cause telescope parts to crash into the tripod or pier (slews do not enforce Meridian flips).

(I am willing to wager good money that some 1,2,3 Simpleton will eventually crash his/her telescope into a tripod when using the all-sky method.)

Because of the possibility of a real telescope hitting the pier, I don't simulate with starting Hour Angles that crosses the Meridian, even though the virtual mount will not suffer any damage. I either start at the Meridian (Pier side east) or hour angle of -4 hours (Pier side west, OTA pointed 60 degrees east of the Meridian).

So, here is the result (remember that the original method gave me 10" of error with the same firmware version):

Caveat Emptor.

Chen

franco

Who are you calling a simpleton!
I only tried it a couple of times and compared it with the usual PA and it was way off. I sent feedback.

At one point I did the usual PA and immediately started the new version - cancelled it before it hit the tripod.....

Kring

In my two nights of simpleton testing ( 🙂 ) I found that the ASPA is off, and it seems enough to make a difference in guiding quality. Goto worked find under either scenario because the AA+ is good at recalculating.

I posted my observations here. https://bbs.astronomy-imaging-camera.com/d/13842-aspa-beta-issue-substantial-variance-in-ncp-polar-alignment Where I did two back-to-back PA vs ASPA and there's a lot of discrepancy. Ran this multiple times and each was consistent within; so we're talking approx 6'-8' minutes of polar alignment error.

w7ay

Kring I found that the ASPA is off, and it seems enough to make a difference in guiding quality.

8 arc minutes of polar alignment error is practically useless for any halfway serious deep sky astrophotography.

Field rotation will eat you alive since ASIAIR autoguiding makes use of multiple stars, some of which can be as far as the corners of the field. Each star's centroid will be moving in a slightly different trajectory due to field rotation, and the weighted average of the centroids (which is what ASIAIR sends to PHD2) will also be moving.

You can use the calculator here to estimate how much field rotation there is, given your exposure time. If the stars at the corners of your field moves by more than 0.5 arc seconds, you could be in trouble (i.e., your total RMS error for guiding becomes greater than 0.5 arc seconds).

http://celestialwonders.com/tools/rotationMaxErrorCalc.html

Even if autoguiding works, unless your exposures are short (you can check with the web calculator link above), your main camera can also show the field rotating around the mean centroid of the stars that ASIAIR had chosen for autoguiding, if the main camera has an FOV that is wide.

Chen

astrosatch

I did new polar align and must say that I was a bit puzzled why it was off and had to tweak it a bit to get as close as possible to asiair numbers. Since my north is completely obstructed I have to do drift alignment and usually get it pretty spot on. My rms numbers were also worse after all sky pa. I guess I will still rely on drift alignment.

franco

I'm actually quite impressed with the normal PA on the AAPro, I'd never managed to get a good PA before I got this little box. My problem is I'm in Oz and my view south is of a steep hill with trees. I found a spot in the garden where I could see the sky peeking through a gap in the trees and that's where I setup my scope each night. Apart from making the all sky mode work properly I'd like to see an option in the traditional PA to return to the home position and recalculate the error, just to check that all the knobs are done up and the mount hasn't shifted.

dickson

Why ASIAIR recommends avoid using star in east or west direction for ASPA? I don't see such advise at NINA 3-star alignment, and the result from w7ay does not show issue using stars near that direction.

mikelockwood

any updates on this thread ? I plan to use the All-Sky soon for the first time at a campsite. Polaris will be obscured.

Have the initial quirks in All Sky been resolved ?

I am running the latest firmware as of the date of this post 11/12/2023

Tech@ZWO

mikelockwood Now, the tracking must be on during ASPA, so you need to keep the mount on the same side of the meridian if your mount auto-turns off track after the meridian.