Re: Polar Align, Zenith method


Roland Christen
 


So it appears that when using the 'zeni th' polar alignment method, the RA drift is corrected by adjusting the altitude and the DEC drift is corrected by adjusting the azimuth (?).
YES, that is correct.

Rolando


-----Original Message-----
From: craig.young.m8@... [ap-gto]
To: ap-gto
Sent: Fri, Aug 25, 2017 2:58 pm
Subject: [ap-gto] Polar Align, Zenith method



In another post, Howard recommended I try the 'zenith' method for final alignment.  Not being familiar with this I decided last night to give it a try.

Two nights ago I used TPoint to measure the error in Polar alignment and found azimith was perfect and altitude needed a small correction.  After a couple of passes I had the altitude within an arc-minute, good enough to try it.  I next used APPM to create a 30 point model in the eastern sky and then did several 10 minute unguided tests in various parts of the eastern sky.  All of the images showed nice round stars.

I then tried the zenith method.  I turned off APPM tracking and moved the telescope to -38 declination (my latitude) and -10 minutes HA.  A plate solve showed this was within a few minutes of the zenith.  My method was to move the scope to -10 minutes HA, take a 10 second image, wait 5 minutes then take another 10 second image.  Sure enough the star drif ted in the RA direction.  After a few iterations I determined the altitude had to be raised.  At the end I had no drift in RA but a slight drift in DEC which I left for now.

I then did another 30 point APPM model and tested it by moving the scope to 0 DEC and -1 hour HA.  A 5 minute exposure showed no drift in RA but a small drift in DEC so the stars were elongated.  The amount of elongation (drift in DEC) matched what I saw at the zenith when I finished the altitude adjustment.

To make sure nothing else had changed, I turned off the APPM model and went back to TPoint.  It showed the altitude was too high by 2.3 arc minutes, so I lowered it back down and ended up with <0.9 arc minutes which is about what I had two nights ago.  I then reloaded the APPM model from two nights ago and again tested using a 5 minute image at 0 DEC and -1 hour RA.  Stars were nice and round again.

So it appears that when using the 'zeni th' polar alignment method, the RA drift is corrected by adjusting the altitude and the DEC drift is corrected by adjusting the azimuth (?).  Unfortunately the clouds came in and I was not able to test this last night.  If indeed this is true then the 'zenith' method is a great way for polar aligning a mount.  There is no refraction involved since the scope is pointed precisely at the zenith.  And all sites should be able to see the zenith compared to those having trouble seeing the eastern sky (drift method) or a pole star (none down here in the southern hemisphere), or visibility of the celestial pole (polemaster).

So before I venture out on the next clear night and try this again, has anyone else tried this method?  Am I doing this correctly?  Is the drift in DEC that I observed actually corrected by adjusting the azimuth?  I was surprised that APPM was not able to correct this small error, which means polar alignment must be very accurate to do long period unguided imaging.

Craig




Join main@ap-gto.groups.io to automatically receive all group messages.