Re: PemPro Polar Alignment differs East to West

Roland Christen

I went thru the same process with our remote scope in Chile several years ago. We had the polar alignment dead on with no movement in Dec for many minutes. The mount is a 1600 and scope was a 12" Mak-Cass astrograph. When we switched the scope to the other side and checked alignment on the west side, it was off, much to my amazement. In both cases the Dec axis was sitting still, not being driven. I have tried to simulate this motion via computer analysis, but no matter what changed, the alignment should remain exactly the same on either side of the axis. Even with the RA and Dec axes out of square the drift is still zero in both cases.

It then dawned on me that at the long focal length of the 12" scope, the effects of atmospheric refraction caused a small change in the star's declination. So on one side it drifts up slightly over time and on the other side it drifts down on the chip. I finally set the azimuth so that the drifts were equal on both sides rather than zero.

I might be totally wrong about this effect and its cause, but I have seen this on all the mounts that I have set up here at my observatory using PEMPRO polar alignment routine.


-----Original Message-----
From: andrew.jones@... [ap-gto]
To: ap-gto
Sent: Sat, Mar 3, 2018 1:08 pm
Subject: [ap-gto] PemPro Polar Alignment differs East to West

Hello Everyone. I am looking for some guidance. I have a Mach 1 on a permanent pier in an observatory that I am in the process of getting setup. I currently have a 11" EdgeHD SCT mounted on the Mach 1. Last night I finally got around to getting my polar alignment dialed it. I have been using a PoleMaster to get me pretty close, but wanted to see if I could further refine the alignment using PemPro 3 since I had recently paid for the upgrade.

I started the PEMPro Alignment Wizard on the West side of the Meridian and did both the Azimuth and Altitude alignments several times until I got both under 1 arc min which was pretty much the limit of my Seeing last night. I then decided to switch to the East side of the Meridian and run the Polar Alignment Wizard again just to confirm I was aligned there as well. To my surprise I was off by quite a bit (3 - 5 arc mins) East of the meridian after letting the drift steps run for 3 - 5 mins. I didn't want to do any more adj ustments as it took me several hours to get it to within 1 arc min West of the meridian. I switched back to the West and ran the polar alignment wizard again for both Az and Alt and after 5 mins both were still under 1 arc min. When I got to the step of moving the star to the end of the pointed arrow I didn't have to do anything as they were both dead on the point of the arrow. All the PEMPro 3 settings I used for where to slew in the sky where left at there defaults.

So my questions for the group is why would my polar alignment be different on one side of the meridian vs. the other? I thought once the mount is polar aligned it really should not matter which side of the Pier it is on. I have no idea why doing a pier flip to the other side of the meridian would cause such a big swing in the alignment. My only guess is that maybe my OTA is not sitting Orthogonal on the mount but I wouldn't think it would matter for drift alignment. The other thought I had is may be I need to adjust my balance to be slightly weights heavy. Currently it is almost perfectly balanced on both axis. The Mach 1 is only about a year and half old so would not expect there to be much backlash as it has the new self adjusting gear mesh mechanism.

Needless to say, I am kind of stumped. I would welcome any knowledgeable advice on what might be going on here as I am not really sure what to do next.

Thanks in advance for the helpful advice.

Clear Skies,
Andrew J

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