Re: RAPAS on Mach2

W Hilmo

Honestly, I’ve never measured my polar alignment error.


Beyond that, I don’t even know how to measure it in a way that would be useful in conversation.  I don’t know if you follow Cloudy Nights or not, but there is a near obsession over there with polar alignment.  I’ve seen people making polar alignment error claims that border on the ridiculous (like people claiming to be within 5” of the pole, etc.)  The drift from a 5” polar alignment error would be so small that I don’t know how you could detect it with a single exposure.  I would think that looking at the total field rotation for an entire night would be a better way to try and see a very small error.  I am pretty sure that people are just running some tool like a PoleMaster or SharpCap and just repeating what the software claims.  The thing is, that the software claims are not repeatable, even in the same session.  It’s made even more confusing because I am pretty sure that different software packages calculate the polar alignment error differently.


Even with PEMPro, which is the gold standard for software aided drift alignments, I can watch the calculated error bounce around by some number of arc seconds with each exposure, just due to seeing effects.


I take a more pragmatic view.  If I don’t get any noticeable drift in a single sub, and if I don’t get any field rotation over a night of imaging the same object, then I’ll call the polar alignment “good enough”.  If your goal is to do longish exposures unguided, especially at a longer focal length, then I would probably do it one of two ways:  If you take the time to run APCC, an APPM model will compensate for any polar misalignment, as well as drift from other factors (as long as they are repeatable).  That would be my preferred method.  If I didn’t want to do an APPM run for some reason, then I would do the RA Drift Alignment that’s described on the Astro-Physics site at the below link.  To do it, I would run PHD2 with my main imaging camera and guide corrections disabled.  This would give me a graph of the guide star during tracking that I could use to know which corrections to make:


The other option, that is new to the keypad firmware that will be available for the Mach2, will be that you can get tracking compensation right from the keypad (and it will also be available for the CP4).  I don’t believe that any of us have that keypad firmware yet, so I don’t know of anyone outside of the Astro-Physics folks that have first hand experience with it.  If I understand what I’ve heard correctly, it will work by calibrating on a couple of stars in the path of your object and building custom tracking rates for your specific target.  I’m guessing that it will be pretty effective.


At the end of the day, I believe that it’s important to understand a few methods to do an  effective polar alignment and also to not obsess over it.


In the meantime, how do you know that your polar alignment is causing problematic drift?  If the Mach2 ships with PEMPro, I would suggest that you start its polar alignment wizard and let it track for a while.  It will clearly show you any drift (and offer the change to make an adjustment to correct it).  If you don’t have PEMPro, then you can do a similar thing with PHD2 (as I described above, using it to facilitate a drift alignment).


And finally, I want to close the loop on a comment that I made earlier about my personal workflow at field sites.  I mentioned that I do both a daytime polar alignment and also use the RAPAS to tweak it.  The purpose of the daytime polar alignment is to get the mount reasonably close to polar aligned before it starts to get dark.  That way, when it gets to be dusk, you are pretty much guaranteed that Polaris will be both inside the view of the RAPAS and also obvious.  If you wait until after dark and start with the RAPAS, it can be tricky to find Polaris correct, especially if it lands outside of the RAPAS field of view.  In that case, it takes longer than 30 seconds (and I’ve seen more than one person polar align to Kochab in that scenario).


Hope that you find this information useful,





From: <> On Behalf Of Terri Zittritsch
Sent: Thursday, April 16, 2020 12:30 PM
Subject: Re: [ap-gto] RAPAS on Mach2


Thanks W Hilmo,

Not sure I ever see field rotation with any of my gross polar alignment methods.  I think you'd have to be pretty far off to see field rotation (but honestly don't know what that would be).  I'm trying to understand what kind of measured repeatable polar alignment people get.    Is 2 minutes, 3 minutes?   30 seconds?   With Polemaster, it's advertised at 30", and I see all kinds of claims on-line (hard to believe claims.. and claims that would be hard to even measure), but who knows.  Maybe I am just the worst polar aligner ever.    If I can do an accurate polar alignment (30") in 30 seconds, I will be exceptionally happy!!   I've never tried the daytime polar alignment.


Join to automatically receive all group messages.