Re: RAPAS on Mach2

Roland Christen

No need to level the mount super accurately. The polar scope will still work well even if you are off by many degrees.

If your mount is not vertical, you can align the polar scope reticle to be vertical:
1) Aim the mount so that the scope points directly at Polaris (put Polaris on the center crosshair).
2) using the mount's altitude adjuster, run Polaris up and down the vertical crosshair. Loosen and turn the eyepiece until the crosshair is aligned with the up-down motion of Polaris.
3) tighten the eyepiece in place and then use the alt and az adjusters to put Polaris at the proper place on the reticle according to the polar app.


-----Original Message-----
From: Terri Zittritsch <theresamarie11@...>
To: main <>
Sent: Thu, Apr 16, 2020 9:59 am
Subject: [ap-gto] RAPAS on Mach2

Hi all,
I've received a RAPAS to use with my Mach2 when I'm doing outreach or in other instances when I don't use a computer.   But I've not been very satisfied with the results from the polemaster I have, once I get down to actually measuring drift.. The actual drift implies much less accuracy than advertised, or that many claim to achieve.  I have not been able to get close to 30 arc seconds, but you wouldn't think this from the screen indication.   Given others experiences I've read about, I am also hopeful the RAPAS will do better, but I'm a bit unclear on this point.

So now I have the RAPAS, for a Mach2.  It is attached on the side of the mount, rather than through the central axis as we know the Mach2 doesn't have this capability.    I've also read that the mount needs to be leveled east-west accurately because the RAPAS needs to be vertical (it is fixed, not rotating in any installation).    But there is only so much accuracy in leveling the mount, or more importantly, getting repeatability.   A bubble level or electronic level is only so accurate.   I've read that .5mm of tripod settling on one foot with 1M separation between the feet will result in adding 2 arc minutes of alignment error.     I don't know how Chris Woodhouse calculated this, and maybe I'll draw this up as a geometry exercise, but you get my drift (pun totally intended).   Leveling by-leg is likely millimeters of inaccuracy or non-repeatability use to use.   0.5mm is totally unrealistic.  

I think the scope load moving side to side may introduce at least a .5mm if not more, or settling.    I'm reading in the RAPAS instructions I received, which implies that accuracy is effected by loading (so may not be repeatable scope-scope unless they have the same weight profile).

So I'm wondering if I'm chasing something that isn't achievable.. like chasing the seeing.   Am I chasing after good polar alignment that just isn't achievable on a portable setup without doing drift alignment?     For instance, for unguided imaging.   

What is the practical accuracy, and repeatability of the RAPAS?  Has anyone here actually checked it on a portable installation.   I've asked Roland and he said he doesn't know, but that I should ask those on the forum.



Join to automatically receive all group messages.