Re: RAPAS on Mach2

W Hilmo

I have found the RAPAS to be quite accurate.


It is suggested to have the mount level because if it’s not level in the east/west direction, it means that the reticule would be rotated clockwise or counter-clockwise, depending on which direction the mount is unlevel.  If you stop and think about it, you would need to have a significant unlevel situation for it to be a problem.  Certainly if the mount “looks” level, then it should be fine.


When I set up in the field, my general workflow is to set up the mount during the day and go through the daytime polar alignment routine.  Then, at dusk I use the RAPAS to do a final tweak to the polar alignment.  If I do this, and then blink a stack of a whole night’s imaging, there is no noticeable field rotation between the first and last images of the session.  The daytime alignment takes me two to three minutes while the sun is up, and the tweak with the RAPAS takes about 30 seconds.


If you are going to image unguided, my suggestion is to do an APPM run before it gets dark enough that you are wasting productive imaging time, so that you can use a tracking model.  You should be easily able to get a 100 point, or so, run before it gets really dark.  The tracking model will compensate for any residual polar alignment error (which, from my experience with the RAPAS, will be really small, if noticeable at all).


I did do one really good PEMPro polar alignment to check the RAPAS calibration.  This is a one time exercise per mount (I have two mounts, so I’ve done it once for each).  To be honest, though, I found that just as shipped (without the one-time calibration), the accuracy of the RAPAS is pretty good.




From: <> On Behalf Of Terri Zittritsch
Sent: Thursday, April 16, 2020 10:28 AM
Subject: Re: [ap-gto] RAPAS on Mach2


On Thu, Apr 16, 2020 at 11:17 AM, uncarollo2 <chris1011@...> wrote:

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.



Roland, I didn't expect you to respond.    The RAPAS feels like the rest of the kit I have from AP, solid and built like a tank!  Being an engineer, I can appreciate it all.

Throughout the instructions I received, in numerous places, are notes to make sure the mount is level which is why I asked.  It didn't take long to think about the implications of the mount not being level, or more accurately, the RAPAS not being precisely vertical, on the accuracy on polar alignment.   The instructions note that you can loosen the 'do not loosen' screws to readjust the reticle if it's moved during shipping, but doesn't seem to be part of the normal (do every time) alignment process.   At least not in the instructions I have, so thanks for sharing this.
I'm just trying to understand whether there is any way to do really accurate polar alignment, 30" or less,  in order to do short unguided exposures, without going to the traditional drift alignment.. I've found drift alignment to be a miserable process to go through.  Life is too short, and clear skies much shorter!   I'm not setting up on a level surface, or on a pier, every night is different.   Since I have all of the equipment, guiding is not a terrible thing to have to do.   I'm just thinking I have this really really nice mount, and is there any way to take advantage of it's full capability in a portable situation.   


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