General Polar Alignment Question

chichiflys

HI - just a general question about using the ASI Air to perform a PA for the mount. Seems to work well enough, but doesn't the accuracy depend highly on how well the OTA is aligned with the mount's polar axis? In other words, if I center a star (say Polaris) in the mounts polar view scope, wouldn't the OTA also have to have that same star centered in the camera's view finder (when in the "home position") in order for the polar alignment to be accurate? So the OTA would have to be precisely aligned with the mount's polar axis....hope that made sense. And the only way to do that is to physically "fine-tune" the OTA mounting bracket where it's attached to the mount, right?

GooE

I made my own bracket for my ota and I guarantee it isn't centered. I also guarantee my RA and DEC aren't centered when I PA. Yet, the ASIair has no issues with the process. I think the only issue, with what you're talking about, is if there was so much magnification that the unit couldn't plate solve.

chichiflys

Hi GooE,
Thank you for the reply. Im in the same boat - for my wide field OTA I have a home-made mount bracket. I agree the ASI does seem to PA and plate solve quite well, but what I'm getting at, and maybe it's more academic than practical, is that the ASI is essentially aligning the OTA with the pole...not so much the mount if in fact the mount's polar scope is not perfectly aligned with the OTA/camera sensor. Even though you physically adjust the mount's dec and RA axis against what the ASI is "seeing" you're not really getting the mount perfectly aligned with the pole, and that is essentially the whole point of a PA. So it's not going to be that accurate if the OTA is not centered with the mount's polar axis....again that's just a thought and maybe Im off...in the end I seem to have relatively good success when auto guiding, but I have an Orion Atlas mount, not the most fine-tuned machine in this hobby so an accurate PA is important especially if I use my ES127 scope.

darkmatter

There is no need for the OTA to be aligned with the RA axis. What the Asiair is does is analyze the arc the stars make when the mount rotates during the polar alignment and from that it calculates where the axis is pointing. It doesn't matter how the OTA is oriented (as long as the pole is within the FOV of course) because that arc is what it's all about, and that doesn't change if the OTA isn't exactly parallel to the axis.

Jarno

w7ay

darkmatter as long as the pole is within the FOV of course

The pole does not need to be within the FOV of the camera. It is simply a virtual point in the coordinate system.

You can actually position the Declination of the mount almost 30 degrees away from the pole, and the ASIAIR will polar align for you.

The accuracy is reduced when the declination is far away from the pole (see one of my earlier postings where I used a mount simulator to measure the alignment error).

Chen

chichiflys

Ah OK then - thanks for clarifying and educating me on how that works. Makes sense I guess, and again it seems to work pretty well - my auto guiding has been good enough for my needs so far. Of course my mount could probably use a hyper tune to make it's RA/DEC adjustments smoother and easier to accomplish, but that's why I get for a "budget" mount. Cheers...