Quick alignment question

[Shibby]

For imaging, I'd like only the RA axis to be driven... Am I right in thinking that, after doing a polar alignment, a 1-star align will result in only the RA axis being active for tracking? (I want to keep Dec connected for positioning etc using Stellarium).

I'm using EQMOD, by the way. The configured 'home position' doesn't affect things does it? It seems to me that synching on one point only would mean everything is relative to that and it won't be trying to compensate on Dec while tracking, only RA. Or am I wrong here?

Ok, the question isn't that quick after all, but hopefully the answer will be

Thanks,

Lewis

[roundycat]

You can usually choose whether one or both axes are driven when tracking. Turning Dec off is risky as you can never polar align perfectly due to atmospheric refraction.

Polar alignment is to do with the mount, getting the RA axis plane parallel to the Earth's axis, nothing more or less. It has nothing to do with tracking. You can get very good PA with all the mounts drives switched off.

Under normal circumstances if you align on one star the mount should know where it is. I never synch on more than one star, there is no point.

Dennis

[Shibby]

Although PA has nothing to do with tracking, it does mean that the tracking should be pretty accurate with just the RA drive right? (apart from atmospheric refraction - hadn't considered that!) EQMOD 'compensates' for any errors in the PA + other factors when doing a multi-point align. Are you saying that when you synch on one star, it still does this and therefore your Dec drive is active??

[Shibby]

...Oh, unless you autoguide of course, this would of course utilise dec

[ollypenrice]

One thought on Dennis's point about one star alignment; if you have cone error (scope not parallel with polar axis) then 3 star can, I believe, compensate.

In practice I do a one-star for imaging and align on a star near the target. If I want to buzz around the sky visually or if I know I have a meridian flip then I'm tempted to do a three star.

As for imaging without the dec drive active in autoguiding, my own experience is that it's a nice idea but I have never got it to work! Not on an EQ6, anyway. I usually correct on Dec but with a low aggressivity and sometimes I only correct in one direction.

There is backlash to consider, a bit of free play and all those other little things that conspire against a good theory, quite apart from the difficulty of getting a polar alignment that is good enough, and that's murder.

Olly

[Merlin66]

If your mount is "properly" polar aligned then it should be able to track objects just by using the RA drive motor. Obviously the further out the polar alignement, the more Dec drift you'll see, that's when the Dec comes in.

On the older (1960's etc) mounts it was not unusual to only fit an RA motor and the Dec was left as a manual adjustment only.

Worked for me for 10 years!!

Ken

[Sam]

I've found that I've had the least amount of problems when correctly polar aligned and a full drift alignment done to where there's little or no movement for 5 minutes. In theory you should be able to get away with RA only but I think in practice it still helps having the DEC going.