Synch or align?

[Ags]

I am struggling to to get the least worst polar alignment of my SE mount. Currently I use my imaging target as one of my two alignment stars, but I'm wondering if I would get better tracking if I aligned on two widely separated stars and then did a synch on my observing target. Any thoughts?

[Demonperformer]

On my 6SE I have found the best results by using an object about 45 degrees (azimuth) on either side of the object I want to image. When I return to it I don't have to 'sync'.

HTH

[Merlin66]

I separate in my mind the issue of polar alignment and that of GOTO sync.

You've got to have good PA to achieve good tracking.

[Ags]

Not true merlin66 - the mount tracks fine even in alt-az mode. Good PA will prevent field rotation. DSS reports very little rotation in my images, but my tracking is shocking.

[Merlin66]

Hmm - you say the mount tracks well in Alt-Az ( which only requires a level mount and a couple of syncs) but tracks poorly (I assume in polar mode, ie with a wedge??) -

that comes down to the accuracy of the drives and the control logic being used....

[Smeg]

With auto-guiding isn't it LESS critical to be Polar Aligned?

Won't it just "adjust" more?

[steppenwolf]

With auto-guiding isn't it LESS critical to be Polar Aligned?

Won't it just "adjust" more?

Autoguiding will correct the pointing of the telescope to keep it on a the guide star but can do nothing about the field rotation imparted by a poor polar alignment.

[Ags]

I folowed Demonperformer's advice and picked two widely separated stars bracketing my target (Caph and Altair, target=M27) and did everything i could to make the alignment accurate... I centered my alignment stars using my highest magnification, and defocused the stars to make alignment even more accurate. In my test exposures I have round stars at exposures of 35 seconds, which is a record exposure length for me... :-) We'll see what the actual pictures show though...

[Merlin66]

Ags,

Are you using Alt-Az or a wedge????

[Ags]

Wedge. Aligning a polar-oriented SE mount is much the same as aligning it in AZ mode. There is an extra step you can take where the scope slews to polaris and acts as a polar scope, but that refinement is beyond me as the ST80 is too long to point to polaris.

I tracked M27 for an hour and a half and it moved very slightly to the east on the sensor. But my subs still show star trails - at right angles to the actual movement of M27. So I presume these residual star trails are coming from the very budget mechanics of the SE mount.

As the trails did not add up to a net movement, perhaps I can push to even longer exposures. The trails are short enough to process away.

[Ags]

So, to clarify, the polar alignment I am doing is very approximate indeed. All I am doing with PA is minimizing field rotation to the point where it will not affect the short exposures I am taking.

[Ags]

I figured out my problem - every couple of minutes my SE mount skips a gear and the image shifts east by about 50 pixels. The jump can be so big I don't get star trails but double dots. It's not too bad, I just have to write off a lot of subs. It also means I need to take subs in twenty minute batches and periodically realign on the target. HEQ5!!! HEQ5!!! I want a HEQ5!