Polar Alignment or Auto Guiding Issue on Long Exposures

Not really. Besides the accuracy of the graph which I would dispute, if your PA is around 3 arcmin guiding isn't really affected at all. Something else is at play, one possible culprit is how solid is the guidescope attached to the scope which is n.1 issue with guidescopes in general.

Simple way to test this (I don't think it's as robust as it should be):

Point the scope at the zenith and take a short exposure without guiding, say 10s, with both scopes at the same time. Then point the scope at the meridian @ 30 deg altitude and take the same shots. Using PI or something similar align the shots (use as reference the one with highest resolution/image scale)  first at the zenith and copy or note down the transformation matrix and then repeat the same with the one at the meridian. If the system is truly rigid the two transformation matrices are going to be identical (or close enough within the error margin). The degree with which they diverge is an indication of the degree of flexibility of the system.

Polar alignment issues will generally show up as image rotation and it will become worse the closer you are to the pole.  The four things that can cause elongated stars are:

1) Mechanical flexure between the guide system and the imaging system (as Andrea has pointed out.)
2) Guiding errors that can be caused by poor guider calibration, warm pixels, drive train errors, and a bunch of other stuff.
3) Mechanical vibration, which can be caused by poor stability where the scope is set up (such as on a balcony), fan induced vibration, focus flop, and other things that cause movement of the image on the sensor.
4) Astigmatic optical errors due to optical misalignment, defocus in an astigmatic system, or field astigmatism.

The biggest mechanical problem that I immediately see in your rig are the ring supports for your guide scope.  Rings are notoriously unstable and that is very likely a big part of your problem.  You must have a VERY mechanically ridged connection between your guide-scope and the imaging scope so get rid of those rings and figure out how to firmly bolt that guide-scope directly to the imaging scope.  There is no need for angle adjustment.  The important thing is rigidity.

Fix that first and then let us know what you get.  My guess is that you’ll immediately get better results.  Then we’ll work through the secondary problems that you might uncover next.

John

I'm extremely confident that the guiding graph/statistics you're showing are not accurate.  The way you have your guide scope attached also has many points of flexure.  The scope supports on those types of rings flex a lot.  The rings themselves are mounted with only one point of contact with the dovetail, which will flex.  The dovetail is mounted to a handle in a cantilever manner which will flex.  The attachment point between the handle and the scope is small and will flex/slide with the expansion/contraction of the metal and the only thing holding it being a hand tightened wingnut.  The whole lot is probably flexing relative to the mount, also, since the scope is attached to the saddle in just one spot with a small shoe.

I have to question your guiding graph for starters, I wonder if you've got the guide scope focal length set up correctly, but I can definately guarantee that your mount is not capable of 0.07" RMS guiding numbers.

If your guide scope focal length is set up incorrectly, you will get RMS correction values that are incorrect, and you could indeed have considerably worse guiding than you are being lead to believe.

Secondly, if you're getting star trailing in long (lets say, 2~10 minute exposures) with a sub 5 arcminute polar alignment, then something else is going horribly wrong… Especially with a 264mm focal length… Honestly with a 264mm focal length and a sub arcminute polar alignment, a 3 minute unguided sub should be perfectly fine or very close to. 

I would 100% be aiming for a 1 arcmin or better polar alignment, but I can guarantee than anything under 5 arcmin is perfectly good enough for a setup like yours. Any star trailing you're seeing is going to be on account of flexture in the system somewhere. This could be in the guide scopes focuser, the guide scopes adjustment rings, the guide scopes mount to the imaging scope and so on. 

Go through all the fixutre screws and make sure they are tight..

Another thing that CAN be an issue, is if your guide scope's axis is not relatively parallel to the imaging scopes axis, that can certainly negatively affect your results. Where possible, you want to be guiding on a star inside the imaging scopes field of view, or just outside of it.

Essentially, what you're experiencing here is why so many people move to an off-axis guider, and never look back. Guide scope differential flexture was the bane of my existence 15 years ago, I moved to an OAG and now I use them on EVERY telescope I use from 300mm focal length to 2840mm focal length. The only time I would use a separate guide scope now is with a camera lens, (135~200mm focal lengths) and that's only because the backfocus limitation would prevent me using an OAG.