Same old newbie alert

Probably nothing... Seeing, altitude and the Jetstream position all play a vital role in getting a decent image..all of which are out of your hands Some things that could cause the issues you suggest is more likely air thermals in the tube, do you store the scope inside? As it needs the reach thermal equilibrium with the dropping temps.. so needs a decent amount of time to do this

If you factor in the polar mis- alignment @ 1.7 arc mins reduce that down far more than it is then your Dec reading will be far better What size threaded bar are you using on the rat cage?

EC means encoders... Some have encoders on just the Ra, others at a much higher cost can be on both axis... Ioptron have EC versions on most of the range they sell

No idea how they work or know of anyone that uses one.. sorry... Nice scope btw, enjoy

I'm getting decent stars at 1280mm focal length .. I've not measured the spacing I just added spacers and platesolved until I got somewhere near 1280 but I think I'm more than 105mm I've just landed back in the UK so when I get a chance I measure it and report back

Have you looked at cone error rather than concentricity?

I'd look for one with a large prism, an 8mm prism is too small, a 12 is much better and with as many solid fittings as poss.. just make sure you have a sensitive guidecam What size tube are you using?

Using a oag and a C8 isn't too difficult, the most difficult part for me was finding the correct adapter so I could incorporate my system into the required backfocus.. you would think using a Celestron scope with a Celestron oag and a Celestron branded guide cam would work wouldn't you? But it never did, and asking the brand for specs and advice made me feel like I was the div... Now Celestron say the required backfocus for their brand of reducer/flatener is 105mm so on platesloving this gave me a focal length of 1320mm and my stars wasn't fantastic, ie pointing inwards... So after much too-ing and fro-ing, I decided to adjust it to work at the spec'd focal length, ie 1280... So I increased the spacing which reduced the focal length.. now I'm working at 1280mm and stars look much better.. adding a better collimation helps enormously .. I've seen the 85mm floating around on cloudynights The reason I think youre getting a vignette look is the size of your sensor, I get dark corners with my atik 460, yours is a 4/3rds size I think, it's all to do with the baffle in the c8 I have no issues with my qhy 290 finding guidestar, never had to rotate the guidecam to find one, always plenty in the fov.. I just aligned the long side of the guidecam with the long side of the prism and imaging cam.. My procedure was to get the spacing of the imaging camera right,measure the distance from the prism to imaging sensor and that figure must be the same from the prism to guidecam incorporating the backfocus of the guide cam.. in PhD loop the camera and look for seagull shaped stars and fine tune until you get them as round as poss... That's it.. everytime you set up it's done ready for those galaxies, nebulae etc

Polar scope method is regarded as a rough alignment

So is it showing just in just the stacked images?

I first thought it was caused by Elon musk satellites but as they stop in the same place I retract that.. Must be the readout from the camera, so might be worth changing the cameras usb.. are you using a hub?

I have yet to identify the direction of the elongation. Astap won’t plate solve the C8 images as the FOV is too small. Astap will platesolve at 1280mm as I use it myself, it's fussy on star shapes thou, so focus need s to be fairly good That glow in the center tells me your spacing is wrong, and if I remember correctly you will need to reduce it to get your focal length to increase... I'm talking of only a few mm

Sorry I assumed you to be using PhD, but looking at the screenshot was you on the simulator? The pulse values haven't been filled, so I'd assume you was guiding with the simulator..

What was the guidestar snr reading? The guide assistant will help iron out the tracking errors a little bit, it switches the guiding off and tracks the star to see how it performs then it calculates the min mo, aggression etc, in the tools tab.. the camera loop depends on how well the mount performs

They say once you have a Tak you don't go back Wouldn't personally know thou.. Its worth a mention that the scope will have several different labels, like from TS products, Technosky.. the Altair might have a different focuser than the others...