Is this tilt, or what?

Take a single sub and use ASTAP to analyze the tilt, it will give you an idea how tilted it is.  It's hard to say much from the resolution posted, and it is also hard to ever say much with an integrated sample especially since it may have shots on both side of the meridian.

Anyone with technical know how with software involved willing to check it out for me? Coming from DSLR's, it's the first time i'm dealing with something like this.

Here's a random unprocessed sub on my Drive.

Hi Piotr,
See below the analysis using ASTAP and PIs Eccentricity script:




As Andrea said, left bottom is the culprit.

Now it is best to turn the camera by 90 degrees without the reducer/flattener and check if the tilt moved with it or not. That'll tell you where the tilt is coming from. Based on the star form the chip should be brought closer the bottom left, thus towards the scope. 

I had this pretty similar with my ASI1600 MM, there it was the anti-dew heating strip which caused a tilt between the filter wheel and the camera.
Took me ages to find this.


CS
David

Thanks for all the input guys.

Gonna rebuild my imagining train this weekend, and provided skies are clear, I'll do some testing you guys suggested to try and figure out if it's the connections in the train, the drawtube, or the sensor.

Piotr Janicki:
Thanks for all the input guys.

Gonna rebuild my imagining train this weekend, and provided skies are clear, I'll do some testing you guys suggested to try and figure out if it's the connections in the train, the drawtube, or the sensor.

yeah that is first thing one should do. If applicable, check your OAG. Especially the ZWO dovetail ones are prone to introduce tilt.
As you said earlier, sensor tilt is unlikely, it is probably a faulty thread since your system is on the slower end and your sensor is not particularly large.
Sensor tilt issues really start to play a role at  <F4 and APS-C and larger, at least in my experience. But every little thing counts.
BTW, it can help to use a bahtinov mask for this, see https://www.astrobin.com/forum/c/astrophotography/equipment/article-sensor-tilt-adjustment/
All the best!

Scott Badger:
I have the ZWO OAG with the dovetail connection and agree it can be problematic. Not only do the thumbscrews work themselves loose pretty easily, but making sure there's no tilt when you tighten them down in the first place can be a bit tricky. It became more of an issue when I switched from a DSLR to the heavier 268M and filter wheel. On the other hand, when my dog knocked my OTA over (it was on a cement floor, standing on end), the point of impact was the filter wheel and the "give" of the OAG connection may have helped prevent damage...... : )

I find that the two thumbscrews holding the prism stem in place are even worse in terms of loosening, which then causes misshapen guide stars. One of the screws is so recessed that getting it tight even to start is tough.

Cheers,
Scott

100% Scott! Couldn't agree more. OAG are to me a necessary evil and I avoid them where I can. Long focal lengths and/or SCTs however kinda mandate these.
CS
David

As computers and cameras improve, allowing shorter more numerous subs, maybe guiding will become a thing of the past as Richard Wright predicts. Or….at subs of 5s or less, guiding and imaging with the same camera?…..

Cheers,
Scott

Linwood Ferguson:

andrea tasselli:

David Stadermann:
Long focal lengths and/or SCTs however kinda mandate these.

I'd agree on the SCT bit. Long focal lenghts, no.

Is your rationale there that mirror movement (relative to the tube where a guide camera would be mounted) is the primary culprit? 

I would have thought the longer focal length itself was a reason to use them.  Otherwise wouldn't people with external focusers go back to guide scopes?  But I see what seems like most still including OAG's?   I ask partly because I just got an external focuser and had not considered going to a guide scope at 2800mm.

As to the OP, I wonder about focusing so heavily on tilt and not backfocus.  When I looked it seemed to have a backfocus issue as well, though neither was terrible.

***

Well, My imagining train is composed as specified by ZWO. Flattener>16,5mm spacer>11mm spacer>t2-t2 adapter 1mm>EFW 21mm> and finally the camera screwed into the EFW, with the sensor being recessed 6,5mm into the casing, bringing us to a total of 56mm, to account for the filter.
What would cause the backfocus out of whack? The flattener being out of spec by a little bit? Now, that you mention it, during plate solving, Asiair insistently claims my focal length is 522mm, as opposed to 510mm itshould reduce to.

Linwood Ferguson:
Is your rationale there that mirror movement (relative to the tube where a guide camera would be mounted) is the primary culprit?

I would have thought the longer focal length itself was a reason to use them. Otherwise wouldn't people with external focusers go back to guide scopes? But I see what seems like most still including OAG's? I ask partly because I just got an external focuser and had not considered going to a guide scope at 2800mm.

As to the OP, I wonder about focusing so heavily on tilt and not backfocus. When I looked it seemed to have a backfocus issue as well, though neither was terrible.

I'd say so. You can't guide with mirror flop/shift but you could even with 3m FL (I did) with a guide scope. You'd only need a 1/5 of the effective pixel scale on the guide scope to guide efficiently. Not saying you must do it that way, just saying you could.

It isn't backfocus distance or all the off-axis stars would show the same aberrations in proportion to the distance from the optical axis, which this one do not.

andrea tasselli:

Linwood Ferguson:
Is your rationale there that mirror movement (relative to the tube where a guide camera would be mounted) is the primary culprit?

I would have thought the longer focal length itself was a reason to use them. Otherwise wouldn't people with external focusers go back to guide scopes? But I see what seems like most still including OAG's? I ask partly because I just got an external focuser and had not considered going to a guide scope at 2800mm.

As to the OP, I wonder about focusing so heavily on tilt and not backfocus. When I looked it seemed to have a backfocus issue as well, though neither was terrible.

I'd say so. You can't guide with mirror flop/shift but you could even with 3m FL (I did) with a guide scope. You'd only need a 1/5 of the effective pixel scale on the guide scope to guide efficiently. Not saying you must do it that way, just saying you could.

It isn't backfocus distance or all the off-axis stars would show the same aberrations in proportion to the distance from the optical axis, which this one do not.

It is not difficult, but it is not trivial to get the 1/5th (which to be fair I've always heard and never tried to test).  For example, I have a 2800mm SCT and a 240mm guide scope, and my guide camera pixel density is lower, so I'm well over 1/10th.   I'd need a significantly longer guide scope if I tried to maintain that 1/5th image scale.   Though it would solve a lot of other problems, especially as I am pretty limited in backfocus with an external focuser after the flattener. 

Anyway....

To me I saw some consistency of star shapes, though I think there is also tilt.  Which would you work on first? 

To the OP - backfocus specs are estimates.  How accurate they are, and how precise they are depend a lot on the vendor and optics quality and construction precision. That's why there is a going market for spacer kits to fine tune.  For example, my SCT quotes a 146.05 back focus.  I think the 146 is about right, but the 0.05 added on the end gives a misleading implication of precision.  Also, manufacturers may or may not build into their "55mm" (or whatever) filter thickness allowance (1/3 the glass thickness gets added back to space you include).  I certainly bow to the majority here that your problem is primarily tilt, but if you head down the backfocus adjustment rat hole, trust your images, and not (completely) the specs of the gear. 

There's also combinations of gear that do not add up mechanically the same as mathematically.  I have a 3mm m-m adapter from Moonlite that is 3mm connected to a Moonlite spacer, but due to thread depth is 5mm connected to a spacer kit from Blue Fireball.  A caliper is handy to have around. 

Simple ZWO example: Camera (ASi6200) + EFW7x2 + OAG-L = 55mm.  Except, with 3mm thick filters it really is 54mm, which their "paths to 55mm" didn't include.