SkyWatcher vs TS Photoline Field Flattener/Reducer - Comparison, Backfocus, Collimation - What to do? :)

[Daniel Karl]

Hey all ... 

So I had another late one last night playing around with my new setup. I had a look at the data today and am trying to interpret it in my sleep deprived state and decide what to
do next - perhaps anyone has some suggestions

The setup is:

130 f/6.6 Triplet Apo
ASI294MM Pro
EFW with ZWO 7nm HSO filters
iOptron CEM70
ASI120MMmini on Skywatcher 8x50 finder/guidescope
ASIAir Pro
 

I chose the Crescent Nebula in Ha as my test subject. (I live in London, so Bortle 8 at least)

The exact optical design of the APO objective is still a bit of a mystery to me. Some guy called Stevie T sold them about ten years ago under the label SET-optics. I found a few old
discussions about them but nothing too illuminating in regards to the optical elements. The tube and cell seem very nicely made and visually the scope seemed fine to my out of 
practice eyes.

This leaves me with the task of finding a reducer/flattener that works and the question of collimation after ten years or so.
I currently have a Skywatcher 0.85 FF from the 120 ED Apo and a 2" TS Photoline 0.79 reducer.
Yesterday I just tried both of them with a standard backfocus to get a starting point. The closest I could get to 55mm with the tubes that come with the ASI294 and EFW is

49.6mm+6.5mm=56.1mm
with a 2mm ZWO filter I should theoretically aim to get 55.7mm right? So I am not quite on target.

I focused with a Bahtinov mask (waiting for EAF) and interestingly the focus point of Skywatcher reducer setup was a whopping 3cm closer than the TS reducer which surprised me.

Don't know how bad the seeing was, but the guiding was between 0.33 and 0.45, so good enough for now I think.

Stacked, registered and stretched the results of 30m Ha were very similar in the image centre:

 

But the corners were a different story - haha.

Top left

 

Top right

 

Bottom left

 

Bottom right

 

Now I'm trying to interpret that - haha.
It looks to me like the TS adapter wants to have a larger distance to the focal plane, whereas the Skywatcher is happier where it is, potentially a tad too close.

Both do not show consistent distortion across all corners, so I suppose there is tilt in the system - either from the focuser or from the optical cell.
I should probably try rotating the camera/reducer setup and see if the distortion rotates or stays in the same position?

TS Corners

 

SW Corners

TS seems to be best bottom left, SW seems to be best bottom right? 
What would you do next? It seems I need to get the distortion evenly across the field first, before I shim the backfocus, right?
Should I do a series of exposures rotating with each reducer to eliminate that it is a problem from mounting the camera etc in the focuser? Unfortunately the focuser does not seem
to have any threads, so I have to use 2" connectors.

Wow - this has gotten a long one - haha.
Well - any thoughts are appreciated