Anyone using this with a 120mm f/7.5 OTA? TS-Optics TSFlat25koll · Bob ten Berge · ... · 2 · 125 · 2

I can't figure out what the adjustment value for this focal length should be.
The OTA I'm using this on is a Sky-Watcher Equinox 120ED Pro f/7.5, with a 900mm focal length.

This is the list that Teleskop Service provides with the required, but I can't for the life of me figure out the correct adjustment for my scope:

The following settings from the innermost position of the adjustment to the sensor are recommended:

aperture 80 mm / focal ratio f/6 - 81.2 mm

aperture 80 mm / focal ratio f/7 - 80.2 mm

Aperture 102 mm / focal ratio f/7 - 75.3 mm

Aperture 115 mm / focal ratio f/7 - 73.0 mm    (focal length: 805mm)

Aperture 125 mm / focal ratio f/7.8 - 69.7 mm    (focal length: 975mm)

Aperture 130 mm / focal ratio f/7 - 69.7 mm    (focal length: 910mm)

Aperture 140 mm / focal ratio f/6.5 - 68.0 mm    (focal length: 910mm)

Aperture 155 mm / focal ratio f/8 - 57.0 mm

I added the actual focal length in Italic behind the examples that are close to my scope.

The backfocus is determined by the last lens element in the imaging train, so it should be this corrector.
Since this corrector is adjustable, I'm assuming the backfocus changes when rotating the adjustment collar, and these recommended positions in the list above show the different backfocus values.

In my image train looks like this:

The rotator takes up 15.2mm and the filter wheel 20mm. The camera is directly threaded onto the filter wheel and adds 6.5mm to the actual sensor.
This totals to 41.2mm, so I just figured I'd set the adjustment ring to 13.3mm and get to a total backfocus of 55mm.
The images I take with this setting do appear sharp, but when I set the adjustment ring to something like 5mm the focus changes, of course, but the image does seem to be just as sharp after refocusing.
So, I wonder, how important is it to get this dialed in perfectly? Can anyone help me out here?

The answer to this question I got from Teleskop-Service was that the adjustment value should be "somewhere around 70mm", which is too vague for me tbh.
But if that is true, I would have to include the 11mm ring on my camera to get to 52.2mm and adjust the corrector to 17.8mm.

Thanks for reading this rant!

Hi, thanks for your reply!

The thing that is confusing me, is that the aperture doesn't seem to be the only deciding factor.
As you can see in the example list from TS, the 125mm and 130mm apertures have the same adjustment value, while their focal differs.

But I guess your opinion/guess is good enough for me to try out the next time there's clear skies.
We'll see how that goes, it may take a while because the forecast is not that good!