You have a de-centering or severe tilt problem. (If you had posted this image earlier, you could have saved us a lot of time with diagnosis.)
Your image shows stars that are so elongated and bloated that they do not look like stars to the plate solver -- so only a small part (to the right) of the plate has solvable stars -- this means that the FOV plate solving is now much smaller than the sensor's FOV. For long focal lengths, this will lead to not being able to solve, since your solvable star field FOV is now too small.
The problem is that you have a significantly decentered or severely tilted image. Because of that, the usable image circle is on the right side of your image, and part of the image circle falls wastefully outside, and to the right of the image.
A perfectly centered image will have stars that emanate from the center of the image if the backfocus is too short (or rotate around the center of the image if the backfocus is too long).
With a perfectly centered optical train, even if the backfocus is incorrect, the optical aberrations (coma, etc) will be balanced on both the left and the right sides, and top and bottom edges.
Your coma is very large on the left, and smaller on the right, because the image circle is skewed to the right. I.e., the optical axis is towards the right side of the sensor frame.
Notice that the stretching of the image makes the left side of the image look like it formed by a longer focal length. This is probably what caused the 2033mm focal length estimation.
Before wasting more time with the back focus, try to first center the sensor.
Make sure the dovetail part of the Optec coupler is perfectly seated before tightening the grub screws. When you tighten the screws, slowly tighten them one by one in small increments, instead of tightening one of them completely first (just do what you do when you tighten the lug nuts of a wheel when you change a car tire).
A good dovetail should have three grub screws so you can properly center the optics.
The best way to do that is to first remove the Lepus from your OTA. Remove all the T2 spacers and camera leaving only the Lepus connected to the Optec T2 coupler (that tube with the large diameter). Place the Lepus and its coupler on a flat table, face down.
Loosen the grub screw that holds the dovetail coupler so that the coupler is loose. Now apply even pressure on top to the coupler so that it seats on the Lepus flange evenly and completely. (I often use weights to apply the pressure, but you have to be careful not to destroy anything.) While applying this downwards pressure, slowly tighten the grub screws in increments.
Again, it is imperative that the coupler is perfectly seated on the flange of the Lepus before you tighten the dovetail screws.
Retest the image with the stars. You should now have an image that has round stars on all sides of the plate and with no stretched stars on one side. You should be able to plate solve correctly, and get acceptable images from your OTA.
If you cannot center the optical train, the lenses inside your Lepus may have been misassembled. Send Optec both the image from the sensor and a picture with the ruler. (Even better to flip the direction of the ruler so it shows the linear distance instead of some hypotenuse because your ruler is slanted -- I had to do some Pythagoras stuff to determine that you indeed have the correct backfocus.
Chen
