Telescope Reviews: What FWHM can I hope for in central London ?

O.K., a few things...

First, there is no reason you can't compare FWHM across different systems. That's the whole point. It's not going to be exact, of course, and is an indication of both the seeing and the quality/alignment/focus/cooling of your optical train. That's the whole point, of course. It incorporates everything that softens your images--both in your optics and in the sky itself. As you mentioned in one of your posts, you just need to use a star with a reasonable SNR that isn't near saturation. It's not exact--and you should probably sample values across several stars, but it will give you a good general result. Also keep in mind that, due to field curvature (even with a flattener), there will be some variation across the field.

Now, on to your issues...

Your stars show coma--it's very obvious in the luminance frames (which are binned 1x1) at 200% magnification. Stars are fan shaped across the field. The most likely cause is that the optical elements in your TOA's objective are not centered with respect to each other.

Another possibility is the same sort of decentering issue in the flattener. I'd recommend taking an exposure without the flattener in place and examine the most in-focus stars and see if they still show coma. If they do, then it's the objective. If not, it's the flattener.

This can't be a tracking issue. If it were a matter of bad tracking, you'd have rugby ball shaped stars, not fan shaped. If it were a focus issue or a result of spherical aberration, you'd have round stars that were bloated. If it were a polar alignment issue, you'd see field rotation as well as rugby ball shaped stars. If your camera weren't square to the optical axis, you'd see variations across the field with stars in one corner much worse than the other three corners.

Coma is quite distinctive, and is not intrinsic in refractors, so you have an optical problem.

Assuming you find the issue is in the objective, not the flattener, I'd recommend sending it back to your dealer for alignment and/or repair. If this is a new scope, I'd just ask for a replacement--let them do the alignment on their own time (assuming the dealer has ready access to a replacement--they may not).

One good thing... Once the alignment issue is resolved it is likely to be permanent. TOA's hold their alignment quite well. Yours may have had a severe blow in shipping, or it may have just somehow slipped through QA.

If this were a doublet design, I'd say you could go ahead and try to fix it yourself. But since it's a triplet, it's probably not a good idea.

By the way, this isn't a collimation issue in the traditional sense, so the push/pull screws on the front of the lens cell aren't going to help. Also, you should be able to see the issue visually as well. Just de-focus a bright star, and you should see that the Fresnel rings are not concentric. That's another way to confirm the problem.

Sorry, I know it's not good news. Hopefully, it's just a defective flattener that you can have replaced, but I think the objective is the more likely culprit.