How do I know if my backfocus distance is right?

In a nutshell, yes, you would have uneven stars near the edges.  A quick and dirty guide:  if the stars are elongating towards the corners, you're too close.  If they are creating a circular pattern around the centre of the image, you're too far away.  It can be very subtle or very obvious, but either way you'll notice it.

If you pixel-peep, you will notice you do have a small spacing "problem". In the M31 image notice how the stars at the top right are slightly commatic and elongated from "North East" to "South West" while the stars at the top left are also slightly commatic and elongated from "North West" to "South East". But the stars at the bottom of the image look okay so I think your problem is a very tiny sensor or optical axis tilt. 

Either way, the aberrations are so small and imperceptible. They are only visible at 1:1 magnification and one must be looking for them, and even then will not find them in the image of M33 which is slightly cropped. So if you are off, it is probably by a small fraction of a mm. I'd say don't worry about it. 

When your spacing is way off, believe me, it's not something you will need to confirm by asking other people, not with a flattener and a large sensor anyway. The aberrations will be very prominent and like you suspected even achieving focus can become a problem.

Cheers,
Dimitris

I wouldn't say you got much to fear, maybe a touch of tilt but that is about it. Looking at you images you'd have to worry more about tracking (noticeable elongation up-down in both) and lateral chromatic aberration (also maybe a touch of LCA as well).

Thanks so much everyone for your guidance on this. Much appreciated

Lynn K:
It's always best to use a scope dedicated flattener from the same manufacture of the scope.

Yes indeed. I have used the dedicated reducer/flattener made specially for this scope by the manufacturer.

John Hayes:
Lynn,
I'm sorry but I have to gently correct you on this one.  A reducer always has positive optical power as do most field flatteners (although that may depend on the particular optical design.)  The design of the system determines where the reducer goes and where the image is formed.  You are correct that the manufacturer will recommend the spacing but I would emphasize that the number one most important design parameter that the manufacturer must supply with any telescope or accessory is the "BWD" (back working distance), which is often called the "back focal distance."  Without that number, it requires a lot of testing to find the position that minimizes aberration across the field.

John

Hi John.  Your expertise is greater than mine regarding this subject. When I referred to the terms "manufacture's number" and "distance", I was referring to the manufacture's recommended "back focal distance" I should have been more spacific.

I was basing my comments on my experience with the Astro Physics 27TVPH reducer.  Rowland recommends a "back focus distance" that renders F7.4.  However he also goes to some length to explaine that different distances will render different foacal ratios.  He give the example of a distance that will render a F8 ratio.  And, it can be used at other distances.  It is not a flattener and varying the distance will have different effect on the flat field.  The recommended F7.4 probably has the least affect on the scope native flat field.   I use it at F4.8 with a AP130GTX, but have also use it at F4.5 with a TMB130SS.  It works well with small to mid-sized chips.  The newer Astro Physics Quad Telecompressor is a combination reducer/flattener and must be used at correct "back focal distance".

The other reducer I have experenced using at different back focal distances is the old SBIG E-Finder reducer. You may (or may not) remember it could be placed in different positions to produce different F ratios.  If I remember correctly, it could be placed on a F10 SCT to produce a F5 or F3.3 ratio, despite that it's flat field was poor.

I was basing my comments on those experiences, and concluding that reducers do not have a fixed back focal distance.

Thank you for your comments.

Lynn K.