actually, since im around 134mm with the reducer, it comes now close to what it should be.
numerically, the unsharp m27 at 105mm has in the ha narrowband channel
fwhm 10.95 excentricity 0.449
the bubble at 135mm has
fhwm 7.43 and excentricity 0.377
one should note that the bubble was made from my balcony with much seeing. the unsharp m27 was made in the field without much seiing.
At first sight it seems m27 shows less eggs. but that is just because it is so unsharp that the blue eggy smear does only get noticed in the raw images.
I think the reducer, at the correct distance, should not affect resolution in any way. It has many lenses to correct its own color error, coma or field curvature.
the reducer has no way, of course, to correct the sphaerochromatism and remaining coma of the aspheric schmidt corrector.
The edge 8 is optimized to minimize these errors at a backfocus of around 134mm.
If it has blur at other distances, the reducer will show blur there too.
So the claim from celestron to go to 105mm for the reducer of the edge 8 seems to make physically no sense to me.
So yes, what i believe is this:
If my edge with the reducer shows an optimum distance at say 138mm or something at f7, then 138 mm will also be the correct backfocus when using it at f10...
My guess is also that at the correct backfocus, the edge with and without reducer is diffraction limited or close to it.
With reducer at 136mm, my ao and phd guiding seem to get the stars bumping around within a circle of 0.5 arcsecond, with seiing on my balcony. That is the scopes resolution.
What is interesting is: At 105mm with the reducer, i have difficulties finding a guide star because everything is so blurred.
At 136mm the stars burn out in the guiding cam.
This means i can now regularly use 0.05 sek exposures and so on, which is nice if you have an active optics.
Its also good for guiding performance if the stars are small dots....
Finally, I guess that these scopes are all mass produced and precise.
I therefore think they have tolerances of 0.1mm. So if celestron writes a wrong backfocus for one scope, the advertisement sheets may have it wrong for all scopes.
These things can happen when a company outsources manufacturing to somewhere else and then some parts have a bit different specs, no one notices the change and the advertisement sheet stays in place..
So if one has here people with measurement devices like optical benches and zygos...
Perhaps it maybe funny to measure with testing devices whether the backfocus is really as advertised or not, with and without reducer for several edge hd specimen.
Then one can put up the correct backfocus values here and people then can look it up.
Testing every distance in milimeters simply wastes clear nights....