I have acquired two 2" diagonals for use with my bare TMB: a cheap Synta
and a TeleVue Everbrite. So far, after about 8 hours of observation, I have
been totally unable to tell them apart.

What difference are you looking for?

Optically, no real difference so far. Maximum magnitude is definitely the
same.

The light loss difference is not large enough for you to notice. The difference
between 90% and 99% is not enough to see visually. That is not really the point
of the Televue anyway.

Andromeda and other DS objects look strikinly similar in my 30mm 2"
Vixen EP. Bright stars seem to flare a bit less in the TV than in the
Synta.

Explain what you mean by "Flare"? If you mean coma, that is not the meaning of
"flare".

Astigmatism on the edge of the field is somewhat present in both
cases albeit in a strangely different manner: the Synta shows astigmatism
on the whole circumference while the TV only shows astigmatism on 1/3rd of
the edge of the field at a time.

Diagonals do not introduce astigmatism over only part of the field. Either all
stars are astigmatic, in which case the diagonal is not truly flat, or all
stars are round. Any astigmatism around the edge of the field is totally due to
the eyepiece. Compare the view straight through with the same eyepiece and you
will see the same astigmatism.

basically my question is what should I look at to
perceive the difference that must exist?

The difference is that the coating on the Everbright is a dielectric which does
not dim with age and is fully cleanable. This last feature is not a concern for
casual users or for deep sky views, but becomes very critical when trying to
eek out subtle detail on the planets. Diagonals get dirty in a hurry when used
regularly, and this really kills planetary contrast. The dielectric coating can
be cleaned repeatedly without introducing sleeks into the coating (which will
happen with a normal aluminum diagonal). Sleeks, will of course permanently
reduce the planetary contrast of your diagonal.

Secundarily, is there any kind of
objective test I can carry out?

No.

Roland Christen