Hi Paul.
You might be seeing a decrease in reflectivity in the >20-yr-old aluminum coating. If able, try comparing your diagonal to a friend's newer one. A well -aligned & -coated prism diagonal will also yield a fair comparison, as its efficiency should be 95+%, too.
Is the extra bit of reflectivity worth the extra $$? That's a subjective decision -- only you can answer it. Based solely on your own brief description of what you experienced: "slight dropoff in brightness" (not even certain it was true), & your experience level (>20 years), I *will* dare to presume this: any well-made diagonal with a smooth, flat, 90%+-reflective mirror (not necessarily 98%) will show you *all* your eyes & your C8 can see. In terms of actual discernibility, we're talking about super-subtle threshold stars & barely-perceived differences in details &/or brightness. If you are one who spends most of his/her viewing time out there at the limits of visibility, then maybe the extra smidge of reflectivity is worth the high $$.
However, there *are* factors beyond reflectivity to consider. Some are purely subjective (which is allowed, after 20+ years of viewing!). Some are:
1. The A-P, Tele Vue, & Lumicon dielectic coatings are *extremely* durable -- significantly moreso than typical hard-overcoated aluminum.
2. Their reflectivity *does not* lessen with age, as they are completely dielectric (non-metallic). Even protected aluminum will eventually oxidize; peak reflectivity will lessen over years (though nowhere near as quickly as with unprotected aluminum).
3. Assuming one starts with an extremely smooth flat, the high-end multi-layer dielectric coating process will indeed yield a smoother mirror (hence a slight advantage in scatter & resulting contrast) than possible via aluminized diagonals. Of course, this begs the ??: how visible is this advantage? Unfortunately, that depends on factors such as visual acuity, seeing conditions, atmospheric scatter, inherent brightness of objects observed, telescopic system itself (accessories, too). For me, the "less scatter" consideration was less important when I purchased the A-P Maxbright. But that's just me.
4. Aesthetics. The Maxbright is beautiful. Finish is wonderful, & it exudes precision & quality. (See? I told you subjectivity/aesthetics may be a factor!) Of course, the A-P is a 2-incher, & you may prefer the 1.25-inch format (a sound option, considering its long f/10 ratio).
Personally, I'd have no qualms buying any of the the high-end 1.25-inchers from Tele Vue, Intes, or Lumicon.
All my babble may not have helped a bit. :-)
Best wishes & luck.
-Dan
