Another aspect of the 685 comes to mind ...
On the Dielectric Diagonal thread there was some question as to whether or not the telescope objective was passing the extra spectrum the BBHS diagonal was capable of.
If the objective was not passing much above 700nm, then using a 685 long pass should result in quite a dim view. Since it does not, it would seem lenses and mirrors a not "gatekeepers" on the 700-900nm range.
Inconclusive by itself.
That's what led me getting the Lumicon and 685 in the first place. I wasn't aware that the 642 is a band pass (the shiny mirror finish was no match for my idiocy) with a cut-off around 850. The diode in my remote is above that cutoff. What passes through it is the reflection of the diode's light on the body of the remote. The remote's plastic body is actually transparent in near-IR.
The sky background is not much different between the Lumicon and the 685, but the nebulae are obviously much smaller. It's really too close. It's 40nms against the hundreds they both pass. What muddies the waters is that the sky background is not much different between the 642 and 685, under my skies. The spectral response of my particular tube is unknown, as is the spectrum of my sky. A panel of various LEDs at different wavelengths, or a tunable laser, is much easier.
Comparing 685 alone, with 685 + 642 (with its cutoff) is something I haven't tried yet. I've dealt with the refactors by not caring anymore and just accepting it. The dob is the issue because I can't just take off the secondary to compare images. It's secondary is dielectric aluminum and its primary is overcoated aluminum. I've been using the 67PP attached to the wheel anyway, so I'll try sliding in the 642 with the slide. There should be a break in the monsoon tonight.
Nebula killer is fun, so the 685 is a keeper. The Lumicon is parfocal with it, so that's also a keeper. The 642 is just a great filter, so that's also a keeper.