I’m looking at a 8"
step lens at the moment - forget where I got it from. Has the very
flat ceramic paint on it’s steps. Likely this would peel up potentially if the cleaning solution were left on for a while. I’m noting that the paint is not just on the inside of the steps but also wrapping the corner by about 1/16". (Kopp #FF-5088, 8 DIA x 11f - wish all lenses were as well marked.)
Paint or no paint shouldn’t
effect anything but it’s feasible and very possible that it does.
Your not noting any visible rings of light is good in the main
field angle but these would be more of a stray light nature.
A halo
effect is typically used to describe the hard
edge of the beam. Your double
image sounds like it is from an out of focus
fixture by way of the lenses and most likely not the lamp. Try running the
lens train, that inner beam should grow larger almost to the extent of the two beams converging. If you cannot by running the
lens train get them to converge, you probably have the wrong
lens train in the
fixture - going back to your wide verses narrow angle
lens. Theoretically a shuttered
fixture that’s far out of focus would have two separate rings to it shuttered or not.
In other words, the
lens is not at the
focal point of the beam right now. A wider angle
lens might need to be closer to the lamp thus a shorter
lens train
barrel. If you have some
Altman 360Q fixtures about, you will note for instance that a 6x9 is a
bit shorter than a 6x12 in this way. Going to be the same with any other radial or
axial Leko of those generations.
As for
bench focus, get a hard
edge (or as near to one as you can) first than you
bench focus. Hard to tell where the
filament is in reference to the
reflector until you have a
image that can show it and what the now hot spot and even beam of light looks like. At that
point you are also looking at the
edge of the hard
edge beam for what the halo
effect looks like - should be hard
edge and the same color all the way around it. Something out of axis will show hot spots, dark spots and often two-colored halo effects. If it helps, if you have gotten this far, your initial focus should be fine for now and is most likely near where it needs to be.
I note that your description does not note what the more narrow
lens did with the
fixture - no doubt it was fine?
Finally, don’t be afraid of bench focusing - one should more be afraid of what a pain in the rear it is than of stuff blowing up - really really rare and contained within the
fixture should it. Did a
bench focus on a
360Q type
fixture not long ago and cranked the lamp against a
reflector until it blew. All that happened was the lamp suddenly went out. That’s it no bang, no spark just went out and I knew that it was like $20.00 down the hole.
In other posts I and others have described procedures for bench focusing both radial and
axial fixtures. Have a read. Starts with opening the
fixture up and eyeballing it centered on the
reflector, than closing it and adjusting some, than opening it again re-centering or inspecting, than re-adjusting until you do find the optical center. If that’s too near the optical center, could be that the
reflector has a
bit of warp to it or something. Possible in which case you would never be able to get the correct beam and your halo on the hard
edge most likely would be say brown on top and blue on the bottom. Good luck, it’s not that bad but seek trained help in doing so - lots of little details or things that only eyes on site can convey in teaching.