My friend with 60 years' observing experience uses an expensive, miserable (…to me…) red light from a reputable astronomics provider to pore at his maps…
I prefer the red/white Coast HX4 flashlight I bought at Canadian Tire for 20 bucks. It provides a wider field of light projection and is significantly brighter, although I can not quote a measurement in lumens for the red light… it is rated at 80 lumens in white…
My friend tells me that even red light can ruin your night vision if it is too bright…
That is not my actual experience.
Is there any actual data or biological plausibility suggesting that red light at a high luminous flux negatively affects night seeing?
Actually, green light is probably better for looking at maps without ruining one's night vision…
https://thehikingaut...r-night-vision/
Is there a lower practical limit to dark adaptation? I've noticed that when I use an eye patch over my observing eye, the sky seems to "blow out" after transitioning to the eyepiece. Kind of like a grainy gray color that darkens and smooths out over the course of about 20 seconds. I thought that might just be dark unadaptation, but there's no advantage before it due to the lack of contrast.
Well, there is dark adaptation and there is dark adaptation.
You can be dark adapted enough to walk around a telescope field filled with telescopes without a light, and that is what I would call level 1. You can tolerate level 30-40 of the light level on a Nexus DSC screen.
This takes about 20-30 minutes without a light to see this well. This improves, I've found, over a few hours. The idea that you achieve full dark adaptation after 30-45 minutes is contrary to my experience.
Then there is the level of dark adaptation you get from walking into the woods and waiting a few minutes until you can make out the path. That is level 2.
(If you turn around and look at the clearing you came from, it will appear very bright, and it is). IF you can maintain this level, you will be able to tolerate level 10 in brightness on the Nexus DSC screen.
Then there is the level you get to with 10 minutes of staring at a black cloth on the ground until you can make out a black eyepiece cap on the black cloth. That is level 3.
If you glance at the sky for a few seconds, even in very dark skies (21.8+), this level is gone and it takes several minutes to regain it.
When I'm going faint, I transition from the ground directly to the eyepiece without looking around or at the sky and it gains me quite a bit.
The Nexus DSC screen has to be at level 1-5 on the Nexus DSC, but it is, preferably, off.
And there is one more level of dark adaptation. I don't think you can sustain it. I call it level 4. You go into a closet at home, AFTER you are already dark adapted, in the dark, with no lights on.
You wait until you can see light coming in around the door. You can begin to see what is around you in the closet from that little bit of light.
You open the door and walk out into the room. You can feel your pupils contract and the room is almost intolerably bright. After a few seconds, you no longer have that level of dark adaptation,
and if you return to the closet you are once again blind.
I don't think you can maintain this level of dark adaptation outside a lab environment, though you might be able to approach it with a black cloth over the head that blocks out all peripheral light from every direction.
It would take several minutes, and you would have to be looking through an eyepiece at high power to reduce the field brightness sufficiently.
I think that many observers, even those at dark sites, never get past level 1. Even some of the better observers never get past level 2.
And why would they? They never look at anything really faint at the very limit of the scope and their visions.
But to achieve level 3 means no light of any kind, red or otherwise--no tablets, red LED lights, and not even any reflective tape on objects around you.
What kind of difference can it make?
I experimented with M14 in the summer. It's horizontal branch centers on magnitude 17.1 (I have a 12.5" scope).
On a normal night at the dark site, I see a slight haze of very faint stars scattered across a round nebulosity on this cluster.
But when I go to the trouble of trying to reach level 3, which takes some time, and I immediately look through the scope, I see hundreds, if not thousands, of stars from edge to edge and the nebulosity is gone
and it resembles a normal globular cluster, only with exceedingly faint stars.
The difference is profound because I have crossed a threshold that allows me to see to, and maybe past, the horizontal branch magnitude.
If I look up at the sky, and trace out a few star patterns, and then look down into the eyepiece again, the hazy nebulous look has returned and I see a scattering of stars across the cluster.
So what level of dark adaptation you want to have, or how far you want to push your scope, is up to you, but just be aware that ANY light--the sky, a red LED on dim, or even the light of someone's LED across a field from you--
will negatively affect your night vision. A too-bright red light is just a sign the observer is not dark adapted, never will be, or doesn't care to be.
By the way, I had a red LED light a few years ago that was 5 lumens on its dimmest setting, and it seemed more than bright enough to see the settings on my Paracorr and the focal lengths on the eyepieces
with the light 15-18" away. I thought it was quite bright. These small lights with 200-300 lumens on bright are simply too bright for astronomy use.
So it's not just that the light has to be red, it has to be dim.
Edited by Starman1, 30 March 2023 - 06:40 PM.