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Old 22-07-2014, 07:09 AM
N1 (Mirko)
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Dark adaptation overrated???

Hi all,

This may sound stupid, but I am having some doubts as to the need for most people to fully dark-adapt when observing at anything other than naked eye or the largest of exit pupils.

I did some casual observing from Tahmoor (semi rural) on the weekend. As I was looking at some globulars at around 3mm exit pupil, dinner time came around. I later resumed my observing. I found that it took very little time (much less than expected at any rate) for my eyes to adjust sufficiently to give the same level of detail on the same objects, Omega Cen and M22. Initially I tought increased altitude of the objects after dinner might have something to do with it. (Or perhaps something I ate ) But the increased elevation thing only applied to M22. Omega Cen had descended considerably during the same period.

Could the explanation be that a 3mm exit pupil (or any other value) only requires a pupil dilated to the same size to reach optimum performance? "Medium sized" dilation appears to be achieved relatively quickly as this discussion suggests (scroll to "Adaptation" about halfway down):

http://michaeldmann.net/mann7.html

As I was not doing any experimenting, it's not possible to tell whether other factors might have something to do with it, or even whether there really was the same detail visible. Also, the above targets aren't exactly faint objects, so I am going to experiment with some other objects later, paying specific attention to level of detail seen versus time elapsed since lights-out. I just thought this was an interesting observation. The reason I had not noticed it before might be that I generally start at low power and large exit pupils at the beginning of a session - (the only?) circumstances where good dark adaptation is essential.

If there was any substance to this, the effects would be profound. I could stop walking around the house with eye patches and sunglasses for ages before going outside I would just need to start the session at medium to high power and everything would be sweet. Or would it?

What are your thoughts?
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Old 22-07-2014, 08:19 AM
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speach (Simon)
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Sounds plausible.

BTW did you have any carrots!
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Old 22-07-2014, 08:20 AM
glend (Glen)
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Well yes I think it does make a difference however it depends on the age of your eyes, the magnitude of the object your trying to view, and the seeing conditions.

Omega Cent is one of the brightest of objects and is easily observed. Try doing the same experiment with the Eye's Galaxies and you should notice that dark adaptation does make a difference.
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Old 22-07-2014, 08:26 AM
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multiweb (Marc)
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When I image I always have a very dark perspex on my laptop. After a while my eyes get used to the dark (couple of hours) and I can lookup or tweak stuff around the scope even without a torch. If I walk around the field and come across other imagers with unshielded laptop my eyes hurt so I can't even look straight at them. So I guess after a few hours your eye must be used to the dark. Maybe the older you are the less flexible they become. The more time it takes to adapt. Everybody's different I guess. But I've used red lights and shields for years now and haven't looked back. Makes it a lot easier for me to function properly around the scope and enjoy the sky at the same time.
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Old 22-07-2014, 08:31 AM
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Quote:
Originally Posted by glend View Post
Well yes I think it does make a difference however it depends on the age of your eyes, the magnitude of the object your trying to view, and the seeing conditions.

Omega Cent is one of the brightest of objects and is easily observed. Try doing the same experiment with the Eye's Galaxies and you should notice that dark adaptation does make a difference.
Yes, try extended objects and very faint ones at that. Try the galaxies at the limit of your telescopes light gathering capability. Then try them on different occasions.

I like to observe galaxies at the limits of visibility, not to detect structure or detail but just to detect the object at all. I find dark adaptation is essential to seeing the object but that is not the only thing. I have not yet invested in a sky quality meter and will do so one day but I can say that on different evenings a particular galaxy may or may not be visible. There may be many factors contributing to seeing an object one day and not the other and for me dark adaptation only helps improve the likelihood of seeing something extremely faint.
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Old 22-07-2014, 09:02 AM
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AstralTraveller (David)
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The exit pupil idea sounds valid and I'd be interested to hear other experienced observers opinions. I also wonder if being under really dark skies makes a difference - I know Tahmoor and it isn't great.

I'm no expert so this may be wrong but years ago I was told that there are two types of dark adaption. The first is the opening of the pupil, that takes only minutes. The second is a chemical change in the retina which increases its sensitivity. I was told that that takes about 1 minute for every year of age. So us old fearts take about an hour to become fully dark adapted whereas young wipper-snappers can be dark adapted in under half an hour. So you need to be more protective of dark adaption as you get older.

There is also an optical illusion with dark adaption but I think it only applies to naked-eye viewing. I used to observe long into the night with nothing more than a faint red light for map reading. I'd then put on some light to pack up and then drive about half a km to the front gate, which I had to get out and open. I'd look up and the sky looked blacker than it did before. I think this was just that I could no longer see the sky glow so clearly and so the space between the stars looked blacker. The other explanation is that every time I packed up the sky improve within 15 minutes. Hmmm, now perhaps that is plausible.
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Old 22-07-2014, 09:20 AM
N1 (Mirko)
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Quote:
Originally Posted by AstralTraveller View Post
The second is a chemical change in the retina which increases its sensitivity. I was told that that takes about 1 minute for every year of age. So us old fearts take about an hour to become fully dark adapted whereas young wipper-snappers can be dark adapted in under half an hour. So you need to be more protective of dark adaption as you get older.
I suppose if the exit pupil experiment did show a difference (all other factors being the same) between "pupil dilated to match exit pupil" and "fully dark adapted", that difference would be (almost) entirely due to said biochemical component of the process.

I think the two DSOs I looked at really are unsuitable for a proper test (too bright), and rather that looking for detail, a better measure could be "detected" vs. "not detected" as astro744 suggests, and doing that for a range of targets.

Interesting.
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Old 22-07-2014, 10:29 AM
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Dark adaptation (retinal sensitivity) and pupil dilation are two separate systems even though they rely on each other. A bright light on the retina triggers the pupil to close to protect the retina. Darken the environment, like walking into a dimly lit room from bright sunlit outdoors, and even though the pupil responds quicker than the retina, the retina's rods and cones are still saturated from the bright situation, and it takes longer to re-establish the chemical sensitivity of the retina.

David mentions the night sky looking darker after packing up under white light and driving for a while. This is the saturation of the eyes to a brighter environment not picking up the fainter glow. Wait a few moments, and the glow becomes visible again.

Now, Mirko, do your 'dinner' trick again, and then look at M22 or Omega Cent. again. Yes you will still see them, and make out the brighter individual stars, but you will not see the fainter deep core glow anywhere as effectively, regardless of the exit pupil that the scope/EP combination gives you. Your eyes are still saturated from the bright lights of the lounge room (like David's eyes from packing and driving). You will also notice the background sky-glow darker - again this is a tell tale sign that your eyes are still light saturated. Try to spy out the arms of M83 after doing the 'dinner' thing, and you just won't. You'll just see the core, and even then with difficulty.

I've also encountered the exact same curious phenomenon you mention, both coming out of lit room AND driving. But, it is an illusion, as David mentioned, as those more subtle details and ques are just not there if you stop to pay attention. Even a careless flash of your dim red light into your eyes will screw them for a short time.

Now, dark adapted eyes have their own pitfalls! Our eyes are poor performers at low levels of illumination. They saturate quickly if the image stays static for too long. This is a real problem with dim extended objects, and if you have a tracking scope, it is even worse. Our eyes are made to pick up changes in light, so look at a scene for too long (light or dim), and our eyes begin to fail to see detail - major part of why our eyes dart around so much. Through a scope, darting eye just does not do it. You need to introduce movement into the scope to giggle the actual image. This 'resets' the sensitivity of our eyes, and for a few moments we see even fainter detail that we missed out while the scope was still. A little tap on the scope to introduce a vibration is all that's needed. This is why I don't have my deep sky scopes with motors - the constant nudging lets me see more.

Still not convinced? Notice when you are panning for a new faint fuzzy how your eye picks up those really faint blobs of light? But then go back with a slower action and you just can't see it any more (the bloody most frustrating thing I know of in astro!!! ). Two things at play here:

first: You are using your central vision instead of your averted vision to pick up the faint fuzzy, so you are screwed once;

second: The action you are using to double back is too slow for our eyes to pick up the oh-so-subtle change in light, so screwed for a second time,

Then out of shear frustration, you just jar the scope while no longer paying attention to the image, and you see the blighter a-freaking-gain! And all you can mutter is 'Oh bloody bull crap!!'

Experience helps. But being human, I still get caught out and frustration taunts my hot latin blood...
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Old 22-07-2014, 10:42 AM
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Quote:
Originally Posted by N1 View Post
I think the two DSOs I looked at really are unsuitable for a proper test (too bright), and rather that looking for detail, a better measure could be "detected" vs. "not detected" as astro744 suggests, and doing that for a range of targets.
Hi Mirko,

Indeed the targets of Omega Centauri and M22 are far too
bright to make some subjective analysis of dark adaptation.

Arguably, better choices are those on the verge of you perceiving
them or not, even after several hours of adaptation.

Quote:
I suppose if the exit pupil experiment did show a difference (all other factors being the same) between "pupil dilated to match exit pupil" and "fully dark adapted", that difference would be (almost) entirely due to said biochemical component of the process.
It is important to keep in mind that pupil dilation, which is a muscular action,
is not the primary mechanism for photosensitivity when the eye is dark
adapted.

The article you cited itself explains it when it describes the function
of the molecular compound known as rhodopsin that acts as a biological pigment.

If you ever read generalized accounts of the chain of events
that take place at the molecular level when a photoreceptor neuron
cell triggers, I think we could all be excused for finding it complex
and seemingly convoluted.

If the mechanism had been designed by an engineer working at a modern
camera company, they would probably take him or her away on the
assumption they had lost their marbles.

Nevertheless it works and is testimony to what a few hundred million
years of evolution can produce.

Suffice to say that rhodopsin absorbs green-blue light and its levels
increases as you dark adapt. The build-up of rhodopsin as you
dark adapt takes place over tens of minutes.

When rhodopsin absorbs photons, a complex set of chemical events take place at the molecular level.
For example, see here - http://en.wikipedia.org/wiki/Rhodopsin

Way down at the molecular level not just the photoreceptor neurons
in your eye but all neural cells use molecules such as sodium, potassium
and calcium to operate. In neurons they exist as charged ions to
create membrane potentials.

Though the build-up of rhodopsin takes place at the tens of minutes
scale, the actual chemical reaction times of when it is doing its job can
take place at the femtosecond and nanosecond scale.

So just as we are all aware that dietary intake of elements such
as potassium and sodium are essential to the human body for
brain and other nerve function, it is not surprising that they will
play a part in dark adaption. Vitamin A for example is essential
for the production of rhodopsin.

There have been studies into how dark adaptation tends to deteriorate
as most of us as we get older.
See http://www.ncbi.nlm.nih.gov/pubmed/10748929

It is also important to remember that the visual system is not just about the
eye but also about the brain where a great deal of the processing takes place.
Many of us turn to metaphors such as digital cameras and CCD's to try and
understand how the human visual system works, but in my own reading
I have come to appreciate that that can be a mistake. For example, many
of us appreciate how colour works in a man-made digital camera and extrapolate to
that the human visual system works in a similar way. As it turns out, they
have little in common.

The take home message is that dark adaptation is not just about pupil
dilation which is a muscular action at the millimetre scale but is
greatly to do with what is happening at the atomic level with the
molecular biochemistry within your eye nerve cells.

Last edited by gary; 22-07-2014 at 01:49 PM.
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Old 22-07-2014, 10:46 AM
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mental4astro (Alexander)
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Gary, beautifully said.
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Old 22-07-2014, 11:27 AM
N1 (Mirko)
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Gary, Alex, and all other contributors: thanks very much for your explanations. I feel I really learned something today. It almost seems like the more obvious reaction of the eye (pupil dilation) is actually the less important. It would be interesting if a setup like the above could measure the impact of each. Am I correct in assuming that seeing or not seeing certain stars e.g. in open clusters (such as M45) is less suitable for this than say nebulae, due to the way stars and extended objects are "processed" differently by the instrument?

Re movement - Alex, again I'm completely with you there. My opinion is reaffirmed that my scopes too should remain hand-operated as long as they are only used for visual work.

I guess seeing the two bright DSOs relatively unchanged by dinner the other day really was an optical illusion (i.e. brain failure according to Neil Degrasse Tyson )
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Old 22-07-2014, 01:05 PM
Renato1 (Renato)
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One thing about dark adaptation, is that one still needs to look up stuff in maps with non GOTO telescopes. For decades I used little red LED flashlights I made myself following instructions in Astronomy magazine. They were fine, I could read everything in Sky Atlas 200o and Herald Bobroff AstroAtlas.

But on the occasions back then that I got together with members at my local club, they were using red flashlights that were about 10 or 20 times brighter than what I was using. I bought an Orion red flashlight, and same story, way too bright. I couldn't see how dark adaptation was possible with them.

I wonder how many other people working solo are using too bright red flashlights?
Regards,
Renato

Last edited by Renato1; 24-07-2014 at 11:22 PM.
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Old 23-07-2014, 01:44 PM
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Originally Posted by Renato1 View Post
One thing about dark adaptation, is that one still needs to look up stuff in maps with non GOTO telescopes. For decades I used little red LED flashlights I made myself following instructions in Astronomy magazine. The were fine, I could read everything in Sky Atlas 200o and Herald Bobroff AstroAtlas.

But on the occasions back then that I got together with members at my local club, they were using red flashlights that were about 10 or 20 times brighter than what I was using. I bought an Orion red flashlight, and same story, way too bright. I couldn't see how dark adaptation was possible with them.

I wonder how many other people working solo are using too bright red flashlights?
Regards,
Renato
Yeah, I think you are right: the fainter the better. Just bright enough to see the star chart is all you need. I find it really annoying when people at star parties wander around with very bright red torches, sometimes with just red cellophane, and even shine them at you as you are trying to look through the scope...

- Dean
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Old 23-07-2014, 10:01 PM
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I am a bit older and my eyes can't focus on red very well. The eye is far more sensitive to green so shouldn't we be using a much fainter green light source over a really bright red light?
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Old 24-07-2014, 10:16 AM
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I am a bit older and my eyes can't focus on red very well. The eye is far more sensitive to green so shouldn't we be using a much fainter green light source over a really bright red light?
Hi Rick,

Our eyes are most sensitive to green (and less sensitive to red), so green light is more likely to muck up our dark adaption than red.

- Dean
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Old 24-07-2014, 08:37 PM
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My point was because the eye is less sensitive to red the red light has to be a lot brighter to be useful whereas a green light could be of really low intensity. I have some laser modules, the green beam is easily visible at 5mw but at 250mw the red beam is only just becoming visible.

Last edited by doppler; 25-07-2014 at 07:27 AM.
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Old 24-07-2014, 11:29 PM
Renato1 (Renato)
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Quote:
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Yeah, I think you are right: the fainter the better. Just bright enough to see the star chart is all you need. I find it really annoying when people at star parties wander around with very bright red torches, sometimes with just red cellophane, and even shine them at you as you are trying to look through the scope...

- Dean
What do you say to them? And how politely do you say it?

When I cover regular flashlights, I use several layers of red insulation tape. Cellophane would be next to useless.

The other method which was recommended in a book about seeing faint DSOs was to use a regular flashlight, but to put a bulb with at least twice the voltage into it. The author claimed that when that dim, one could happily used white lite instead of red light. That worked pretty well too - except that nowadays, it's hard finding small flashlights with bulbs, and then finding the bigger bulbs.

I suppose a way of replicating that would be to use lots of white insulation tape over an LED flashlight.
Regards,
Renato
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Old 25-07-2014, 10:55 AM
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Alexander, regards tracking scopes, I wonder if there would be virtue in a program change for them if you are hunting stuff at the edge of perception. Introduce a random "Jitter" into the tracking to keep faint objects moving around a little.

Regards red lights, I have an LED Maglite with as many layers of red gel in the lens as I could cram in. it is dim enough to comfortably look directly at the LED with reasonable dark adaptation.
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Old 25-07-2014, 09:24 PM
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I get no difference after about 3 minutes, and I can see more than anyone else nearly every time there is a meet-up, no matter how long we wait.

I'm not sure if it is unusual to dark adapt quickly or not, but my wife stumbles around a room when I can see clearly enough to read the pattern on her PJs.
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Old 26-07-2014, 08:50 AM
N1 (Mirko)
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Quote:
Originally Posted by Poita View Post
I get no difference after about 3 minutes, and I can see more than anyone else nearly every time there is a meet-up, no matter how long we wait.

I'm not sure if it is unusual to dark adapt quickly or not, but my wife stumbles around a room when I can see clearly enough to read the pattern on her PJs.
This phenomenon is being discussed at length here.

But seriously, given the shape of the graph in one of the articles referred to below, you'd expect a big change in the first few minutes of being in the dark, then a small change over a long period of time. So whether it's really "no difference" you get after a few minutes, only a proper test could show I think.
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