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Deep Dark Adaptation

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#1 Araguaia

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Posted 19 January 2019 - 05:11 AM

I woke up early today, saw the Southern Cross out the bathroom window, and was off with the red light and binos to the little clearing by the house where I left a confy chair last night.  I spent practically the entire session looking at one of the finest binocular fields in the sky, taking in the whole Carina Nebula and the bright open clusters NGC 3293 and 3532.  

 

Although my retina had not seen a white photon since I went to bed last night, and although I was very careful with my dim red flashlight, and even kept one eye closed while scanning for snakes, it was about fifteen minutes before I really started to see detail.  Both clusters were well resolved, and Eta Carina was bright red.  The nebula extended well beyond the two central patches that barely fit into the widest telescope field.  Four huge lobes around this turned it into a galactic flower, with arches of nebulosity reaching all the way out to the open clusters.  To the east, near the edge of the field, a large and richly textured dark nebula, the beginning of the Great Rift.  I have yet to see Barnard's Loop, but this must be comparable, although much brighter - easily seen with no filters, just 10x42s and a dark sky.

 

I found a comfortable crouch and lost myself in the scene, but after a while, the view deteriorated.  An annoying glow was fading out the fainter emission detail and washing out the contrasting textures in the dark nebula.  Soon I could not find things that I had just been observing with direct vision.  I figured it was about 4:30 AM - well before first light.  What the hell?  I dropped the binos, and sure enough, there it was  - Venus rising to the ESE, wrecking the dark sky like a distant Wal-Mart.  End of session.

 

When I was teaching myself amateur astronomy, all the online sources, including CN, seemed to agree that it takes 15-30 minutes to dark adapt, with night vision "improving marginally for up to an hour".  They also seem to suggest that brief exposure to weak light is OK, especially if it is red.  Not so!  Amateurs will be well served to know that deep dark adaptation takes well over an hour, and that any bright source of photons - even a red-mode Stellarium or Venus - will degrade it significantly.


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#2 happylimpet

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Posted 19 January 2019 - 05:36 AM

I think its well known that real dark adaption takes several hours.

 

You sure it was Venus ending things and not just the start of dawn - end of astronomical twilight?


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#3 happylimpet

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Posted 19 January 2019 - 05:44 AM

according to the app on my phone astronomical twilight in cantao ends at about 3:50am (though i dont trust it 100%!) so maybe it was just this?


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#4 Asbytec

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Posted 19 January 2019 - 06:08 AM

I am no expert on human physiology or how our eye operates, but I am pretty sure there is a chemical reaction required for the rods to perform. Sometime I think we just expose most of that chemical over time and it needs to be replaced. It could just be you used up your supply of chemical night vision and needed a refill. Sometimes after a period of time observing, I notice the image fade. This is when I back away from the exit pupil for a short period (still under the hood) to re-energize my eyes and relax for a bit and get some O2 into my bloodstream. It's back in short order. Eat more carrots. :)



#5 Araguaia

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Posted 19 January 2019 - 06:15 AM

according to the app on my phone astronomical twilight in cantao ends at about 3:50am (though i dont trust it 100%!) so maybe it was just this?

The apps all work on Brasília time, which is in daylight savings right now.  Our state remains on solar time.  Twilight would then be at 4:50 AM, and I did start to notice it at about 4:45.  This was at about 4:30 AM, with Venus still below the treetops.

 

I think that, more than LP in the sky from Venus, what washed out the view was stray light from the planet hitting the lenses.  Still, it shows that even bright planets can degrade observing conditions - let alone the crescent Moon, which some seem to think is almost harmless.


Edited by Araguaia, 19 January 2019 - 06:18 AM.

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#6 Dick Jacobson

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Posted 19 January 2019 - 08:18 AM

According to an ophthalmologist at the Mayo Clinic, initial dark adaptation takes 8 minutes, while 20 minutes is required for maximum adaptation.



#7 viewer

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Posted 19 January 2019 - 08:36 AM

According to an ophthalmologist at the Mayo Clinic, initial dark adaptation takes 8 minutes, while 20 minutes is required for maximum adaptation.

Supported by Wikipedia as well: https://en.wikipedia...daptation_(eye)

 

Cones take approximately 9–10 minutes to adapt to the dark.
The sensitivity of the rod pathway improves considerably within 5–10 minutes in the dark.
The eye takes approximately 20–30 minutes to fully adapt from bright sunlight to complete darkness and becomes 10,000 to 1,000,000 times more sensitive than at full daylight.

Have found with my LP I'm getting to the maximum quite fast at my balcony.


Edited by viewer, 19 January 2019 - 08:45 AM.


#8 Araguaia

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Posted 19 January 2019 - 08:53 AM

With just 20 minutes I can't even see the arms of M83 properly - and that is coming out of our romantically-lit home, with just a few dim lights.



#9 bunyon

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Posted 19 January 2019 - 01:59 PM

There is absolutely nothing physiological that has a single value for everyone. I’m sure time to full dark adaptation varies as much as jumping ability or pitch.
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#10 Redbetter

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Posted 20 January 2019 - 02:48 AM

I have only observed the Eta Carina nebula once, naked eye, then in an 8" SCT with ~1.2 deg of field.  The actual size of the nebula is considerably wider from 2 to 3 degrees depending on the axis.  However, it is quite bright in the central regions and nothing like Barnard's Loop.  Barnard's Loop is enormous (far wider than typical binocular fields) and of very low surface brightness. 

 

Maximum dark adaptation does not seem to take that long if one is already reasonably dark adapted to begin with.    One is going to hit the effective max quite quickly looking at bright objects like the Carina or Orion nebulae because they will considerably limit dark adaptation. 

 

It isn't that one is replacing the rhodopsin (which is already in large excess after fairly short adaptation) it is that the photochemical products of rhodopsin bleaching continue to produce an elevated background brightness until they are reconverted back into rhodopsin.  The eye will detect plenty of light with 95, 98, 99, 99.5 or 99.9% of the rhodopsin concentration...but the remaining 5, 2, 1, 0.5 or 0.1% bleaching byproducts increase the observed background brightness acting the same as light pollution haze in reducing the effective threshold/contrast sensitivity of the eye. 

 

My understanding is that the only time vitamin A supplementation is helpful for night vision is if one is actually considerably deficient in vitamin A.    Keep in mind that the scotopic range is from roughly 18 MPSAS to perhaps 26 MPSAS with threshold detection at maybe 28 MPSA--although the eye itself when fully adapted will produce a background brightness of ~26 MPSAS.    From 18 to 26 magnitude is a factor of about 1600 times as bright/dim.  So one would have to be seriously depleted of vitamin A and rhodopsin before one would be effectively "running out" in dark sky conditions.


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#11 viewer

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Posted 20 January 2019 - 06:55 AM

It could take 40 minutes for the max adaptation according to this:

 

https://www.ncbi.nlm...books/NBK11525/

https://www.ncbi.nlm...port=objectonly

 

The curve asymptotes to a minimum (absolute threshold) at about 10−5 cd/m2 after about 40 minutes in the dark.

 

But then you have this…

 

https://www.ncbi.nlm...port=objectonly

 

Indicating that for astronomy, when you really are pushing it, several hours may matter. When will the yellow curve reach the dark red?

 

Don't stare into direct light the hours before going out! wink.gif 


Edited by viewer, 20 January 2019 - 07:09 AM.

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#12 Redbetter

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Posted 20 January 2019 - 02:42 PM

The starting point is the key.  All of the curves illustrate that it takes a long time to go from full bleach (daylight).  But when one is going from semi-adapted the level of bleach will be much less.  When driving to a dark site in the evening, there is typically little traffic along the last several minutes of the way, and one will emerge from the vehicle with some dark adaptation already.   Much of the same is true waiting for the sky to darken through twilight and the time for this latter is long. 

 

An observer can spend hours under 21 - 22 MPSAS skies, and be fully adapted most of that time, but the eye will still need to adapt more when one goes to the eyepiece either with a nebula filter at low power, or unfiltered at medium or high power because the background sky is now orders of magnitude darker.  Fortunately, this final adaptation is from an already relatively dark starting point, so the maximum at the new level is achieved in a few minutes. 


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#13 Araguaia

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Posted 21 January 2019 - 03:20 AM

Those curves show that there is a quick improvement in the first 5 minutes, followed by a plateau, and then more up to about 20 minutes, after which the rate of improvement falls off quickly.

 

That might explain why so many sources say it takes 20 minutes, and why so many people think they are good after 5 minutes.

 

Redbetter shows us that even after you are dark adapted, there is further adaptation that happens when you look to a dim object.  It takes a few additional minutes, and presumably can be lost in a few seconds.  I hadn't thought of that, but it makes perfect sense.  So deep dark adaptation is not something you achieve, but something you manage throughout the night.  


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#14 Asbytec

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Posted 21 January 2019 - 04:42 AM

Without doing a thesis on the topic and curves aside, I find I am well dark adapted in short order. Enough to see the area around me, but probably not optimum for deep sky work. It does take a little time, I believe, to really get going on a productive observation. Maybe 15 or 20 minutes under the hood. I am definitely up and running well before an hour passes, even if there is some marginal gain theoretically at that point. By then, most times I have already drained the image for all it's worth and ready for a short break. Sometimes when I am observing at a small exit pupil, I kind of hesitate to drop down in magnification to larger exit pupils for fear of losing any gains in dark adaption due to the slightly brighter sky. 


Edited by Asbytec, 21 January 2019 - 04:43 AM.


#15 happylimpet

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Posted 21 January 2019 - 07:18 AM

Interesting. I have always thought that while 30 mins or so does provide pretty good dark adaption, there are definite benefits after a further 2-3 hours. I had thought this was a widespread belief, but perhaps not.



#16 Redbetter

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Posted 21 January 2019 - 02:11 PM

One must be cognizant of what that "plateau" means.  It is only present for high bleach levels--that is coming from a truly non-dark adapted stated corresponding to well over 20% bleaching of the rhodopsin.  This is probably in the bright end of the mesopic range.  The plateau should not be an issue when already dark adapted and experiencing very low level bleaching from the night sky, short exposures to red or even dim white light.  I don't see how the plateau will factor in again unless one is observing the Moon or bright planets with sufficient aperture and or sufficient time to cause an extensive bleach. 

 

 

Interesting. I have always thought that while 30 mins or so does provide pretty good dark adaption, there are definite benefits after a further 2-3 hours. I had thought this was a widespread belief, but perhaps not.

 

I suspect in most cases that roughly an hour is sufficient to go from near full bleach to full/near full adaptation.  The literature from actual studies indicates something in this range, however, they might not be capturing the extremes of the threshold adaptation.  One can see a much slower and higher starting point for the final "S3" adaptation line extending from the change in slope of the rapid "S2" line that follows the plateau.  Comparing the S3 portions of different bleach levels they seem to represent more of an asymptote transition from S2 to final threshold.  The S3 portion is not well understood in terms of the responsible photochemistry, but it has a much slower time constant if treated as linear.  However, this probably fades into the threshold at the very low bleach levels that are typical of dark sky--and this appears to be what is experimentally set as a threshold in the studies.

 

Since this time of year I typically arrive at a dark site after sunset or even after twilight, I notice that I am reasonably adapted when I step out of the vehicle and further adaptation is rapid.  The main limitation to being nearly fully adapted over the next half hour is the use of a red LED to unload/setup/collimate.  Once that is off, things progress rapidly.

 

While it might seem that there will or should be even greater sensitivity possible with another 1, 2, 3 or 4 hours of dark adaptation, I have not been able to quantify that.   Instead I find I hit an NELM limit early that doesn't change.  I reach about 7 in dark sky and it doesn't improve after that.   Staring into an eyepiece with small exit pupil/high magnification, with a hood/cover reaches a point of diminishing returns after a few minutes.  If there was significantly more adaptation to be achieved, it should instead reveal dimmer telescopic limiting magnitudes after an additional 15 minutes or half an hour.  Instead I see rapid improvement for the first several minutes and then nothing further.  The exception is when the background is again darkened via exit pupil staring into a dim star field in the eyepiece, or through blocking the relatively bright night sky by going into a forest canopy, dark building, etc.

 

The rapid improvement is the 1.7 to 1.8 minutes per visual magnitude time constant I was able to derive from the S2 portion of one of the experimental graphs from "50 Years of Dark Adaptation."  It has virtually no S3 curve/line because it is already at the visual threshold, at least per the study.  This does seem to correspond to what I find visually.  I would love to get another half or full magnitude of NELM or TELM from longer deep dark adaptation, but I have not seen it so far.


Edited by Redbetter, 21 January 2019 - 04:00 PM.

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#17 Araguaia

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Posted 22 January 2019 - 03:57 AM

Thanks for that, Red.

 

So in the real world, the process of dark adaptation does not consist of sitting in absolute darkness, but of slowly switching to dimmer lighting as the dusk progresses and we go out and set up.  The final stage doesn't start until you are looking through an EP at a dark field.  This in itself might stretch those theoretical 20-40 minutes into an hour.

 

Then there is the possibility that the chemical adaptation at the retina is only half the story.  Once the retina can do no more, it may be up to neural networks to tune into the fainter signals, process them from the noise, and integrate them, and that may improve after a few minutes of observation.

 

If this is the case, then unlike chemical adaptation, it might become counterproductive after too much observation as the neurons "tire" and need some recovery.  Hence the diminishing returns we often experience, and the advantage of breaks.

 

On the other hand, neural networks can be trained...



#18 Asbytec

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Posted 22 January 2019 - 06:01 AM

Good question. I often need a short break. I just seem to lose the entire FOV at times. Not sure why. A short recovery is all I need to get back at it.

As to the above, once an observation is well underway, I do not notice any additional dark adaption occurring on the time frame of a hour.
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#19 redhawk

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Posted 30 January 2019 - 07:23 AM

At a very dark spot, my eyes continue to adapt for roughly forty-five minutes. Even a very dim red light, mostly covered by my hand with only a small bit of light allowed to shine through  my fingers, compromises my dark adaptation. You can see that for yourself, the sky appears much darker immediately after the light use, though it comes back fairly quickly. While waiting for full darkness to fall one night, we attracted a kangaroo rat. We would toss it a small piece of a cookie (it was right at our feet, they are fearless), and after it got completely dark, the k-rat just would disappear while he was eating and not moving. Then we would toss him another tidbit, and with his movement he could be seen again. Same principle applies to seeing a very dim object when the telescope moves. Of course averted vision is required to enhance the view. I would sleep in the bed of my pickup truck when done for the evening (cowboy camping!), having a pair of binoculars within reach for the times when I would wake and  found the stars just brilliant and calling out for me. That's the time when you have become fully dark adapted, and even naked eye views of star clusters are wonderful, and the large, dark areas of the sky take on a new depth. The large dark nebula, such as Le Gentil 3, north of Deneb, can be truly appreciated in all their dark glory.


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#20 vsteblina

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Posted 30 January 2019 - 09:49 AM

At a very dark spot, my eyes continue to adapt for roughly forty-five minutes. Even a very dim red light, mostly covered by my hand with only a small bit of light allowed to shine through  my fingers, compromises my dark adaptation. You can see that for yourself, the sky appears much darker immediately after the light use, though it comes back fairly quickly. While waiting for full darkness to fall one night, we attracted a kangaroo rat. We would toss it a small piece of a cookie (it was right at our feet, they are fearless), and after it got completely dark, the k-rat just would disappear while he was eating and not moving. Then we would toss him another tidbit, and with his movement he could be seen again. Same principle applies to seeing a very dim object when the telescope moves. Of course averted vision is required to enhance the view. I would sleep in the bed of my pickup truck when done for the evening (cowboy camping!), having a pair of binoculars within reach for the times when I would wake and  found the stars just brilliant and calling out for me. That's the time when you have become fully dark adapted, and even naked eye views of star clusters are wonderful, and the large, dark areas of the sky take on a new depth. The large dark nebula, such as Le Gentil 3, north of Deneb, can be truly appreciated in all their dark glory.

**** cowboys are that soft that they sleep in the back of the pickup???

 

What ever happened to sleeping on the ground like real Foresters do??

 

I tend to agree with you on dark adaption.  I was camped on the Beaverhead National Forest in Montana and decided to sleep outside. Got my contacts in, doused the eyes with saline, and settled down into my LaFuma recliner (ok, it was during the Perseid meteor shower, so I had to use the LaFuma).

 

I awoke after four hours to the darkest sky I have ever seen in my life. What really caught my attention was the dim circular object that was about 15 degrees up in the sky. I had to look it up in the star atlas and sure enough it was M33. I was 45 at the time and have never even come close to seeing M33 as disc shaped since. 

 

Even in that sky use of the red flashlight did affect my night vision. But waking up and then seeing the night sky in a truly dark, high elevation setting is total magic.

 

People should do it before their eyes get old.


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#21 MikeP

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Posted 30 January 2019 - 01:06 PM

Too late for me. My eyes are old already. But I try.



#22 MOwen

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Posted 30 January 2019 - 03:29 PM

Check out this article from S&T July 2006; very informative...

 

 https://www.skyandte...a-pupil-primer/



#23 BGazing

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Posted 30 January 2019 - 03:51 PM

I woke up early today, saw the Southern Cross out the bathroom window, and was off with the red light and binos to the little clearing by the house where I left a confy chair last night.  I spent practically the entire session looking at one of the finest binocular fields in the sky, taking in the whole Carina Nebula and the bright open clusters NGC 3293 and 3532.  

 

Although my retina had not seen a white photon since I went to bed last night, and although I was very careful with my dim red flashlight, and even kept one eye closed while scanning for snakes, it was about fifteen minutes before I really started to see detail.  Both clusters were well resolved, and Eta Carina was bright red.  The nebula extended well beyond the two central patches that barely fit into the widest telescope field.  Four huge lobes around this turned it into a galactic flower, with arches of nebulosity reaching all the way out to the open clusters.  To the east, near the edge of the field, a large and richly textured dark nebula, the beginning of the Great Rift.  I have yet to see Barnard's Loop, but this must be comparable, although much brighter - easily seen with no filters, just 10x42s and a dark sky.

 

I found a comfortable crouch and lost myself in the scene, but after a while, the view deteriorated.  An annoying glow was fading out the fainter emission detail and washing out the contrasting textures in the dark nebula.  Soon I could not find things that I had just been observing with direct vision.  I figured it was about 4:30 AM - well before first light.  What the hell?  I dropped the binos, and sure enough, there it was  - Venus rising to the ESE, wrecking the dark sky like a distant Wal-Mart.  End of session.

 

When I was teaching myself amateur astronomy, all the online sources, including CN, seemed to agree that it takes 15-30 minutes to dark adapt, with night vision "improving marginally for up to an hour".  They also seem to suggest that brief exposure to weak light is OK, especially if it is red.  Not so!  Amateurs will be well served to know that deep dark adaptation takes well over an hour, and that any bright source of photons - even a red-mode Stellarium or Venus - will degrade it significantly.

Eta Carina was bright red in the binos?

I saw it once in the binos...do not recall it being red.



#24 Araguaia

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Posted 31 January 2019 - 04:08 AM

Yes.  Bright red-orange, I should have said.  I was just looking at it again an hour ago.  Distinctly red, but more orange than Antares.

 

The nebulosity around it does not look orange at all in binos.  White.  If I try hard I can imagine it is a bit greenish, like M42, but that is probably an illusion, as it is a faint orange in a larger scope.



#25 Astro-Master

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Posted 15 February 2019 - 11:09 PM

What I have found works for me is using an eye patch on my observing eye to read my charts, or anytime I'm not looking in the eyepiece and also using a dark hood.  Its also important to take deep breaths to keep the oxygen flowing to the eyes, at higher altitudes this becomes even more important.  If the view in the eyepiece starts to dim, a few breaths will bring it back.

Taking bilberry can help with night vision, World War II pilots used it on night flights to gain an edge.  Anything that reduces strain while observing will help, a good observing chair, taking a stretch break lying on a yoga mat will do wonders after standing at the eyepiece for an hour.

Using a dark towel or hood to block all skyglow from the sky, can add 1/2 magnitude or more.

Taking eye vitamins with lutein and zeaxanthin will help keep your eyes in good health.

 

And don't forget to: KEEP LOOKING UP!




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