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Old 18-04-2016, 09:18 AM
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Stonius (Markus)
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Dark adaption has little to do with the diameter of your pupil

Everything I've ever read about dark adaption has mentioned that it's all about getting the largest pupil diameter possible, which only happens after the muscles fully relax after about half an hour of darkness.

I mentioned this to a friend of mine the other day who happens to be a professor of neuroscience, and he told me that was wrong, it's actually a biochemical process, not a biomechanical one.

A quick look on Wikipedia proved him right. I was stunned, stunned I say.

To condense / summarise this page (Adaptation (eye));

"The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude.

[...] A minor mechanism of adaptation is the pupillary light reflex, adjusting the amount of light that reaches the retina. [...] Changes in the sensitivity of rods and cones in the eye are the major contributors to dark adaptation.

[...]Rhodopsin, a biological pigment in the photoreceptors of the retina immediately photobleaches in response to light. Rods are more sensitive to light and so take longer to fully adapt to the change in light. Rods, whose photopigments regenerate more slowly, do not reach their maximum sensitivity for about half an hour. Cones take approximately 9–10 minutes to adapt to the dark."

https://upload.wikimedia.org/wikiped...Adaptation.png

And from this page on Accelerating Dark Adaptation in Humans;

"This adaptation period is different for both rod and cone cells and results from the regeneration of photopigments to restore retinal sensitivity.[1] Cone cells are able to regain maximum retinal sensitivity in 9–10 minutes of darkness whereas rods require 30–45 minutes to do so."

And according to that page, if you want to accelerate your dark adaption, make sure you're staying fit and getting enough Vitamin A and Anthocyanins.

Thinking about it, it's obvious. The pupil alone does not change diameter by nine orders of magnitude. There has to be something else at play here.

And of course, pupil size is still important for exit pupil calculations.

Anyway, I thought I'd put it up here in case anyone else was similarly misguided.

Markus
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Old 18-04-2016, 11:34 AM
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AstralTraveller (David)
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I heard this in the 70s thought it was quite well known. I was under the impression that the time to achieve chemical dark adaption increased as one got older but your info does mention that. Perhaps that is just an excuse for us old fellows to be more grumpy about light trespass, seeing as how we believe we have put more time into getting dark adapted. Looks like I should give up on cups of tea while observing and drink berry juice instead.
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Old 18-04-2016, 11:34 AM
raymo
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Thank you for your thoughtfulness Markus. I'm pretty sure that the
overwhelming majority of we astro hobbyists are aware of these facts,
but equally, I'm sure that some newbies are not.
raymo
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Old 18-04-2016, 11:46 AM
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Like I said, I was suprised. Perhaps because I'd never heard any different, perhaps because of my own assumptions. I figured a lot of the serious people *would know this, but it caught me off guard having been into astronomy (on and off) for 30 years. So I figure if I could help clarify it for someone else, I would. Hopefully it helps someone else out, even though it has no effect on the practicalities of dark adaptation. :-)

Markus
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Old 18-04-2016, 12:44 PM
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You've just been reading the wrong sort of "everything".

You can dilate your pupils permanently with eye drops designed to do that, but if a bright light shines in your eye, you'll be blind as a bat for a couple of minutes under the night sky, dilated pupils notwithstanding. It's intuitively kind of evident from everyday experience, e.g., persistent after-images from bright lightsources.

The way rhodopsin works is quite fascinating. It's an integral cell membrane protein: a molecular machine that is triggered by a single photon. The photon causes the molecule to pull one proton (hydrogen ion) through it's tubular interior from one side of the membrane to the other. During this process it changes conformation (shape) and needs to reset itself before it's ready for another photon. This requires some biochemistry (and energy) and takes some time.

http://www.nature.com/nature/journal.../406653a0.html
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Old 18-04-2016, 11:02 PM
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I've had those drops when I had Lasic surgery. I didn't measure them at the time, but I do wonder if they would give a wider pupil than naturally possible, thus making it possible to use eyepieces with larger exit pupils. Do you happen to know if this is the case?
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Old 19-04-2016, 08:16 AM
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Having gone through laser correction too, I don't know that I would be willing to go through the eyedrops just to get a bigger/quicker dilation of the pupils for astronomy.

I don't think they gave any greater dilation, they just produced it quickly so they could measure your max pupil diameter and figure out how wide a circle they had to correct in the surgery and if it was possible to correct your whole field of view.
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Old 19-04-2016, 09:26 PM
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barx1963 (Malcolm)
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As I understand it the pupil opens up very quickly. If you walk outside into a dark area after being indoors in a bright room, your eyes will often be unable to pick up any stars (or anything else), but wait a few seconds and they start to appear. But you are nowhere near dark adapted, all that has happened is your pupils have opened up. But try walking around (as I have done at star parties a few times) and you are dangerous. Ten minutes later when the bio chemical dark adaption starts kicking in you can start seeing much more and 30 minutes later it is about as good as it gets.
Anyway that is my experience.

Malcolm
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Old 19-04-2016, 10:15 PM
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Yes, drugs can open your pupils wider than they would by themselves. I've seen special eyedrops sold for astronomy. They're probably atropine based or hopefully something related, but safer.(??) Atropine is the main toxin in deadly nightshade and was used as a beauty product in 18th/19th century: to make ladies' eyes' pupils bigger and supposedly more attractive - though it probably made them look more like they're tripping off their heads...
https://www.youtube.com/watch?v=BceHGzsDVLY&t=1m26s

Last edited by janoskiss; 19-04-2016 at 10:31 PM.
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Old 19-04-2016, 10:23 PM
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Out of curiosity, I looked it up just to see if it is theoretically possible to augment dark adaption with artificially dilated pupils. I'm not suggesting anyone actually *do this, of course, but I thought it was an interesting thought experiment.

The typical drugs used are phenylephrine and tropicamide (usually together). In this study, most of the dilations achieved were in the normal range with the exception of 0.5% tropicamide solution where the maximum dilation achieved was a whopping 8mm (but perhaps this individual had an 8mm natural dark adapted dilation as well! We'll never know.)

So I guess it's unlikely that we'll see 'Livestrong' branded telescopes anytime soon :-)
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Old 19-04-2016, 10:35 PM
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The eye is most sensitive for detail in DSOs at around the 2mm EP exit pupil mark (e.g., 10mm EP in an f/5 scope), so there is no need to dilate the pupils more than nature intended (even if you're old, you'll have 4-5mm pupils in the dark).
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