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Druid

How thoroughly dark-adapted are you when observing?

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I was just reading Ch2 of Roger Clark's 'Visual Astronomy of the Deep Sky' and was struck by the graphs he presents showing the 5-6 magnitude/sq. arc-sec difference made by half an hour of effective dark adaptation.

That's crudely equivalent to the difference between a 4" and a 32" scope given equal levels of adaptation, if I've done my sums right (which I probably haven't, so please feel free to put me right here)

I don't think I've ever succeeded in completely adapting when observing at home due to the plethora of unshielded lights in my suburban environment.

I hadn't really realised how much of a difference it was likely to have made to my observing though. 

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It very much depends, and sometimes it does not matter (when observing planets, etc). I have been able to see mag 13.9 objects through my 8" scope in (relatively good) suburban settings. This is close to the limit often cited for an 8" scope. I might be able to push it further in really dark settings. I start observing relatively easy stuff at the beginning of the session and move to harder stuff as the session progresses.

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Not very I suspect. I only observe from our back garden and like you that are a lot of unshielded lights around. I have started put a dark cloth over my head while at the eyepiece to try and cut some of it out but can never get really properly adapted.

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I can only adapt for about 10 minutes max before something ruins it. I live in an area where it's inevitable. It's maddening.

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When I drive out into the wilds, and get really dark adapted, it really makes a difference. The views you get after an hour without lights are so much better. I don't care if it's red either - try an hour somewhere dark without any light...

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I suspect the answer depends somewhat on how dark your ambient surounding levels are.  Inside a black painted dome in a dark sky site it might be worth fully adapting for half an hour or so? Everywhere else (even ouside on a dark sky site?) the extraneous light levels may prove too high for there to be huge benefits.

I sit looking upwards in my observatory sometimes (SQM ~21.85) thinking 'there's actually a lot of light out there- it's not dark at all.......'

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I have not looked through a scope for over 40 years but have often wondered what dark adaptation actually is if it is no more than the change in pupil size would it make any difference at all if say you had a scope/eyepiece combination that gave you an exit pupil size of 2mm for example?

Alan

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I have not looked through a scope for over 40 years but have often wondered what dark adaptation actually is if it is no more than the change in pupil size would it make any difference at all if say you had a scope/eyepiece combination that gave you an exit pupil size of 2mm for example?

Alan

There's a useful article in this month's Sky at Night mag. The first 5 minutes or so of dark adaptation is pupil dilation, which improves the eye's light gathering ability by around 16 times. Over the next 20 minutes or so there are chemical changes that occur in the retina in the absence of light, which can improve sensitivity by up to a million times!

Interestingly further down in the article it pointed out to be careful to avoid the blindspot when using averted vision. I have just realised - after a couple of decades of observing - that I have been using the "wrong" side (though avoiding my blindspot by about 10 degrees) and so I can only imagine what I've been missing out on! Must try harder.

Paul

Sent from my iPad using Tapatalk

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I certainly find that as my session progresses I get to see fainter and fainter stars.

My experience is that I get to about 0.5 magnitude above the most possible from my site in about 30 minutes and then as the remainder of the time before the optics dew up passes I get slowly towards the limit possible.

I certainly find that putting a black towel over my head helps a lot regardless of what stage my eyes are at! Although a towel is annoying as it does not stay put. I must design a more appropriate garment!

I am sure that if I made more effort with dark adaptation I could go deeper still..

Mark

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I certainly find that as my session progresses I get to see fainter and fainter stars.

My experience is that I get to about 0.5 magnitude above the most possible from my site in about 30 minutes and then as the remainder of the time before the optics dew up passes I get slowly towards the limit possible.

I certainly find that putting a black towel over my head helps a lot regardless of what stage my eyes are at! Although a towel is annoying as it does not stay put. I must design a more appropriate garment!

I am sure that if I made more effort with dark adaptation I could go deeper still..

Mark

Imagine if a neighbor saw somebody in the yard with a towell over their head sitting in the darkness.  I think it would be cause to call the police in my neighborhood. 

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Imagine if a neighbor saw somebody in the yard with a towell over their head sitting in the darkness.  I think it would be cause to call the police in my neighborhood. 

I've spoken to the police a few times while out observing. They're normally pretty interested once they know what you're up to.

Returning to the topic, I have found that if left long enough, you really can start to see things by starlight. It was a bit of a shock the first time, looking up from the scope and suddenly everything around me being brighter.

Even looking at bright stars through the scope seems able to destroy that adaption, though.

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What I guess I'm still not entirely clear on is how a relatively bright suburban sky impacts dark adaptation.

Does it limit the adaptation that can be achieved in any significant way? A couple of people above are talking as though it does. 

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To echo Laser Jock, whose SQM seems to be similar to mine, the great surprise when fully adapted is that a dark site isn't all that dark. I have no need of a red torch to walk around when I'm really adapted. When I'm not I do need the torch. This is a rare treat for me because I'm gawping at screens far to much!!

Olly

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I guess a better question to ask is whether anyone knows how sky brightness relates to the level of dark adaptation, assuming direct light pollution is blocked.

I think that breaks down into: a) how sky brightness relates to the proportion of rhodopsin bleaching and B) how that bleaching relates to contrast sensitivity. 

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Living next to a carpark I can't ever get adapted much. When I have been out at star parties it is amazing how much you can see around under DARK skies.

Cheers

Peter

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I've spoken to the police a few times while out observing. They're normally pretty interested once they know what you're up to.

Returning to the topic, I have found that if left long enough, you really can start to see things by starlight. It was a bit of a shock the first time, looking up from the scope and suddenly everything around me being brighter.

Even looking at bright stars through the scope seems able to destroy that adaption, though.

Yep I have experienced this to in which the actual starlight can become almost a little glaring to your night vision sensitive eyes.  When I feel that I have become fully dark adapted, the distant light dome over Tyneside, alters from  being a dim glow of slight distraction to becoming a feature that is noticeably brighter, to the extent of maybe having a marginal impact on my otherwise full dark adaption. This however is from a fairly lofty observing spot that I occasionally use, which is still a good distance from the City.

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What I guess I'm still not entirely clear on is how a relatively bright suburban sky impacts dark adaptation.

Does it limit the adaptation that can be achieved in any significant way? A couple of people above are talking as though it does. 

It's not the relative brightness of  the sky -- it's the neighbors that turn on porch lights, the random cars that shine lights through my fence, the security lights across the street.  Any glimpse of those things can ruin all of the adaptation.

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Sure, I get that. 

It's not the relative brightness of  the sky -- it's the neighbors that turn on porch lights, the random cars that shine lights through my fence, the security lights across the street.  Any glimpse of those things can ruin all of the adaptation.

Sure I get that.

What I'm trying to understand is the impact of sky brightness when direct light pollution of the sort you describe is removed from the equation. 

I suspect that the impact is twofold, firstly, your eyes don't adapt as far as they would under dark skies, because more photons are still hitting your rod cells. 

The other factor is how much adaptation down into the 20-plus magnitudes / sq. arc-sec range is going to help when your sky is only about 18 mags / sq. arc-sec.

I think magnification is also going to make a difference there, so you're not comparing the surface brightness of a DSO with the sky, but rather with that of the magnified bit of sky you're seeing through your eyepiece.  

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Dark adaption is primarily down to the levels of neurotransmitters within the rods in the retina. They're sensitive to blue light, because that's the most useful to see with at night. Only light containing this blue component will stimulate them which is why red objects appear grey and why red light is recommended to astronomers.

When the rods are exposed to bright light (and white light is very bad for this because it contains a large blue component) they over-stimulate the rods causing the chemicals to bleach them, over-stimulating them and rendering them useless. Dark adaption is a reversal of this effect as the levels of unbleached neurotransmitters return to normal, usually taking about half an hour.

So what does this mean for our purposes? Firstly you don't see colour because the cone cells, the ones sensitive to colour, need bright light to work. Secondly there's a threshold of light above which the rods cease to function. In practical terms, light levels in any area not directly lit are unlikely to affect them once you're adapted, not to the level where you'd notice beyond placebo. Street lamps do bleach the rods so if you can get away from them so much the better. In my own observing location in the park there is some light from the Laboratory and Cricket Club, but this wouldn't be enough to seriously mess up my dark adaption. Dog walkers and cyclists shining torches in my face do mess things up for me though because of their proximity. Viewing the moon will also bleach your rods which is why moonlit nights are rubbish for Deep Sky.

Really, the reasons light polluted areas are hard to use is because the light scattering around makes the sky brighter than faint objects, rendering them invisible. Only high surface brightness objects can punch through, or using filters which block specific wavelengths.

DD

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Sit in a blindfold for 30 mins. That would really get the neighbours talking.

Blindfolded with a black sheet draped over my head. 

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I do have a few problems with un-shielded lights when observing from my back garden. One of the things I do find helpful though is put on an old fashioned photographic dark room light in the kitchen so that I can set up without bright lights where I store my kit as well tea/coffee making facilities without ruining dark adaptation.

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I do have a few problems with un-shielded lights when observing from my back garden. One of the things I do find helpful though is put on an old fashioned photographic dark room light in the kitchen so that I can set up without bright lights where I store my kit as well tea/coffee making facilities without ruining dark adaptation.

This is a good idea.  I'm going to get one. 

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Sitting in the Elan valley watching the Stars get brighter and brighter is simply bliss and its light its that dark!

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