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To dark adapt or not?

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#1 REC

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Posted 26 March 2020 - 12:15 PM

I know, we all preach dark adapting eyes for maximum benefit, but I find sometimes that is not the case. I was out last night and it was very clear and the transparency must have been 5/5. I live in a red zone, the  light pollution didn't seem that bad as it normally is for some reason. I spent a lot of time on Orion, Gemini and Auriga. After I was finished I put my gear away in the house and then went out the front door to take out the trash. Looking up I saw Orion and it looked better than before when I was out for an hour and had dark adapted eyes. I mean the stars in Orion and Gemini really stood out against a blacker sky. Everything looked better when I blocked the direct light from the street lights. So my pupil was smaller than before as it closed down to adjust the the lights in the house.

 

Anyone else feel the same way?


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#2 RyanSem

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Posted 26 March 2020 - 12:20 PM

For bright stars you are correct to a point. With a blacker background (i.e. less dark adaptation) the contrast is greater and therefore the bright stars will stand out more. But getting dark adapted actually makes the sky less black in order to pick out dimmer objects and more detail through the eyepiece.
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#3 jupiter122

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Posted 26 March 2020 - 12:55 PM

Could be that during the initial time that you spent observing, Orion moved to a less light-polluted part of your sky, making it more visible. How long was it between the two times you spent observing it?

 

Tim


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#4 Cames

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Posted 26 March 2020 - 01:06 PM

I noticed the exceptional transparency as well.

I wonder if the improvement is due to the recent reduction in the number of airline flights?

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C


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#5 Tom Polakis

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Posted 26 March 2020 - 02:12 PM

I know, we all preach dark adapting eyes for maximum benefit, but I find sometimes that is not the case. I was out last night and it was very clear and the transparency must have been 5/5. I live in a red zone, the  light pollution didn't seem that bad as it normally is for some reason. I spent a lot of time on Orion, Gemini and Auriga. After I was finished I put my gear away in the house and then went out the front door to take out the trash. Looking up I saw Orion and it looked better than before when I was out for an hour and had dark adapted eyes. I mean the stars in Orion and Gemini really stood out against a blacker sky. Everything looked better when I blocked the direct light from the street lights. So my pupil was smaller than before as it closed down to adjust the the lights in the house.

 

Anyone else feel the same way?

 

Since your eyes were not dark adapted, the sky probably appeared inky black, making bright stars seem to stand out better.  With dark-adapted vision, the sky should appear medium gray at the darkest places in the world.

 

Before you step inside the house next time, look without a scope at the faintest extensions of the Milky Way, or find a star at the edge of detectability.   Then look at this part of the sky after returning from inside your  house.  You will not be able to see them until your eyes have dark-adapted again.

 

Tom


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#6 Starman1

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Posted 26 March 2020 - 04:55 PM

I know, we all preach dark adapting eyes for maximum benefit, but I find sometimes that is not the case. I was out last night and it was very clear and the transparency must have been 5/5. I live in a red zone, the  light pollution didn't seem that bad as it normally is for some reason. I spent a lot of time on Orion, Gemini and Auriga. After I was finished I put my gear away in the house and then went out the front door to take out the trash. Looking up I saw Orion and it looked better than before when I was out for an hour and had dark adapted eyes. I mean the stars in Orion and Gemini really stood out against a blacker sky. Everything looked better when I blocked the direct light from the street lights. So my pupil was smaller than before as it closed down to adjust the the lights in the house.

 

Anyone else feel the same way?

If you drive to a dark site, and step out of the car, the sky is black.

20 minutes later, the sky is charcoal grey.

45 minutes after leaving the car, the sky is almost silver, kind of a silver grey.

The black sky is because your eye is not sensitive enough to light to see the real color of the night sky, which is not black.

If you looked for some really faint stars, they'd only appear after a while away from the car.

 

I see Tom already made the same answer.  "Great minds run in the same gutters", as my mother used to say.


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#7 BillP

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Posted 26 March 2020 - 05:38 PM

I know, we all preach dark adapting eyes for maximum benefit, but I find sometimes that is not the case. ...

 

Anyone else feel the same way?

I do because experience shows me so.  Basically it all depends on the task you are doing whether full dark adaptation is necessary.  Lunar and planetary it is just not needed unless you will be working at exit pupils less than 0.35mm or so (i.e., generally extreme mags for the particular aperture).  If the plan is double star observing then this too generally does not need full dark adaptation unless again you are trying some fainter doubles/companions for the aperture you are using.  Good example here is Sigma Ori where the faintest of the 4 can be difficult in small apertures if sky conditions are not good or not dark adapted. 

 

And then there are cases where you do not want dark adaptation.  Cases relevant here are when observing some of the brighter open clusters, basically the Messiers, then if I purposefully observe some of them immediately after being inside, so no dark adaptation, yes they show less stars but then all the stars have vivid and distinct colors in some clusters!  Quite beautiful to see when the cluster is filled with various colored and hued stars as opposed to mostly blue-white with a few yellow-orange.



#8 Tony Flanders

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Posted 26 March 2020 - 07:50 PM

Right, a number of things look better when you're not dark-adapted. The Moon is perhaps the prime example. Bright color-contrast double stars often look more colorful when your not dark-adapted. And as you just discovered, the bright stars marking the outlines of the bright constellations are more prominent when you're not dark-adapted. If you had been looking at Cancer rather than Orion or Gemini, you would have had a different reaction.


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#9 gwd

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Posted 27 March 2020 - 03:40 AM

When I lived in light pollution I noted star colors for my star-hopping.  When I moved to a darker sky sight at high altitude the colors seemed to disappear.  I don't think my eyes fully dark adapted in the light polluted city.   When the moon is out, look at the moon through your telescope with only one eye, or just patch one eye and expose the other to light.  Then look at something faint through the telescope and alternate eyes. 



#10 Jon Isaacs

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Posted 27 March 2020 - 05:49 AM

My two cents:

 

Like others have said, when I walk out the door from a lit room at our place in the high desert, the skies look black, I can't see anything.  If I am not careful, I will run into something.  

 

In less than a minute, my pupils will have dilated and the slow process of dark adaptation has begun.  In 30 minutes, the skies will be bright and the only light I need is to read and I will be viewing objects I couldn't see in those first minutes.  

 

Under bright skies, they may be more aesthetically pleasing with a small exit pupil and the rhodopsin bleached but there's definitely more to see in the way of DSOs with my eye dark adapted.  

 

For the planets and double stars, as Bill said, a dark adapted eye can be counterproductive, resolution and color are jobs for the cones, not the rods.

 

As far as stars and their brightness, the brightness of a star is a function of the area of the aperture and the brightness of the sky glow is a function of the area of the exit pupil.  When the aperture and the exit pupil are the same, then the contrast of the stars against the night sky does not change, the stars and sky glow dim equally.  

 

Jon



#11 jaraxx

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Posted 27 March 2020 - 08:13 AM

The reason your eyes respond to dark conditions is not to see the bright stuff. It's to see the dim stuff, and how much more of it you can see is either a tribute to wonder or to evolutionary pressures. Actually it could be both, divinity and dire wolves working together...

Go outside in the high desert and take a walk under clear skies with no moon. You'll be amazed how much you can see after a short time, and even more amazed at what you can see after 15 minutes. Then look up - and you're under a silvery sky with light coming from everywhere, not a dark sky with bright lights in it. The effect can be disorienting to those of us who watch the sky under less than ideal conditions. Those constellations that are marked by bright stars need a second look because there's so much more stuff around them. The sky you know so well becomes a sky where you need to look closer. And those constellations that you can't see at all in the suburbs start to make sense.



#12 jrbarnett

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Posted 28 March 2020 - 09:31 AM

I know, we all preach dark adapting eyes for maximum benefit, but I find sometimes that is not the case. I was out last night and it was very clear and the transparency must have been 5/5. I live in a red zone, the  light pollution didn't seem that bad as it normally is for some reason. I spent a lot of time on Orion, Gemini and Auriga. After I was finished I put my gear away in the house and then went out the front door to take out the trash. Looking up I saw Orion and it looked better than before when I was out for an hour and had dark adapted eyes. I mean the stars in Orion and Gemini really stood out against a blacker sky. Everything looked better when I blocked the direct light from the street lights. So my pupil was smaller than before as it closed down to adjust the the lights in the house.

 

Anyone else feel the same way?

If you are like me your eyes are imperfect.  Naked eye with a dilated pupil bright stars are spikey and sloppy.  However with a more constricted pupil those stars tidy up.  The smaller pupil excludes astigmatic portions of my lens, and casual scans of the constellations do look better.  But you're also NOT seeing many stars and other celestial objects that you would be seeing naked eye with dark adaptation, and at the telescope on deep sky targets, the difference in what you see in the eyepiece will be...er...night and day.  :)

 

Best,

 

Jim




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