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How important is night vision/dark adaptation?


smulx
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I know it makes a difference, but is it really noticeable? When I'm observing, I'm frequently going in and out of the house, mostly to look at Stellarium. I've never went to the trouble to get adapted properly as I couldn't be bothered to sit in the dark for half an hour. Is this something I should really be going out of my way to achieve?

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In my experience it does make a big difference. The other night I popped my head out the back door and was convinced it was not worth venturing out (I could not even see The Plough!), my wife was in the kitchen so the light was on, but when I actually shut the door and went out properly I could see loads of stuff. I ended up having a good night.

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Short answer: Yes

Long answer : Yes!!!

It makes a tremendous difference! Once your eyes are adapted take any precaution you can to keep them that way - dark red screens -eyepatches anything to allow you to see more in the dark!

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I suppose it depends on your local light conditions.

I you live in a town / city with lots of street lights etc then there isnt alot to gain , however if you live in a nice dark rural area :o then its going to make alot of differance

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Try a little test and you can check out the results for yourself. Go outside and stay out for a good 20 mins. Avoid all lights if you can. Come back indoors again but keep one eye shut. Stay a few minutes then go back out again and check out the stars, do the "camera 1, camera 2" thing (only open left eye, close, only open right eye, close, etc.) and you`ll see some difference :o When i go out into the garage at dark i can`t see anything really yet after 10-20 minutes in the dark i can make out stuff sitting around easily.

ATB

will.

PS. You need the house lights on for this to work:D

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Technically when you are dark adapted only the rods around the edge of the retina are firing. They are very good at picking up black and white and edges between the two. In light conditions the enser cones at the center of the retina are triggered and picking up colour.

To test this all you need do is look up on a full moon night and see how many stars you can see. Then look up on a moonless night an see how much more you can see of the stars. In practise this means you are missing that much more of the view in your eyepiece when you aren't dark adapted :o

Edited by brantuk
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I wrote a little blurb about the rods and cones, and dark adaptation, for the U.S. National Park Service's Grand Canyon Star Party web site. Here is the link:

Grand Canyon National Park - Grand Canyon Star Party (U.S. National Park Service)

Short answer is, the cones need quite a bit of energy relative to the rods to work. As has been wisely stated above, cones give detail but there is not enough energy available for full benefit, but we are diurnal so need night vision for survival as well. The rods around the outside of the eye help us survive in the wild; they only detect shades of gray, but you don't need to know the eye color of the wolf stalking you; you need the motion. But the rods are peculiar little buggers; they need rhodopsin, or visual purple, to work! They can't detect light energy without it. So, when it gets dark, your pupils start to open but that happens within minutes. The true magic is the production of rhodopsin, which takes from about 20 to 40 minutes. NOW your rods can see motion and minor detail. But rhodopsin is photoreactive; even a small amount of light breaks it down and it is gone. So, rods are now back to non-functioning until more rhodopsin is produced.

Funny thing about rhodopsin, it is photreactive up away from the red end of the spectrum, so red light, if sufficiently dim 'cause unless it's laser, it's not pure red, is used at night to retain night vision provided by rhodopsin. But excessively bright red will also break down the rhodopsin.

Once you shelter your eyes and let the pupil open, and then patiently allow the rhodopsin to form, you have the gift of night vision. But a few seconds of any bright light, and you are back again to needing the white cane with the red tip. And the active rods being around the outside of the eye is why averted vision works at night; the information needs to be off axis to get the best response.

Your cones don't get enough energy to work in the dark and your rods need rhodopsin to get you the vision you need; cell phones at night will sure kill that capability! Get thee dark adapted, and protect it.

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Some sort of hood/cloth is also a real help when using eyepiece filters. The filters are rejecting some of the light in order to see the important frequencies like O-III, H-Beta, etc., so if this light enters over your shoulder it will enter the eyepiece and bounce back at you. Shielding the eyepiece from this back-scatter is very important in order to get the benefit of the narrow band filters.

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It is HUGELY important. I would never use a computer when doing visual observing, red acetate or not. Print off paper finder charts for your evening and light them with a sopt red torch. I need half an hour myself and have red lights in the house for necessary visits. You can, if living with normal people(!!) keep some sunglasses handy to put on if you have to go into an illuminated area.

It takes half an hour to get adapted and half a second to lose it...

Olly

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It is HUGELY important. I would never use a computer when doing visual observing, red acetate or not.

Indeed. A really deep red - like the Wratten #29 - passes only very long wavelength light which doesn't have the energy to unlink rhodopsin, the sensing mechanism used by the rods which are responsible for dark adapted vision, but most red filters do.

I actually use a white LED torch with a Baader 685nm "infra red" filter over the LED, this gives a very dim deep red light which is ideal for reading charts etc. when observing visually.

It takes half an hour to get adapted and half a second to lose it...

Eyes do vary, as do observing conditions - you just aren't ever going to get properly dark adapted when there's any more than a thin crescent moon in the sky - but when conditions are ideal, dark adaptation actually continues into the second hour - as measured by what you can see & how easily you can see it. Looking at a bright object, even a planet or a bright star, can undo all the good work.

if living with normal people(!!) keep some sunglasses handy to put on if you have to go into an illuminated area.

A "pirate" eye patch to protect the observing eye is also a Good Idea .

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I'd better accept that as a car-less city dweller, I'm never going to see the sky properly. The space behind the building where I live is shaded from street lighting, but has footpath lights and an adjacent lit-up building.

Roll on retirement (2 years) when I'm moving to a darker location.

Mary

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Have you ever noticed how little you can see when you go to bed and turn the light off?.

Have you ever noticed how much you can see in the bedroom 20 mins later if you are still awake and the light is still off?

Same thing.

A good analogy, I like this.

I'd like to add that even after 20 minutes, dark adaption can continue. I noticed this while on a meteor watch once.

Edited by Vega
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There have been some reports that wearing the patch, then removing it, takes some small amount of time to get the signals flowing again. The bedroom analogy is pretty good, with the refinement that if you are dreaming of sugar plums and gradually awake, your eyes and brain get tuned to the view. If, however, you are deep asleep and Rover comes up and slurps your cheek, causing you to jump awake thinking a whale has swallowed you whole, it's still a few seconds of mystery while your gyros restabilize and the optic chain comes alive.

As far as dark adaptation, the observation that even a few days Moon can inhibit the full adaptation is dead on, from my experience. I do many star parties for schools in urban areas during the month, and one or two personal deep sky nights as well. The deep sky site has a 4,000 foot ridge between the site and the city, blocking much of the light. The difference is quite incredible in the views in the same instrument at both sides of the ridge, just due to the city glow. Using a 10" SCT, I can only get a bare hint of a smudge for The Whirlpool, even when the local school lights are completely dark and using an extended dew shield to limit stray light down the aperture. Just the eyes never getting shifted to night. But at the schools on the opposite side of the ridge, and at our deep sky site, on most nights I can get the spiral arms clearly on The Whirlpool and, on rare nights of good transparency (no recent agricultural activity to kick our fine desert sand in the air, and our typical humidity under 18%), then it is possible to see the entire Whirlpool looking like a VCR tape. The only difference is the diffuse city light, blocked by the ridge. The rhodopsin is photoreactive, so even the diffuse light restricts its generation and thus the rods just don't get the chance to do their best. I've seen some observers set up in a tent with only the aperture of the scope sticking out of a flap at the top. I tried it one night and it was remarkable. Like many inches of aperture gain.

Edited by Skylook123
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For planet watching (looking for detail on the planets) it is actually better to make sure that you are not dark-adapted and that you are operating in the normal (daytime) range.

If anyone has a good goto mount, try to follow a planet such as Mars, Jupiter or Saturn into daylight. You may be amazed at the difference in detail that you can discern.

For deep-space stuff, definitely try to get dark adapted and to stay dark adapted - hoods, dark light-proof shrouds etc. It will make a huge difference to seeing the dim extensions on many popular objects - it'll make the difference between seeing a galactic core, or the core+spiral arms. It's really really worth the effort.

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Fascinating thread. Some super info in here. In my experience, for searching out and observing faint DSOs, the longer I can keep my eyes dark adapted the better. 20 mins is fine to begin with, but if I can stay 'dark' for more than an hour I really start to see the benefit - quite literally!

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Jupiter is quite bright, and detail can be lost in the glare when dark adaptation takes over. But using a neutral lunar filter really recovers a lot of detail. Unfortunately, Jupiter is leaving us for a time. When it comes back, try a lunar filter and be surprised.

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