16 replies to this topic

[yamicat]

The 'Averted Vision" comes up from time to time but like many astronomy words/phrases we read over it and turn a 'blind eye'.

 

What is averted vision?

 

Why and when is it best to use averted vision?

 

 

[Love Cowboy]

Averted vision is viewing an object "out of the corner" of one's eye, as opposed to looking directly at it.  In visual astronomy, faint objects frequently are visible with averted vision even when invisible with direct vision.  This is because our eyes have more rods on the edges than in the center, and the rods are what are responsible for our being able to see in low light levels.  I'm sure someone else is going to come along and explain it in more detail than that, but that's the basic gist.

 

As far as when to use it... all the time on deep sky objects!  If you can't see an object with direct vision, try averted and it might pop out.  And, even if you CAN see an object with direct vision, it would appear brighter with averted vision and could give up more detail. 

[sg6]

Averted vision is when you do not look driectly at an object. I think there are 2 reasons.

First is that looking directly at an object means the image falls on the center of the rear of the eye and that is where the blind spot is.

 

The other (I think) is that the central portion of the eye has the colour sensitive cones and these need a level of brightness to see anything.

 

So a dim object looked at directly can be difficult to "see".

 

The rods are (I think) not central and are more sensitive but they are not colour. So to bring these into play you need to look a bit off to one side and then they are used. So a dimmer object becomes "apparent" but it is grey and if you try to look at it then it disappears.

[S.Boerner]

There is even an averted vision scale:

http://www.astronomy...on Scale.pdf   

 

Since Love Cowboy is an AL Awards person I'll mention that averted vision scale level is one of the requirements of the AL's Local Galaxy Group & Neighborhood Observing Program.

[ChristianG]

First is that looking directly at an object means the image falls on the center of the rear of the eye and that is where the blind spot is.

 

The other (I think) is that the central portion of the eye has the colour sensitive cones and these need a level of brightness to see anything.

 

So a dim object looked at directly can be difficult to "see".

 

Hi. 

 

'First': no, the blind spot is off to one side and somewhat below the fovea. The fovea is the color vision part and is less sensitive than the peripheral monochrome vision, so 'The other' is quite correct!

 

Just try it, naked eye will do! Cheers...

 

--Christian

[jtsenghas]

The naked eye object I like to teach people to use averted vision on is the Galaxy in Andromeda--M31.  Most people get the most pronounced effect looking below it to exploit the part of the retina with the most rod receptors.  It can be found 8 degrees from Mirach, which is two full Telrad circle diameters if you want to help newbies locate it with your Telrad and then view it naked eye.  On a Sky and Telescope Pocket Atlas a penny subtends exactly 4 degrees just like a Telrad circle. (Just my $0.02 worth...)

 

 

Edited by jtsenghas, 18 January 2016 - 08:10 PM.

[havasman]

http://www.telescope...al_response.htm

 

Section 13.9 contains a short section on receptor distribution on the retina. That's the driver for averted vision's effectiveness.

 

 

For more on dark adaption and other factors in play see the physiology of vision tab here:

 

http://www.reinervog...et/index_e.html

[REC]

Maybe that's why I think I see more when using a binoviewer? Maybe I'm using parts of my averted vision from two eyes than one? Interesting thought?

 

Anyway, I use it all the time when looking at dim fuzzballs with one eyepiece. I have been doing it forever and it just natural for me I guess.

[Philler]

Actually I have found that looking just slightly-- sometimes ever so slightly--to one side of a object like a galaxy will not only help reveal it but will sometimes also bring out details. Also, sometimes if you vary the position of the object whether it is above or below or right or left in your field of view can make a difference. Also, going to higher powers helps with contrast to reveal faint objects and details in them. Don't be afraid to experiment with magnification.

[kfiscus]

My favorite deep sky object (DSO) to show the value of averted vision to newbies is NGC-404, the galaxy right next to the star Mirach in Andromeda (in the same neighborhood as JT's M-31).  It has the nickname of "Mirach's Ghost".  Direct vision isn't very effective at revealing it.  Moving the field of view to avoid the glare of the star really helps, too.

[Jon Isaacs]

Some good stuff so far.

 

A good way to practice is to look at open clusters that have both faint and brighter stars, the double cluster in Perseus is good for this.

 

Look at it directly and then shift your glance off to the side, suddenly the cluster will seemingly light up, the stars you are seeing brighten and unseen stars come to life.. With experience, averted vision becomes second nature, you just do it with thinking about it.

 

Jon

[kfiscus]

Jon is right about using clusters.  If you show a newbie the cluster M-35, it is a real beauty.  Without saying anything, see if they happen to notice the much dimmer cluster NGC-2158 lurking at the edge.  AV is the key.

[lamplight]

NGC 6826, the "blinking planetary" is fun. Look at it dead on there's just a star. With averted vision, looking off to the side (or try a circular motion of looking around the object) and the rods responsible for night time vision will pick up the nebulosity and the surrpunding nebylosity will "blink" into visibility. I understand it's named for this averted vision "blinking" effect.  

 

 Mirachs ghost might be easier to locate.  

 

The double cluster (ngc 869 and  884) is a great cluster example with star magnitudes that escape direct vision and wink out when observed directly.  Kinda fun

[REC]

Some good stuff so far.

 

A good way to practice is to look at open clusters that have both faint and brighter stars, the double cluster in Perseus is good for this.

 

Look at it directly and then shift your glance off to the side, suddenly the cluster will seemingly light up, the stars you are seeing brighten and unseen stars come to life.. With experience, averted vision becomes second nature, you just do it with thinking about it.

 

Jon

That's a great idea to use that one to practice with!

[yamicat]

I'm getting a image of reverse tunnel vision to try explaining my intake. I mean focus dead on, then keeping same focal distance, shift to side and use peripheral vision. Then expect to fight urge to see what's in side vision, cause you'll lose what you want to see. In short, don't look at what you want to see.   HEY! Now I get it!, experienced are having fun with the newbies. Some things never change.   

Actually starting to make some sense.  Sounds like fun.

[jtsenghas]

You've got it! That's the art of seeing objects by "not looking at them"!

 

I sometimes find it works better for me by slowly scanning small circles or figure eight shapes in small regions near the edge of the field of view when the desired faint object is centered.  This both offers different parts of the retina to the faint object and helps to discipline myself not to aim my vision directly at it.

[Jon Isaacs]

I'm getting a image of reverse tunnel vision to try explaining my intake. I mean focus dead on, then keeping same focal distance, shift to side and use peripheral vision. Then expect to fight urge to see what's in side vision, cause you'll lose what you want to see. In short, don't look at what you want to see.   HEY! Now I get it!, experienced are having fun with the newbies. Some things never change.   

Actually starting to make some sense.  Sounds like fun.

 

At first it is something of a chore, it takes some concentration.. That is why practicing on targets like the double cluster that will suddenly "light up" can be helpful.  Soon enough it will be second nature...

 

Jon