I have a 6cm aperture newtonian refractor telescope through which I saw the Orion Nebula yesterday night. As all you can hope with a telescope of such a small aperture, I saw some faint cloudy structure, but when I looked away from the cloud through the telescope it appeared to be more distinct and dense. Why is it so?
This is called the Averted vision. It involves not looking directly at the object, but looking a little off to the side.
How does it works?
Because the retina is less sensitive at the center, and this is very remarkable on the very faint objects (like nebulas or galaxies)
This is a common technique used by astronomers.
More info here
How to make the most of Averted vision?
To maximise the effects of your Averted Vision, you first need to know which eye is your dominant eye. To do so:
- Point with your finger a distant object
- Then blink an eye, then the other eye.
- The eye in which your finger is perfectly aligned with the object is your dominant eye.
Then, for the best Averted vision:
- Look at the top-right of the object if your dominant eye is the right one
- Look at the top-left of the object if your dominant eye is the left one
The effect you describe is called "averted vision". It works because the center of the retina is optimized for details. We use it for reading and anything that needs sharp vision. The other parts of the retina are more sensitive to light and and motion but not to detail
Digital cameras are like this too. Having many tiny pixels makes for a sharp image but the small pixels are less sensitive to low light and create more noise or grain in the image. A digital camera is more sensitive to light if the pixels are physically larger but of course then you can have fewer of these in a same size sensor
The eye in effect has smaller "pixels" that are closer together but less sensitive to light in the center of the visual field and larger more sensitive "pixels" in the off center areas. So, by looking off to the side you can cause the image of the nebula to fall on the part of the retina that is more sensitive to light.
"Averted vision" is a nice term, and perhaps expertly technical as well. It's the same principle we are taught as shooters/small arms handlers to utilize for night-work when enhanced vision equipment is not available; we simply know it as a function of our peripheral vision, right up there with using the same visual technique to more easily detect motion - especially in a darkened environment. The phenomenon is due to both the distribution and the disparity in numbers of rods versus cones within the human retina. In any event, it's pretty cool once you get used to using it to your advantage :-)