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Old 20-11-2011, 06:23 AM
astrospotter (Mark)
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astrospotter is offline
Join Date: Jan 2010
Location: San Jose, CA, USA
Posts: 146
On learning about your own eyes

Some years back in doing some reading on averted vision and following a few links on the web I became aware of the veins we all have on the back of our eyes naturally lead to 'sweet spots' that may vary in each individual to some extent. For averted of course the rods are great for low level light and are the most dense away from the central vision as we know.

So to better understand where my averted vision sweet spots were I found that in the eyepiece go to some starfield that is not very populated (not in the milky way for example) and find an area that has a star or two that are at the very limits of your perception in averted vision.

Now move your eyes relative to that tiny dot and you will fine that the star disappears and re-appears. Try to note the fraction of the field of view and if you are for example looking 'up and to the right 1/6 fov' for example for the very best and widest sweet spot. (I am a right-eyed observer).

You will find sweet spots in different places. It is well worth the time and from time to time I refresh my memory as to where they are. I tend to favor about 3 of them (sometimes below object as well so try all over the place but of course away from the object just a bit)

Too far away and of course averted looses it's ability to form an image, too close and you loose it entirely (The star starts to image on your central eye which means the image is on higher and higher density of cones that are lousy for dim light)

One really nice and detailed discussion of the effect of cones/rods is here

So learn about your observing eye(s). It will pay off to be sure.

For extreme challange objects I first identify the field and set my big dob to track. (if you cannot track then just sit near your scope and take a few seconds a couple times a minute to push your scope to the field while doing the next bit. Next I cover my entire head in a dark blanket for 10 minutes and hope nobody comes by to make fun of me ...
Then I find you will have that extra edge that is sometimes required for extremely dim objects. While viewing I wrap the light/dark blanket that is over my head already around the eyepiece so that my mouth has an open area and I don't breath into this tiny cone I have formed to see the eyepiece.

This may sound very funny but I can assure you it works and is sometimes even required for extremely dim objects.
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