"Useful" magnification

[Maideneer]

I haven't really tried it yet because I'm waiting for my new scope pieces to arrive but I'm sitting here wondering if you go beyond the useful magnification guideline of your scope, does that mean you'll start to see 'nothing'?  What exactly will you see once you blow past that line?

I have crazy ideas I like to try, like using a 3x barlow with a 2.3mm EP on a 9.25HD to see a crater on the moon...not saying that'll work of course but these are things I wonder about.

[rl]

It's not that you see "nothing" so much as "nothing extra". Leaving the usual seeing limits out, once you've matched the resolving power of your scope to the resolving power of your eye, all you end up doing is spreading the same amount of detail over a larger part of your retina. The view doesn't necessarily fall apart, but it loses sharpness because the limited amount of detail is oversampled by the cells on your retina. 

The resolving power of your eye is a very individual thing; I've seen numbers between 1 and 4 arcminutes (= 60 to 240 arcseconds) in the literature. It also depends on illumination..whether you are using the rods or cones. So for a 4" scope with a resolving power of 1 arcsecond, a magnification somewhere 60 and 240 will allow the eye to see all that the scope can reveal. 

[vlaiv]

Larger darker blurrier image that is the same as one at lower magnification.

People seem to think that somehow magnification plays a part in telescope image - but it does not - or at least not in that way (it only impacts image after it has been formed by telescope - like if EP is not sharp enough).

Telescope forms image of certain resolution at its focal plane - regardless what sort of eyepiece is in the focuser - or if there is even any EP in focuser.

Eyepiece really acts as magnifying glass. You can tale EP alone and whatever you place at its focal plane (field stop of EP) - it will be seen clearly and magnified - you can try this at your computer screen - unscrew 1.25" barrel of simple EP and place it against computer screen showing some image - you'll be able to see individual pixels!

What happens when you magnify too much - well nothing really except that image gets darker as light is spread over large area. This happens:

If you see this with normal magnification:

then you'll see this with x3 that magnification (roughly, I'm just simulating light loss - not doing proper calculation):

Some low contrast feature will simply vanish because image is darker and there is less contrast, but they are still "there" and if you use camera instead of eye - you can enhance contrast and still see them.

[mikemarotta]

You got some good replies and on the surface, they are unarguable. It does depend on what you want the magnification for.  Allow me to suggest Ronald Stoyan's "Visual Astronomer" webpage. 

(It is not https secure, but here it is http://visualastronomer.com/ ) He recommends making the most of all of your available magnification. Just for example, if you are chasing binary stars, image quality is not the most important factor. All you want is two Airy disks that separate the objects. I would not normally view at 220X, but I must put a 6mm ocular in with a 2X Barlow to split the "double-double" in Lyra using my 102mm refractor from my city skies home. It is not a loss but rather a benefit to be able to verify what I have read about. 

2 hours ago, Maideneer said:

...  if you go beyond the useful magnification guideline of your scope, ...

I do agree with rl that much of how you see is individualized. Numbers are guidelines. The eyes of young people have different pupil responses than do those of adults and older adults. As we age, we can get near-sighted and then far-sighted again. Expectation has a lot to do with what you see. So does experience. 

1 hour ago, rl said:

It's not that you see "nothing" so much as "nothing extra".  The resolving power of your eye is a very individual thing; ... 

I agree with that. 

1 hour ago, vlaiv said:

Larger darker blurrier image that is the same as one at lower magnification. ... Some low contrast feature will simply vanish because image is darker and there is less contrast, but they are still "there" and if you use camera instead of eye - you can enhance contrast and still see them.

We do most of our seeing with our brains. The eye just supplies information. The brain makes sense of it.