Is the cosmos invisible to the unaided eye?

[ostren]

This has been bugging me for some time and I'd love to get a clearer picture. We know that photos from the various moon missions show a black sky without stars, and that has been explained to me before as resulting from the fact that those photos were taken in broad daylight, plus maybe the camera equipment was set for optimal recording of local features.

But it still tickles me a bit about the fact that surely the Earth's atmosphere is held to be responsible for the stars seeming "twinkling", as clearly that same atmosphere does a great deal of diffusing and diffracting of starlight. Even though stars are big and bright, the distances are so unimaginably large that to the unaided eye they must surely be infinitessimal pinpoints -- and I mean ULTRA-infinitessimal! Sure, the Hubble Space Telescope can see and photograph stars because of its great magnifying power. But I'm stuck on the notion that the unaided human eye cannot see stars. If I were sitting atop that Hubble scope, wouldn't I just see unadorned blackness out there?

 

[PRodQuanta]

Good question. I'm just commenting so I can see what others have to say.

Paden Roder

 

[Chronos]

Of course we can see stars, heck you can even see other galaxies with the unaided eye. Riding on the Hubble, you would really get an amazing view [dress warm and bring life support].

 

[Nereid]

Your eye records photons; once there are more than a certain number detected (per rod or cone), your brain 'sees' light (colourless for really dim light; cones - from which come our colour perception - are less sensitive than rods).

So, if you can see stars from here on Earth, you will surely be able to see the same stars from in space, because the atmosphere absorbs some of the photons (you can see this effect by looking towards the horizon; faint stars which are just visible when overhead become invisible when they are close to the horizon; more air for the photons to go through, more absorption).

The angular size of a distant object has little to do with its perceived brightness, if that size is smaller than what your eyes can resolve (approx 8'?). You can see this when you're flying over a town or city at night - you can make out individual street lights, but they are surely far too small to resolve! (care to do some calculations? how small?)

 

[ostren]

What are you saying? that 8 minutes of arc is the smallest that the eye can "resolve" because that corresponds to less than a single rod or cone on the retina? But you're quite certain that the eye perceives it anyway?

 

[Janus]

What are you saying? that 8 minutes of arc is the smallest that the eye can "resolve" because that corresponds to less than a single rod or cone on the retina? But you're quite certain that the eye perceives it anyway?

Yes, that about it. All the rod and cone has to do is detect enough photons from the source, regardless of its angular size, and the eye will percieve the light from the source.

 

[Nereid]

The 8' resolution comes from basic optics - wavelength of light, size of the pupil (lens), focal length, etc. There may also be an 'image processing' component - the signals from neighbouring rods (or cones) are integrated in the visual processing centre of the brain (or pre-processed upon leaving the retina?); I read somewhere that this system has a 'response function' that is strikingly similar to the PSF! i.e. a kind of sombrero hat (to the first Airy ring).

Another way of saying this is that a point source at infinity is imaged onto the retina as a smeared-out blob which covers several rods at least, and this blob is then processed by the brain as a point.

 

[turbo]

As an optician, my job was to try to fit people with lenses that would best correct for the errors in their eyes lenses and errors in their eye geometry. What many people don't know, though, is that when your pupils are constricted (in the presence of very bright light) your visiual acuity increases, and when you are in darkeness (pupils opened to the max) the errors in your eyes' lenses become the most apparent. Astigmatism is a particularly bad one (cylindrical error), since it can cause you to see flares, even in an otherwise well-focused image. If you are using a telescope and you have no astigmatism, you should focus for best sharpness without your glasses. If you have astigmatism, by all means wear your glasses, since the scope's ocular cannot correct for that.

 

[Phobos]

Sure, the Hubble Space Telescope can see and photograph stars because of its great magnifying power.

FWIW, Hubble images also show local stars as points (except for one or two notable exceptions where it was able to resolve the image as a disk).

Betelgeuse... http://antwrp.gsfc.nasa.gov/apod/ap990605.html
Mira... http://antwrp.gsfc.nasa.gov/apod/ap010121.html