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Question about stars viewed through optics


pipnina

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At the most recent PAS meeting on the 11th this month, David Wilton talked about different "telescope terms explained simply", at one point he touched on exit pupil and said that nebulae & extended objects get dimmer the less exit pupil you have, and also the sky background dims, but stars do not.

This doesn't make a lot of sense to me, but it leads into two questions:

1) Do stars really stay the same brightness at different exit pupils and this is why higher mag results in dimmer stars being visible because the sky background dims but the stars don't?

2) Do stars get brighter with aperture? In my 250px stars are a heck of a lot brighter than in my 130/900, but the 250px I use at 5mm exit pupil, the 130/900 at 3.6mm, if the above is true, does this mean that stars get brighter with aperture?

 

Thanks!

    ~pip

 

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Pip your speaker was correct. When you view a star in a telescope it is essentially a point source and as such what you see is the diffraction pattern of a point source. While an extended source is the overlap of infinitely many diffraction patterns. When you increase the magnification you view at you reduce the exit pupil size (mag = entrance pupil/ exit pupil). I the case of the star it magnifies the diffraction pattern but the total light stay the same as it all remains within the exit pupil. For an extended object it magnifies the overlapping patterns and some of the light now falls out side the exit pupil reducing the intensity. This is equivalent to the fact that field of view is smaller at higher magnification and the sky background is more spread out.

To your questions yes and yes.

Regards Andrew

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4 hours ago, michael.h.f.wilkinson said:

Stars do not get dimmer, because their light is not spread out because they are point sources. Only when your magnification is so high you start seeing the Airy disk clearly does the perceived intensity get lower

 

4 hours ago, andrew s said:

Pip your speaker was correct. When you view a star in a telescope it is essentially a point source and as such what you see is the diffraction pattern of a point source. While an extended source is the overlap of infinitely many diffraction patterns. When you increase the magnification you view at you reduce the exit pupil size (mag = entrance pupil/ exit pupil). I the case of the star it magnifies the diffraction pattern but the total light stay the same as it all remains within the exit pupil. For an extended object it magnifies the overlapping patterns and some of the light now falls out side the exit pupil reducing the intensity. This is equivalent to the fact that field of view is smaller at higher magnification and the sky background is more spread out.

To your questions yes and yes.

Regards Andrew

Cheers guys! Thanks for the info!

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