Viewing stars through a refracting telescope?

[21joanna12]

Hello! I am learning about refracting telescopes and am slightly confused. I have read that a refracting telescope will magnify planets, but not change their brightness, but will not magnify stars as these will remain as point objects. It just makes the stars appear brighter. I am slightly confused by this because reading through my textbook, it seemed like the refracting telescope produces a magnified image of a star and the image is inverted and virtual,,, although I can't understand how you produce an enlarged image of a point object either!

I am also confused because I understand that the stars will appear brighter because the objective lens collects more light from the star than the pupil, but then why do planets not appear brighter by the same principle?

Thank you in advance!

 

[Doug Huffman]

Brightness is intensity/area.

 

[wabbit]

The image of a star in a telescope has a certain size, but this is due to diffraction in the optics (as you say, the star is a point source). You need high magnification to see it, and with good optics it looks like this : http://cdn.cambridgeincolour.com/images/tutorials/airydisk-rings.jpg. The smaller the telescope, and the higher the magnification, the bigger that image is.

As for planets, a bigger scope does collect more light and the total intensity increases with a larger objective lens. However if you then use the same eyepiece on both telescopes, the bigger scope will have higher magnification, and the total light is spread out over a larger area - the two effects exactly cancel out.

If you use the bigger scope at the same magnification as the smaller one though, then the surface brightness of the planet does increase.

When you compare naked eye (say 7mm pupil, 1x magnification) with for instance 50x on a 80mm refractor, the surface brightness changes by a factor $(\frac{80}{7}\cdot\frac{1}{50})^2\simeq0.05$: it gets 20x less bright per area.