Weather Forecast Tool - FWHM

If your small aperture telescope cannot split a close double then YOU ARE seeing overlapping Airy Discs.

But you don't actually see an Airy disc when you look through a telescope at a focussed star. You are seeing, what for the point of the FWHM explanation, is a point of light.

That point of light will be smaller and brighter under good conditions and dimmer and wider under worse conditions. This can be measured by taking the width of the peak at the midpoint of its intensity.

The wikipedia link you gave on Airy discs isn't the friendliest of articles. A better one to start with would be this en.wikipedia.org/wiki/Angular_resolution . That explains as Peter pointed out why you cannot resolve double stars in a small aperture telescope no matter how much magnification you use.

You dont see stars as points of light ever, even without any atmosphere.

If you see Airy discs when you take images or look through a telescope that is focussed you either need a new telescope or your eyes looked at

In any case Seanie asked about FWHM, not the complete theory of light.

Thanks Al!

You guys ever wondered why a Keck image shows stars as "smaller" than they do in an image taken through an SLR camera with a 50mm lens.

Congratulations,now you know.

I have just told you.

You guys ever wondered why a Keck image shows stars as "smaller" than they do in an image taken through an SLR camera with a 50mm lens.

Congratulations,now you know.

I have just told you.

:roll: