Weather Forecast Tool - FWHM

pages.unibas.ch/geo/mcr/3d/meteo/seeing/nmm22/CASE.html

I usually use the above site to predit the local weather conditions (it's for Casement Aerodrome I think). I noticed recently however that it now carries a FWHM estimate. Cool!

For other parts of the country (and Europe) try this URL:
pages.unibas.ch/geo/mcr/3d/meteo/seeing/nmm22/

Regards,

I noticed recently however that it now carries a FWHM estimate. Cool!

A whats-that-now?

I noticed recently however that it now carries a FWHM estimate. Cool!

A whats-that-now?

Full width half maximum. Measure of seeing basically if all else is equal.

Say you have an image of a star on a CCD. Assuming the camera and scope is set up perfectly and you had perfect seeing (no atmosphere!) you'd expect to see the star as a point of light.

With the atmosphere you get a central few pixels that have most of the light, but pixels surround them get light also.

Think of the image on the CCD as being like a mountain. The pixels with the most light are the highest. Ideally you want a high narrow mountain, that will give a sharp star. A fat low hill will give you a bloated star.

FWHM is the measure of the width of the hill/mountain on the ccd image half way up. So the smaller number the better.

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

You see an "Airy Disc".

A diffraction pattern.

The larger your aperture the smaller your Airy Disc and the more detail you can see in an image.

Explained here:

en.wikipedia.org/wiki/Airy_disc

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.

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