Sadr, Diffraction & Nebulosity

Well, I didn’t stay away from Cygnus for long! To be honest I had some scope time to spare whilst trying out my new hyper-expensive techno gismo. I jest, it’s a diffraction focusing device to help me get the imaging just so. Take one homemade dew shield (tube of photo mount board plus tank tape) add 4 small holes, 90 degrees apart, as close to the telescope end as possible. Now thread 2 long fine knitting needles (thank you Jane) through holes so that they are at right angles to each other & thus form a cross at the front of the telescope. Now when you try to focus the telescope for the camera (i.e. looking through DSLR viewfinder) it is much easier since bright diffraction spikes appear when focus is reached. See picture below:

Sadr in Cygnus Click image for a larger view.

This is Sadr, a beautiful star at the centre of Cygnus. Normally once all properly focused you would gently remove the knitting needles before imaging, but I felt like experimenting with the effect, so in they stayed. Now if we look at the full image after processing …

Sadr in Cygnus Again click image for a larger view. can see that much of the area around Sadr displays significant nebulosity. Imaging Details: Canon 20D C6SGT Prime f10 ISO1600 Exposure 44 mins 16s in 16 subs

So what is ‘Diffraction’ anyway? To quote GCSE Physics sources, “Diffraction is the spreading out of waves when they pass though a gap or around an object”; a little crude but that is essentially it. Light can be thought of as either a particle (photons) or a wave form i.e. it has duality. If you consider a wave front of light as it approaches a small ‘gap’ such as a diffraction grating or the ridges & troughs on the back of one of your music CDs; flat parallel waves of light may approach but waves that are curving appear from the other side of the gap. If there is more than one ‘gap’ then these curved waves can interfere with each other and this causes the colours that you see when looking at a CD or DVD surface. This same principle applies to my two knitting needles – parallel light from the stars curves (diffracts) around the needles, these resulting waves interfere with each other, producing a pattern that helps me focus. There are loads of uses for this, holographic imaging for one, analysing crystal structure is another; it is also the limiting factor for focusing good quality telescopes & cameras, see Airy Disc.


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