From: '''newspam'''@nonad.co.uk   
      
   On 03/01/2026 10:50, albert@spenarnc.xs4all.nl wrote:   
   > In article ,   
   > john larkin   wrote:   
   >> Maybe two can be stacked for double the magnification.   
      
   > The main thing is the surface area, that account for the power   
   > collected.   
   > Getting the image of the sun smaller, increases the temperature   
   > but that is not that important.   
      
   Having dark absorber material makes a big difference. White cotton wool   
   is much harder to ignite than something that has already been charred once.   
      
   > You realize that a Fresnel is thin,irrespective of focal length?   
   > If you want to make the focus length smaller, you just have to   
   > redesign the lens. No need to stack.   
      
   It gets difficult for a Fresnel lens to be anything faster than f 0.2   
   because the ridges start to interfere with the light path to the outer   
   edges of the aperture. Aberrations also get really bad at fast f ratios.   
      
   https://www.edmundoptics.com/f/fresnel-lenses/12434/   
      
   Commercial ones with decent optical performance tend to be expensive   
   unless they are mass produced for some other purpose.   
      
   Flat OHP condenser lenses are a good source of cheap big fast ones.   
      
   --   
   Martin Brown   
      
   --- SoupGate-DOS v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)