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Mircea Pteancu

27 Iul 2008 20:47



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Chiar in aceste zile,pe  ATM Email List se discuta si despre lentile singulare pentru observatii solare.Mai exact  George Anderson isi construieste un telescop solar cu care vrea sa observe doar intr-o singura lungime de unda de 0,8A si se intreaba ce rost are sa foloseasca un obiectiv compus.Jarvis Krumbein aminteste de un prieten care a folosit un asemenea obiectiv slefuit din sticla rubinie.Apoi ,in josul paginii intervine Vladimir Sacek (un fel de Zircon de-al lor) care clarifica marimea relativa a  aberatiilor functie de forma lentilei si de indicele de refractie a sticlei:forma de aberatie sferica minima-biconvexa inegala,plan-convexa (bunicica,doar 10% extra aberatie sferica fata de prima la n=1,5) si biconvexa (70% aberatii extra).De retinut ca pentru o aberatie sferica de lambda/4 o lentila de 10cm din BK7 trebuie sa fie la F/14 iar  aberatia cromatica se neglijeaza in aceasta aplicatie.
 Acuma asa ,ca sa ma dau smecher,pun de la Mircea o idee pe care nu am apucat sa o pun in practica :ma gandeam sa slefuiesc o lentila plan convexa de vreo 5-6cm din filtru de sudura pentru o luneta solara.La un focal de 1-1,5m ar trebui sa functioneze.Mai amintesc ca Jean Dragesco,alias Ioan Dragescu,a folosit intr-o vreme un refractor cotit cu obiectiv lentila simpla ,tot pentru observatii solare.Mircea
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George, a single element lens is superior for this application.  40 or
more years ago, a friend built a solar telescope and ground and polished
a ruby glass lens for the scope.  This combined the initial broad band
filtering and the image formation in one piece of glass.  He used one of
the very early "Daystar" Ha filter setups and the whole thing worked
quite well.  Another advantage if you do your own objective lens is that
you can figure the lens for near perfect on axis performance at the
desired wave length.  No need for extra elements and the corresponding
extra reflections.

Jarvis Krumbein

On Sat, 26 Jul 2008 23:20:05 -0400 George Anderson
<tillerman1@videotron.ca> writes:
> I am currently in the process of building a 0.8A solar scope. At 
> present 
> I am using commercial achromatic lenses in the optical train but 
> have 
> been thinking that I may change that in the next year or so.
> Question: Since I am only interested in a single wavelength (656nm) 
> is 
> there really any need for multiple glass types? Wouldn't a single 
> element lens be more than enough for the narrow passband that I am 
> interested in seeing?
> 
> Regards
> 
> George Anderson
> Montreal Canada
> 
> Clear skies and good health


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George Anderson wrote:

>Since I am only interested in a single wavelength (656nm) is 
> there really any need for multiple glass types? Wouldn't a single 
> element lens be more than enough for the narrow passband that I am 
> interested in seeing?

Depends on the lens. Single lens cannot be corrected for spherical
aberration. For given f.l., it is minimized for bi-convex shape with the 
rear surface radius longer 6 and 14 times than the front radius for 1.5 
and 1.6 refractive index (n), respectively. At n=1.5 the aberration is 
some 10% larger for plano-convex lens, and over 50%  for bi-convex.
At n=1.6 there is no appreciable difference between the optimum
shape and plano-convex in the size of aberration, and it is nearly
70% larger for the bi-convex shape.

Actual aberration for plano-convex lens, as the PV wavefront error of  
primary spherical at best focus is given by W=9.3D/2048F^3 for n=1.5, 
and W=6.8D/2048F^3 for n=1.6, with D being the lens diameter, and F 
the focal ratio denominator. In units of the 656nm wavelength, that gives 
about 0.65 wave PV for a single 4" f/10 BK7 lens. It would have to be
nearly f/14 in order to get it down to 1/4 wave.

Coma and astigmatism are comparatively negligible.

Vlad

