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Mircea Pteancu

13 Iun 2009 15:26



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Incitat de articolele despre sistemele Duplov ,am intrebat inteleptii tribului ATM ,pe ATMList
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Hello
I have only mirrors,the most ''complex'' lense I own is a 60mm dublet of 310mm f'.l.
I joined this list one and half years ago and I don't remember  references to such apochromats using normal,or only slightly exotic optical glasses.Maybe some of you will find this article  as interesting.It was brought to my attention by zoth,a colleague moderator on our Romanian forum ''astronomy.ro''. 
http://bossanova9.org/astro/optics/normal%20glass%20apo%20-%20duplov.pdf
 
Regards,Mircea 
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Iata raspunsul lui Bob May,care considera sistemul Duplov adecvat pentru lunete planetare ,si inca relativ mari ,de 15-20cm diametru si pe la 5 metri focal si care vor cere oculare de diametru mare.
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The problem with optical glasses (glass in general actually) is
that the dispersion curves are just that, curves, and as a
result, it is difficult to get two curves that match their slopes
so that the light is properly refracted to the same spot.  The
errors aren't large but they are there.  The exitic glasses do
have their curves different from "normal" glasses (the normal
glasses are glasses that conform to a curve in the Nd/Vd chart)
in that the dispersion is different from what the normal line of
glasses is.  This makes it possible to better match the amount of
dispersion of the light to another glass over the spectrum of
light.  Needless to say, the obtaining of an exact match between
the dispersions of the various glasses is an almost impossible
task.
What makes things easier tho is to use more glasses which can
then compensate better for color errors.  Using 5 or more glass
pieces improves this a lot over the 2 glass doublets.  The
article shows 3 sets of glasses to obtain a high quality of
compliance of the dispersion of the light.  Unfortunately, what
happens is that each surface of the glass (uncoated glass)
reflects about 4% of the light so having 10 surfaces will mean a
large loss of light.  For planetary viewing, that isn't so bad
but still losing almost a magnitude of light is a big loss for a
star viewing telescope, not so bad with a planetary telescope.
I'll note that you do want to do large magnifications when
looking at planets so that means that a design of a larger
(150-250mm) aperture and a longer FL.  The larger aperture makes
matching glass harder but the longer FL makes it easier and I
will comment here that focal lengths on the order of 5 meters
will make for an excellent telescope for planetary viewing as the
image scale (how big things are at the focal plane of the
telescope) is larger which means that you can use larger EPs that
have better control of the aberrations that happen in the EP.
Good luck!
Bob May
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Iata opinia lui Jan Bentz,care scoate la lumina parametrii asupra carora articolele nu insista si anume ca lungimea fizica este mult mai mare ca focala rezultanta,curbura de camp este puternica,campul imaginilor de inalta calitate este mic,concentricitatea elementelor optice trebuie sa fie criticadatorita distantelor mari intre elemente.Va fi deci un excelent instrument planetar,daca va fi bine construit,un mare ''daca'' zice Jan.
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Mircea -- I raytraced all three of the systems described and they all appeared to do what is claimed in the paper.  Some problems were left out of the description however.  Aside from the obvious construction problems these systems all have a fairly small field of high quality and are all 1.5 to 2.5 times longer than their focal length.  The curvature of field ranges from about -150 to -200mm but is largely compensated for by alot of astigmatism.  I have not done a tolerance analysis but the concentricity would be very critical because of the widely separated components.  Some of the elements are cemented so the loss of light would not be quite so high as Bob May implied.   The usual trouble with widely separated component systems where each component is not seperately achromatised  is the large chromatic differance of magnification (lateral color) but  these designs ingeniously correct for that.  They would certainly be suitable for a planetary scope if properly constructed - a big if.

Jan Bentz
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Si in final precizarea lui ''Richard in the UK'' ref. la marimea/micimea de fapt/ a campului imaginilor de calitate si la dezamagirea sa legat de faptul ca un Newtonian F/7 are aberatii extra-axiale mai mici decat sistemele Duplov avand 14 suprafete optice.
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I would call it a very limited field. The errors of the 90mm f/7 start
increasing very quickly once the field is more than 5mm diameter. Do a
direct comparison of the 90mm f/7 with a 90mm f/7 Newtonian and, over
the very limited field that this "apo" has, you will see little
difference. In fact, paraxially, the newt outperforms it. For 14
optical surfaces, I would have expected more.

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Best regards,
Richard in the UK
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Va reamintesc,nu impuscati mesagerul!Mircea

