----------------------------------- Tavi F. 20 Iul 2008 12:38 ----------------------------------- "Oglinzile parabolice din pyrex sunt caracterizate de rapida adaptare la temperatura mediului ambiant, sticla din pyrex adaptandu-se de 10 ori mai repede decat sticla normala la schimbarea de temperatura." Citat de pe astroshop.ro Ai gasit locul "ideal" pentru informare, in privinta asta. Intoxicatia comerciala nu are limite! Mai demult am adus problema asta in discutie, pe topicul "Discuri de sticla", pornind de la afirmatiile unui mare constructor de telescoape- Jean Texereau: Plate glass, though much less attractive in thermal behavior than Pyrex, is nevertheless widely used by European amateurs. In part, this stems from the difficulty of obtaining Pyrex in Europe, even in small disks. A good plate glass does have the advantage of taking a beautiful optical polish. Moreover, the thermal distortion effects are almost imperceptible in reflectors under 10 inches in diameter. Invariably, air currents in the telescope tube are the larger, more troublesome factor. Cite ceva de la alte surse: http://www.geocities.com/smalldob/equipment_reviews/pyrex_or_plate.htm Sau un alt aticol bun, care marita citit: http://www.skyandtelescope.com/howto/visualobserving/3305656.html?page=2&c=y Desprind citatul: This relative insensitivity to temperature changes also has its advantages when the optician carefully refines (or figures) the curve on the mirror's polished surface. When the glass is worked, a certain amount of heat is generated by friction between the tool and the mirror. Because this heat will deform Pyrex less than plate glass, the optician doesn't have to wait as long to test the mirror and proceed to the next figuring step. Since a commercial firm presumably wants to minimize the time spent on an individual mirror, some amateurs assume that a Pyrex mirror will turn out better than one made from plate glass. This sounds logical enough, but I have seen no evidence of it in the dozens of mirrors I have tested over the years. In fact, our tests of six 8-inch Dobsonians (S&T: January 2000, page 60) showed no correlation between glass types and optical quality. In any case, these attributes are more of an issue for the optician than for the mirror user. If you're making a mirror, Pyrex does have its advantages, but if you're simply buying or using one, these characteristics really shouldn't come into play. Si o discutie interesanta: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2393296/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1 din care dau un citat (din partea unei persoane care a atins perfectiunea in domeniul slefuirii oglinzilor, strehl>0.98 ) referitor la adaptarea la temperatura: ……If I recall correctly, plate acclimates more rapidly to temperature changes; even though the expansion coefficients don't look as favorable. But all considered, the faster acclimation makes up for that small deficiency. Mark In utilizarea unor telescoape avind oglinzi bine executate, cu diametre de pina la ~200mm, sticla (plate sau pyrex) nu are nici o importanta, conteaza gradul de detensionare al ei, si lipsa unor incluziuni cum ar fi bulele de aer. "Sticla de borcan" Saint Gobain e buna in privintele astea. ps.- Pyrexul are coeficientul de dilatare de ~2.3 ori mai mic decit cel al sticlei obisnuite. Subsemnatul, care a cules aceste informatii, are experienta in domeniul slefuirii (inclusiv a parabolizarii oglinzilor).