Sunday, January 16, 2011

Testul lui Lockyer

In cartea sa ''Stargazing:Past and Present'', din a carei editie din 1878 citez mai jos , Norman Lockyer ne propune sa testam telescoapele noastre pe o secventa de dificultate crescanda de stele duble si multiple din constelatia Orion. Toate stelele principale ale acestor stele duble si multiple sunt vizibile cu ochiul liber, ceea ce face ca acest test sa fie usor de facut.
Testul verifica atat capacitatea de colectare a luminii cat si cea de rezolvare a detaliilor fine si se adreseaza in primul rand posesorilor de telescoape mici,sa zicem de la 60mm la 100mm diametru, desi una dintre duble este atat de stransa incat necesita, pentru a fi rezolvata in componente , un bun telescop de 150mm diametru.

Am parcurs acest test in dimineata zilei de 10 Decembrie 2010, cu telescopul meu Dobson de 125mm F/7 si am gasit stelele cu usurinta , cu grosismente chiar si de peste 300x , folosind doar cautatorul acestuia de 8x35mm.Stelele sunt toate usor de gasit,fiind vizibile cu ochiul liber,poate cu exceptia stelei 52 Ori care se pierde usor in pacla atmosferica si in poluarea urbana.
Asadar iata secventa de stele de observat:
Delta Ori
Beta  Ori
Zeta  Ori
Sigma Ori
Lambda Ori
52 Ori

Rezultatul testului este unul singur: Lockyer ne spune ca telescopul care da imagini clar rezolvate,fara aberatii,cu spatiu negru vizibil intre componentele chiar si ale ultimei stele duble din secventa,ei bine...''acel telescop nu este unul de dispretuit''.
Ceea ce  inseamna mult atunci cand vine de la un astronom cu experienta,competenta si exigenta lui Lockyer,si,prin asumare,ale tuturor numelor mari pomenite in descrierea testului:Amiralul Smyth,Kitchiner cel ciudat,John Herschel si Struve.
Sublinierile si impartirea in alineate imi apartine si ajuta la intelegerea acestui test.


Star test your Objective Glass

Quotes from ‘’STARGAZING: PAST AND PRESENT’’ by J.Norman Lockyer

London. MACMILLAN AND CO. 1878

Book II,CHAP. XII,page 164-166

‘’The convenient altitude at which Orion culminates in
these latitudes renders it particularly eligible for observation
; and during the first months of the year our
readers who would test their telescopes will do well not
to lose the opportunity of trying the progressively
difficult tests, both of illuminating and separating power,
afforded by its various double and multiple systems,
which are collected together in such a circumscribed
region of the heavens that no extensive movement of
their instruments an important point in extreme cases
will be necessary.

Beginning with DELTA, the upper of the three stars which
form the belt, the two components will be visible in
almost any instrument which may be used for seeing
them, being of the second and seventh magnitudes, and
well separated. 

The companion to BETA though of the same
magnitude as that to DELTA , is much more difficult to observe,
in consequence of its proximity to its bright primary,
a first-magnitude star. Quaint old Kitchener, in his
work on telescopes, mentions that the companion to
Rigel has been seen with an object-glass of 2 3/4-inch
aperture ; it should be seen, at all events, with a 3 -inch.

ZETA the bottom star in the belt, is a capital test both of
the dividing and space-penetrating power, as the two
bright stars of the second and sixth magnitudes, of
which the close double is composed, are exactly 2.5" apart,
while there is a companion to one of these components
of the twelfth magnitude about 3/4 seconds distant. The small star below, which the late Admiral Smyth, in his charming book," The Celestial Cycle," mentions as a test for his object-glass of 5*9 inches in diameter, is now plainly to be seen in a 3 3/4 inch. The colours of this pair have been variously stated ; Struve dubbing the sixth magnitude which, by the way, was missed altogether by Sir John Herschel "olivaceasubrubicunda."

That either our modern opticians contrive to admit
more light by means of a superior polish imparted to the
surfaces of the object-glass, or that the stars themselves
are becoming brighter, is again evidenced by the point of
light preceding one of the brightest stars in the system composing SIGMA . This little twinkler is now always to be seen in a 3 3/4-inch, while the same authority we have before quoted -Admiral Smyth -speaks of it as being of very difficult vision in his instrument of much larger dimensions. In this very beautiful compound system there are no less than seven principal stars ; and there are several other faint ones in the field. 

The upper veryfaint companion of LAMBDA is a delicate test for a 3 3/4-inch,
which aperture, however, will readily divide the closer
double of the principal stars which are about 5" apart.

These objects, with the exception of ZETA, have been given
more to test the space-penetrating than the dividing
power ; the telescope's action on 52 Orionis will at once
decide this latter quality. This star, just visible to the
naked eye on a fine night, to the right of a line joining
ALPHA and DELTA, is a very close double. The components, of the
sixth magnitude, are separated by less than two seconds
of arc, and the glass which shows a good wide black
division between them, free from all stray light, the
spurious disk being perfectly round, and not too large,
is by no means to be despised.
Cartea ''Stargazing:Past and Present'' a lui J. Norman Lockyer poate fi descarcata prin linkul de mai jos,sponsorul  digitalizarii fiind Google:

Atasez ,din cuprinsul vast al acestei foarte interesante carti, doua  ''optick tubes'' ,pentru a intelege cum erau gandite si executate telescoapele   in epoca in care a fost publicata cartea . 
''Optick tubul'' din partea de sus a figurii este al unui telescop Newton de 25cm diametru iar cel de jos  este al marelui telescop din Melbourne de 120cm diametru, construit de firma Grubb din Irlanda.
Telescopul din Melbourne a fost ultimul ''dinozaur'' dintre telescoapele de observator cu oglinda principala realizata din bronz de telescop, ca de altfel si  oglinda sa secundara Cassegrain.Acest ultim telescop a beneficiat si de luxul de a avea doua seturi de oglinzi:un set era in uz,adica ''de serviciu'' in timp ce celalalt set era in curs de relustruire.
Norman Lockyer depicted observing at his large refracting telescope

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