Newsgroup: it.scienza.astronomia
Titolo: Fotografia in parallelo...

Ciao a tutti...devo accingermi a fare alcune foto in parallelo. La
macchina dovrebbe aver un obiettivo di 50mm . Siccome devo fare dei
test farò alcune pose con tempi dai 30 secondi ai 10-12 minuti.
L'inseguimento sarà di tipo manuale, sfruttando il mio 114 con
montatura equatoriale. Che consiglio potete darmi sull'oculare da
usare? Chiaramenete più corto è l'oculare, meno errori si commettono,
però stare 10 minuti a fissare una stella a 300x potrebbe essere un pò
stancante 8-) Quale potrebbe essere secondo voi un buon compresso tra
una buona guida e la possibilità di osservare qualcosa durante la
posa? 
Ciao e 73-51 de Tartaruga .
.oO-=> TARTARUGA (* Gabriele *) <=-Oo.
     E-Mail: 
ru...@ita.flashnet.it
http://www.geocities.com/SiliconValley/Peaks/4731/
"Chi dorme non piglia pesci, ma chi non dorme, alla fin fine...muore..." (C) Tartaruga 1999 ;-)

ciao
non dovrebbero sussistere controindicazioni a "guidare" sull'oggetto che
stai fotografando (se proprio ti stufa guardare una stella), e poi con
l'ottica da 50mm non serve guidare con ingrandimenti spropositati, ne
bastano 100
evita poi tranquillamente una posa di mezz'ora, che non serve a niente se
non a bruciarti l'emulsione; un 50mm č almeno f/2 e con una pellicola ISO400
per dia gia con un quarto d'ora si ottengono ottimi risultati (sempre se il
cielo sia OK)
saluti
roberto
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roberto


 posa?
Con un 50mm non hai grossi problemi visto che ti puoi permettere un 'errore'
di guida di oltre due primi e per quanto riguarda i tempi di esposizione
c'e' una formuletta facile facile ... mai oltre 20% in più del quadrato del
rapporto focale dell'obbiettivo ... espresso in minuti.
Dunque con un f2 non andare oltre i 5 minuti con una 400ASA; meno se il
fondo cielo non e' più che scuro ...
Ciao e buon divertimento ... io con il 114 ho fatto esposizioni con un 135mm
f3.5 inseguendo quella piccola stellina  per 15 minuti e ti assicuro che
alla fine ottenere una buona foto ti da una soddisfazione in più ;) .
    Carlini Alessio

    Carlini Alessio


 posa?
Ciao,
Ti allego una serie di formulette che penso possano esserti utili (ne
sistono altre più precise però!).
MAXIMUM ALLOWABLE TRACKING (SLOP) ERROR
S = about 8250/(F*E)
where S is the error ("slop") in seconds of arc
F is the focal length in mm
E is the amount of enlargement of the print (3x is the standard for 35-mm
film)
The slop is derived from the formula Theta = K*(h/F), with K = 206265 (the
number of seconds in a radian) and h = 0.04 mm of image-drift tolerance (an
empirical value from astrophotographs).
CONVERSION OF PLATE SCALE TO EFFECTIVE FOCAL LENGTH
EFL = mm per degree * 57.3 = 206265/seconds of arc per mm
where EFL is the effective focal length in mm
57.3 is the number of degrees in a radian
206265 is the number of seconds of arc in a radian
RESOLVING POWER OF A PHOTOGRAPHIC SYSTEM
Resolving Power = 4191 seconds of arc/F (For Tech Pan R = 660 seconds/F)
where Resolving Power is that of a photographic system with Kodak 103a or
color film
F is the focal length of the system in mm
MAXIMUM RESOLUTION FOR A PERFECT LENS
Maximum Resolution = 1600/f
where Maximum Resolution is the maximum resolution for a perfect lens f is
the f-number (f/) of the lens
Most films, even fast ones, resolve only 60 lines/mm; the human eye resolves
6 lines/mm (less gives a "wooly" appearance). 80 lines/mm for a 50-mm lens
is rated excellent (equal to 1 minute of arc); a 200-mm lens is rated
excellent with 40 lines/mm. 2415 films yields 320 line pairs (160 lines)/mm
(equal to 1 second of arc); Kodak Tri-X yields 80 lines/mm.
MINIMUM RESOLUTION NECESSARY FOR FILM
Minimum Resolution = Maximum Resolution * Print Enlargement
where Minimum Resolution is the minimum resolution necessary for film
Maximum Resolution is the maximum resolution for a perfect lens
Print Enlargement is the amount of enlargement of the print (3x is standard
for 35mm film)
APPARENT ANGULAR SIZE OF AN OBJECT
Apparent Angular Size = (Linear Width / Distance) * 57.3
where Apparent Angular Size is the apparent angular size of the object in
degrees
Linear Width is the linear width of the object in m
Distance is the distance of the object in m
A degree is the apparent size of an object whose distance is 57.3 times its
diameter.
SIZE OF IMAGE (CELESTIAL)
h = (Theta*F)/K
Theta = K*(h/F)
F = (K*h)/Theta
where h is the linear height in mm of the image at prime focus of an
objective or a telephoto lens
Theta is the object 's angular height (angle of view) in units corresponding
to K
F is the effective focal length (focal length times Barlow magnification) in
mm
K is a constant with a value of 57.3 for Theta in degrees, 3438 in minutes
of arc, 206265 for seconds of arc (the number of the respective units in a
radian)
The first formula yields image size of the sun and moon as approximately 1%
of the effective focal length (Theta/K = 0.5/57.3 = 0.009).
The second formula can be used to find the angle of view (Theta) for a given
film frame size (h) and lens focal length (F). Example: the 24 mm height, 36
mm width, and 43 mm diagonal of 35 mm film yields an angle of view of 27
degrees, 41 degrees, and 49 degrees for a 50-mm lens.
The third formula can determine effective focal length (F) required for a
given film frame size (h) and angle of view (Theta).
SIZE OF IMAGE (TERRESTRIAL)
h = (Linear Width / Distance) * F
Linear Width = (Distance * h) / F
Distance = (Linear Width * F) / h
F = (Distance * h) / Linear Width
where h is the linear height in mm of the image at prime focus of an
objective or telephoto lens
Linear Width is the linear width of the object in m
Distance is the distance of the object in m
F is the effective focal length (focal length times Barlow magnification) in
mm
LENGTH OF A STAR TRAIL ON FILM
Length = F*T*0.0044
where Length is the length in mm of the star trail on film
F is the focal length of the lens in mm
T is the exposure time in minutes
0.0044 derives from (2*Pi)/N for minutes (N = 1440 minutes per day)
EXPOSURE TIME FOR STAR TRAIL ON 35-MM FILM
T = 5455/F
where T is the exposure time in minutes for a length of 24 mm (the smallest
dimension of 35mm film)
F is the focal length of the lens in mm
MAXIMUM EXPOSURE TIME WITHOUT STAR TRAIL
T = (1397/F)
where T is the maximum exposure time in seconds without a star trail
1397 derives from 1 minutes at reading distance (254 mm), the smallest
angular quantity that can be perceived by the human eye without optical aid
("limiting resolution") and is equal to 0.1 mm. This quantity also applies
to the moon. 2-3x yields only a slight elongation. Use 20x for a clock
drive.
F is the focal length of the lens in mm
The earth rotates 5 minutes of arc in 20 seconds, which yields a barely
detectable star trail with an unguided 50-mm lens. 2-3 minutes (8-12 secs.)
is necessary for an undetectable trail, 1 minute of arc (4 secs.) for an
expert exposure. Divide these values by the proportional increase in focal
length over a 50-mm lens. For example, for 3 minutes of arc (12 secs.), a
150-mm lens would be 1/3 (1 minute and 4 secs.) and a 1000-mm lens would be
1/20 (0.15 minutes and 0.6 secs.). Note that to compensate for these values,
the constant in the formula would be 1000 for a barely-detectable trail, 600
for an undetectable trail, and 200 for an expert exposure.
N.B. The above formulae assume a declination of 0 degrees. For other
declinations, multiply lengths and divide exposure times by the following
cosines of the respective declination angles: 0.98 (10 degrees), 0.93 (20
degrees), 0.86 (30 degrees), 0.75 (40 degrees), 0.64 (50 degrees), 0.50 (60
degrees), 0.34 (70 degrees), 0.18 (80 degrees), 0.10 (85 degrees).
EXPOSURE DURATION FOR EXTENDED OBJECTS
E = f^2/(S*B)
where e is the exposure duration in seconds for an image size of >= 0.1 mm
f is the f-number (f/) of the lens
S is the film 's ISO speed
B is the brightness factor of the object (Venus 1000, Moon 125, Mars 30,
Jupiter 5.7)
Thus, a 2-minute exposure at f/1.4 is equivalent to a 32-minute exposure at
f/5.6 (4 stops squared times 2 minutes). This ignores the reciprocity
failure in the film, which would mean that the 32-minute exposure would have
to be even longer.
SURFACE BRIGHTNESS OF AN EXTENDED OBJECT ("B" VALUE)
B = 10^0.4(9.5-M)/D^2
where B is the surface brightness of the (round) extended object
M is the magnitude of the object (total brightness of the object),
linearized in the formula
D is the angular diameter of the object in seconds of arc (D^2 is the
surface area of the object)
EXPOSURE DURATION FOR POINT SOURCES
e = (10^0.4(M+13))/S*a^2
where e is the exposure duration in seconds for an image size of >= 0.1 mm
M is the magnitude of the object
S if the film 's ISO speed
a is the aperture of the objective
FOCAL LENGTH NECESSARY TO PHOTOGRAPH A RECOGNIZABLE OBJECT
F = (Distance / Linear Field) * Image Size
where F is the focal length in mm necessary to photograph a recognizable
object
Distance is the distance of the object in m
Linear Field is the linear field of the object in m
Image Size is the image size in mm (equal to 24 mm divided by the amount of
enlargement of the print [3x is the standard for 35mm film] for the smallest
dimension of 35mm film)

sistono altre più precise però!).
Grazie mille...mi saranno certamente utili 8-)


 problemi...almeno quello...8-)
Risolvi il problema visto che il 114, sotto un cielo tranquillo, ce la fa a
separarle senza grossi problemi .... non e' che osservi dal balcone di casa
!? Questo peggiorerebbe la turbolenza dell'aria limitandoti nelle
osservazioni.
Spero di esserti stato utile ... buon divertimento
    Carlini Alessio

    Carlini Alessio


 da lì si scende 8-)
Per un fondo cielo "perfettamente scuro" il limite è posto dall'emulsione
fotografica, dunque dipende da essa,  che oltre un certo periodo di
esposizione perde sensibilità ... e' il cosiddetto 'Difetto di reciprocita'
'
Se non ci fosse si potrebbe pensare (Come sognai all'inizio della mia
avventura astrofilica :) ) di acquistare un bel tele f11 ed esporre per
alcune ore fino a raggiungere mag. limiti impensabili .... purtroppo le cose
non vanno in questo modo infatti dopo un po la sensibilità della pellicola
crolla rendendo inutile lo sforzo successivo :(( .
Dunque solo l'esperienza con una pellicola puo' darti quello che cerchi.
Ciao ciao
    Carlini Alessio

Ciao ciao
