Newsgroup: it.scienza.astronomia
Titolo: Consiglio + formulette....

Salve a tutti. Mi sono avvicinato da poco all'astronomia ed č giunto il
momento di acquistare un buon strumento. Dopo un certo periodo di
acculturamento e sperimentazione teorica :-), lurkando anche qui, č giunto
il momento di
passare alla pratica. So che questa domanda č tediosa
per i piů, ma purtroppo...., spero abbiate pazienza. Dunque: ho messo gli
occhi sui seguenti oggetti: un simpatico Celestron Firstscope 80 EQ Wide
Angle e, sempre
Celestron, Firstsope 114 EQ. Il mio bilancio per ora č quello....piů avanti
magari una motorizzazione e le prime foto sfuocate... Rifrattore o
riflettore? Il cielo di casa mia č rosso-aranciato a 360° per 45 di altezza
:°-(((((((((
a causa di, non 1, ma ben 2 parcheggi di supermercato a meno di 1 km l'uno
dall'altro e abitazioni intorno,
dunque č d'obbligo qualcosa di pratico da muovere per spostarmi nelle serate
in "vera missione" :).
Importante: oltre ai pianeti mi piacerebbe osservare il piů possibile il
deep-sky perciň consigliatemi bene: quali oculari? Lente Barlow si'? no? da
quanto?
Propenderei per il rifrattore, per via del trasporto (eviterei il pericolo
di
dover riallineare gli specchi ogni volta, meno manutenzione in generale
ecc.) ma forse per il
deep-sky č piů versatile un newton...mah! Ditemi voi!
Ok, questo era il consiglio...ora passiamo alle formulette: ho letto da
qualche parte le formule per ricavarsi la magnitudine limite di uno
strumento (newton). Purtroppo non riesco piů a trovarle, potreste postarmele
cortesemente sia per rifrattore che
riflettore? Non credo siano le stesse a causa dell'ostruzione del
secondario.
(magari cosě innalzo anche l'interesse culturale del mio post... :-))
Spero di poter fare presto un resoconto dei miei risultati cosě forse sarň
utile a qualche astrofilo dilettante, nella speranza di restituire le
cortesie che ora vi chiedo.
Ciao a tutti gli astrofili cibernauti e cieli sereni !

=1D in mm)
   where m is the approximate limiting visual magnitude
         Delta is the pupillary diameter in mm (accepted value 7.5)
         D is the diameter of the objective in mm
----------------------------------------------------------------------------
----
Angular Radius of Airy (Diffraction)
Disc
r = (1.12*Lambda*206265)/D
  = 127.1/D
   (the second formula is based on Lambda = 0.00055 for yellow light)
   where r is the angular radius (one-half the angular diameter) of the Airy
   disc (irreducible minimum size of a star disc) in "
         Lambda is the wavelength of the light in mm
         206265 is the number of " in a radian
         D is the diameter of the objective in mm
         The Airy disc in visual appearance is brighter at the center,
dimmer
         at the edges.
----------------------------------------------------------------------------
----
Linear Radius of Airy (Diffraction)
Disc
r = 0.043*Lambda*f
   where r is the linear radius (one-half the linear diameter) of the Airy
disc in mm
         Lambda is the wavelength of light in mm (yellow light 0.00055)
         f is the f-number (f/) of the objective
----------------------------------------------------------------------------
----
Dawes Limit (Smallest Resolvable Angle,
Resolving Power)
Theta = 115.8/D
   where Theta is the smallest resolvable angle in "
         D is the diameter of the objective in mm
   Atmospheric conditions seldom permit Theta < 0.5".  The Dawes Limit
   is one-half the angular diameter of the Airy (diffraction) disc, so that
the
   edge of one disc does not extend beyond the center of the other).  The
working
   value is two times the Dawes Limit (diameter of the Airy disc), so that
the
   edges of the two stars are just touching.
----------------------------------------------------------------------------
----
Magnification Needed to Split a Double
Star
M = 480/d
   where M is the magnification required
         480 is number of seconds of arc for an apparent field of 8 minutes
of arc
         d is the angular separation of the double star
   About the closest star separation that the eye can distinguish is 4
minutes=20
   of arc (240 seconds of arc).  Twice this distance, or an 8-minute
  (480-second) apparent field angle, is a more practical value for
  comfortable=20 viewing.  In cases where the comes is more than five
magnitudes
  fainter than=20 the primary, you will need a wider separation:  20 or 25
minutes of
  arc,=20 nearly the width of the moon seen with the naked eye. =20
----------------------------------------------------------------------------
----
Resolution of Lunar Features
Resolution = (2*Dawes Limit*3476)/1800)
Dawes Limit * 38.8
   where Resolution is the smallest resolvable lunar feature in km
         2*Dawes Limit is the Airy disc (a more practical working value is
twice this)
         1800 is the angular size of the moon in "
         3476 is the diameter of the moon in km
----------------------------------------------------------------------------
----
Light Grasp
Light Grasp = (D/d)^2*Pi
            = 7*D^2
   where Light Grasp is times that received by the retina
         D is the diameter of the objective in mm
         d is the diameter of the eye's pupillary aperture in mm (accepted
           value 7.5) pi is the transmission factor (approximately equal to
62.5% for the
           average telescope, up to approximately 180 mm)
         To compare the relative light grasp of two main lenses used at the
         same magnification, compare the squares of their diameters.
----------------------------------------------------------------------------
----
Estimating Angular Distance
Penny, 4 km distant .......................................  1"
Sun, Moon ................................................. 30'
   (The Moon is approximately 400 times smaller in angular
   diameter than the Sun, but is approximately 400 times
   closer.)
Width of little finger at arm's length ....................  1=B0
Dime at arm's length ......................................  1=B0
Quarter at arm's length ...................................  2.5=B0
Width of Orion's belt .....................................  3=B0
Alpha Ursae Majoris (Dubhe) to Beta Ursae Majoris (Merak) .  5=B0
   (Height of Big Dipper's cup.  These are the "pointer
   stars" to Polaris.)                                      =20
Alpha Geminorum (Castor) to Beta Geminorum (Pollux) .......  5=B0
Width of fist at arm's length ............................. 10=B0
Alpha Ursae Majoris (Dubhe) to Delta Ursae Majoris (Megrez) 10=B0
   (Width of Big Dipper's cup.)
Height of Orion ........................................... 16=B0
Length of palm at arm's length ............................ 18=B0
Width of thumb to little finger at arm's length ........... 20=B0
Alpha Ursae Majoris (Dubhe) to Eta Ursae Majoris (Alkaid) . 25=B0
   (Length of Big Dipper.)
Alpha Ursae Majoris (Dubhe) to Alpha Ursae Minoris
   (Polaris) .............................................  27=B0
----------------------------------------------------------------------------
----
Estimating Magnitudes
Big Dipper, from cup to handle=20
   Alpha (Dubhe)     1.9
   Beta (Merak)      2.4
   Gamma (Phecda)    2.5
   Delta (Megrez)    3.4
   Epsilon (Alioth)  1.7 (4.9)
   Zeta (Mizar)      2.4 (4.0)
   Eta (Alkaid)      1.9
Little Dipper, from cup to handle
   Beta (Kochab)     2.2
   Gamma (Pherkad)   3.1
   Eta               5.0
   Zeta              5.1 (4.3)
   Epsilon           4.4
   Delta             4.4
   Alpha (Polaris)   2.1
----------------------------------------------------------------------------
----
Range of Useful Magnification of a
Telescope
D = diameter of aperture in mm
Minimum useful magnification .................... 0.13*D
                               0.2*D for better contrast
Best visual acuity .............................. 0.25*D
Wide views ...................................... 0.4*D
Lowest power to see all detail (resolution of eye
   matches resolution of telescope) ............. 0.5*D
Planets, Messier objects, general viewing ....... 0.8*D
Normal high power, double stars ................. 1.2*D to 1.6*D
Maximum useful magnification .................... 2.0*D
Close doubles ................................... 2.35*D
Sometimes useful for double stars ............... 4.0*D
Limit imposed by atmospheric turbulance ......... 500
----------------------------------------------------------------------------
----
Geographic Distance
Geographic distance of one second of arc = 30 m * cos of the latitude
   where cos(Latitude)=1 on lines of constant longitude
----------------------------------------------------------------------------
----