Assorted Formulae for Telescopes

where a is the aperture of the objective F is the focal length of the objective f is the f-number (f/) of the objective

where M is the magnification Alpha is the apparent field Theta is the true field Apparent Field: the closest separation eye can see is 4', more practically 8-25', 1-2' for good eyes. The Zeta Ursae Majoris double (Mizar/Alcor) is 11.75'; Epsilon Lyrae is 3'. True Field (in o) = 0.25 * time * cos of the declination (in ') = 15 * time * cos of the declination where time is the time to cross the ocular field in minutes A star therefore moves westward at the following rates: 15o /h (1.25o/5 min) at 0o declination 13o /h (1.08o/5 min) at 30o declination 7.5o /h (0.63o/5 min) at 60o declination.

where M is the magnification F is the focal length of the objective f is the focal length of the ocular At prime focus (ground glass), magnification is 1x for each 25 mm of F

where M is the magnification D is the diameter of the objective d is the exit pupil (5-6 mm is best; 7 mm does not produce a sharp outer image) The scotopic (dark-adapted) aperture of the human pupil is typically 6 (theoretically 7, 5 if over age 50) mm. Since the human pupil has a focal length of 17 mm, it is f/2.4 and yields 0.17 per mm of aperture. 2.5 mm is the photopic (light-adapted) diameter of the eye.

(by substituting F/f for M) where d is the exit pupil f is the focal length of the ocular f-number is the f-number (f/) of the objective By substituting d=7 (the scotopic aperture of the human pupil) and multiplying it by the f-number, the longest useful focal length of the ocular is given.

(by substituting 6 mm for d and taking the reciprocal) where M is the minimum magnification without wasting light for a dark- adapted eye (17x per mm of aperture) D is the diameter of the objective in mm

(by substituting 0.63 mm, the minimum diameter to which the average human pupil can contract, for d and taking the reciprocal) where M is the maximum theoretical magnification (158x per mm of aperture); the maximum practical magnification is +50%).

m = 2.7+5 log D (assuming transparent dark-sky conditions and magnification >= 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

r = 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 arcsecs Lambda is the wavelength of the light in mm 206265 is the number of arcsecs 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.

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

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.

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 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 viewing. In cases where the comes is more than five magnitudes fainter than the primary, you will need a wider separation: 20 or 25 minutes of arc, nearly the width of the moon seen with the naked eye.

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 arcsecs 3476 is the diameter of the moon in km

Light Grasp = 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. Formulae for Astrophotography

where f/ is the f-number of the system (objective) F is the focal length of the objective D is the diameter of the objective

where f/ is the f-number of the system F' is the effective focal length of the system Fe is the focal length of the ocular (divided by any Barlow magnification) D is the diameter of the objective M is the magnification Fc is the focal length of the camera F is the focal length of the objective Fc/Fe is the projection magnification M/D is the power per mm The diameter of the first image equals the film diagonal (44 mm for 35 mm film) divided by the magnification.

where f/ is the f-number of the system F' is the effective focal length of the system D is the diameter of the objective F is the focal length of the objective (times any Barlow magnification) B is the secondary image ("throw"), the distance of the ocular center from the focal plane of the film, equal to ((M+1)*Fe)/A A is the primary image, the distance of the ocular center from the focal point of the telescope objective M is the projection magnification, equal to (B/Fe)-1 Fe is the focal length of the ocular

where f/ is the f-number of the system F' is the effective focal length of the system D is the diameter of the objective B is the distance of the Barlow center from the focal plane of the film A is the distance of the Barlow center from the focal point of the telescope objective B/A is the projection magnification (Barlow magnification)

(the ratio of intensities of illumination is squared according to the inverse square law) where Exposure Compensation is the exposure compensation to be made to the example system f/S is the f-number (f/) of the subject system f/E is the f-number (f/) of the example system

where Exposure Compensation is the exposure compensation to be made to the example system De is the objective diameter of the example system Ds is the objective diameter of the subject system

(the light-recording power is directly proportional to the square of the radius of the objective and inversely propertional to the square of the f-number) where Power is the light-recording power of the system r is the radius of the objective f is the f-number (f/) of the system Example: a 200-mm f/8 system compared with a 100-mm f/5 system (100^2)/8^2 compared with (50^2)/5^2 156.25 compared with 100, or 1.56 times more light-recording power

where Print EFL is the print's effective focal length Camera F. L. is the camera's focal length Print Enlargement is the amount of enlargement of the print (3x is the standard for 35-mm film)

where Guidescope M is the magnification needed for guiding astrophotographs f is the photographic focal length in mm Experience indicates that the minimum guiding magnification needed is about f divided by 12.5, precisely what a 12.5 mm guiding ocular used in an off- axis guider for prime-focus photography yields. (Since visual magnification is the ratio of the objective to ocular focal length, the combination of prime-focus camer and off-axis guider with a 12.5-mm ocular gives a guiding magnification of f/12.5. f/7.5 (as with a typical focal reducer that reduces the effective focal length by a factor of 0.6) is a significant improvement. f/5 or higher magnification is for top-quality guiding. Guidescope M = Guidescope EFL / Print EFL where Guidescope M is the guidescope's magnification (should be >= 1, preferably 5-8) Guidescope EFL is the guidescope's effective focal length, the guidescope's focal length times any Barlow magnification (should be >= to the focal length of the primary and the guidescope's magnification, 0.2x per mm of focal length of the objective, 0.1x per mm of the camera lens Print EFL is the print's effective focal length

where Guiding Tolerance is in mm 0.076 is one " at a 254-mm reading distance from the print (a crosshair web is usually 0.05 mm)

where S is the error ("slop") in " 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 = 206256 (the number of seconds in a radian) and h = 0.04 mm of image-drift tolerance (an empirical value from astrophotographs).

where EFL is the effective focal length in mm 57.3 is the number of degrees in a radian 206256 is the number of " in a radian

where Resolving Power is the resolving power of a photographic system with Kodak 103a or color film F is the focal length of the system in mm

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); Tri-X yields 80 lines/mm.

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 the standard for 35-mm film)

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.

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 27o, 41o, and 49o for a 50-mm lens. The third formula can be used to find the effective focal length (F) required for a given film frame size (h) and angle of view (Theta).

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 im F is the effective focal length (focal length times Barlow magnification) in mm

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)

where T is the exposure time in minutes for a length of 24 mm (the smallest dimension of 35-mm film) F is the focal length of the lens in mm

where T is the maximum exposure time in seconds without a star trail 1397 derives from 1' 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' in 20 s, which yields a barely detectable star trail with an unguided 50-mm lens. 2-3' (8-12 s) is necessary for an undetectable trail, 1' (4 s) for an expert exposure. Divide these values by the proportional increase in focal length over a 50-mm lens. For example, for 3' (12 s), a 150-mm lens would be 1/3 (1' and 4 s) and a 1000-mm lens would be 1/20 (0.15' and 0.6 s). 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 0o. For other declinations, multiply lengths and divide exposure times by the following cosines of the respective declination angles: 0.98 (10o), 0.93 (20o), 0.86 (30o), 0.75 (40o), 0.64 (50o), 0.50 (60o), 0.34 (70o), 0.18 (80o), 0.10 (85o).

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), ignoring the effects of reciprocity failure in the film, which would mean that the 32-minute exposure would have to be even longer.

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)

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

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 35-mm film] for the smallest dimension of 35-mm film)

where H is the hour angle Theta is sidereal time Delta is right ascension The Hour Angle is negative east of and positive west of the meridian (as right ascension increases eastward).

where n is the serial order of the planets from the sun (Mercury's 2n =1, Venus's n = 0, Earth's n = 1, asteroid belt = 3)

where Theta is the angular size of the object in degrees h is the linear size of the object in m d is the distance from the eye in m e.g., for the width of a quarter at arm's length: (55*0.254)/0.711 = 2o

where the larger the relative brightness value, the better the instrument (e.g., binoculars) is for viewing in twilight or for astronomical use after dusk (low light conditions only) d is the diameter of the exit pupil D is the diameter of the objective M is the magnification

where L is the linear size, or actual length in space, in km A is the maximum angular length as observed in degrees D is the known altitude of the meteor as it enters the atmosphere in km

where Efficiency is the efficiency of the lens for photographing an average (in a meteor shower) F is the focal length of the lens f is the f-number (f/) of the lens

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 .................... 1o Dime at arm's length ...................................... 1o Quarter at arm's length ................................. 2.5o Width of Orion's belt ..................................... 3o Alpha Ursae Majoris (Dubhe) to Beta Ursae Majoris (Merak) . 5o (Height of Big Dipper's cup. These are the "pointer stars" to Polaris.) Alpha Geminorum (Castor) to Beta Geminorum (Pollux) ....... 5o Width of fist at arm's length ............................. 10o Alpha Ursae Majoris (Dubhe) to Delta Ursae Majoris (Megrez) 10o (Width of Big Dipper's cup.) Height of Orion ........................................... 16o Length of palm at arm's length ............................ 18o Width of thumb to little finger at arm's length ........... 20o Alpha Ursae Majoris (Dubhe) to Eta Ursae Majoris (Alkaid) . 25o (Length of Big Dipper.) Alpha Ursae Majoris (Dubhe) to Alpha Ursae Minoris (Polaris) .............................................. 27o

Big Dipper, from cup to handle 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

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 of one second of arc = 30 m * cos of the latitude where cos(Latitude)=1 on lines of constant longitude