TAF - resolving power
| RESOLVING POWER of an optical system (Pr) |
| Minimum Angle of Resolution (a) - Minimum Visible Detail |
capability of an optical system to make cleary visible two objects angularly near (i.e.: the components of a multiple star system) or to visualize objects with very small angular size (i.e.: Cassini division of Saturn rings or the edge details of a small lunar crater).
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the Resolving Power (Pr) of an optical system, intended as the capability of an optical system to make clearly visible two objects angularly near, is identified with the minimum angle of resolution (a) showed by an optical system and increase when the latter decrease.
• the higher is the Resolving Power (Pr) the smaller is the minimum visible detail or minimum angle of resolution (a)
• the lower is the Resolving Power (Pr) the bigger is the minimum visible detail or minimum angle of resolution (a)
the Resolving Power (Pr) -that is the minimum angle of resolution (a)- of an optical system is given by the following Dawes' formula
Dawes' formula:
 |
a |
minimum angle of resolution in arc-seconds or minimum visible detail in arc-seconds |
115 |
Dawes' limit (theorical value) |
D |
optical system diameter (aperture) in mm. |
|
the Dawes' limit = 115 is applied to optical systems assumed as perfect
considering the phisical defect that may be present in optical systems a Dawes' limit = 120 is used
therefore this Dawes' formula is usually applied:
 |
a |
minimum angle of resolution in arc-seconds or minimum visible detail in arc-seconds |
120 |
Dawes' limit (practical value) |
D |
optical system diameter (aperture) in mm. |
|
the Resolving Power (Pr) is proportional to the aperture in mm. of lens because the higher is the aperture of lens the lower is the minimum angle of resolution (a) and consequently the higher is the Resolving Power (Pr)
• doubling the diameter (aperture) in mm. of lens (D) will double the Resolving Power (Pr) of the optical system
• halving the diameter (aperture) in mm. of lens (D) will halve the Resolving Power (Pr) of the optical system
• to double the Resolving Power (Pr) of the optical system, double the diameter (aperture) in mm. of lens (D)
• to halve the Resolving Power (Pr) of the optical system, halve the diameter (aperture) in mm. of lens (D)
the Resolving Power (Pr), identified as the minimum angle of resolution (a), can be defined from the following formulae :
 |
Pr |
Resolving Power in arc-seconds of the optical system
(minimum angle of resolution in arc-seconds or minimum visible detail in arc-seconds) |
115 |
Dawes' limit (theorical value) |
D |
optical system diameter (aperture) in mm. |
|
 |
Pr |
Resolving Power in arc-seconds of the optical system
(minimum angle of resolution in arc-seconds or minimum visible detail in arc-seconds) |
120 |
Dawes' limit (practical value) |
D |
optical system diameter (aperture) in mm. |
|
under construction
for the Resolving Power calculation of optical systems not listed mail to Maurizio De Marco the request will be inserted in TAF