Sony A900 - Alpha 24.6MP DSLR Camera Manual - Página 20
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FINE FOCUS
Conrady deduced a formula for depth of focus (DoF) based on the geometric circle of confusion. For a 1/4wave wave-
front error the depth of focus is given by:
DoF = 4 x Lambda x f/#^2 where Lambda is the wavelength (22 micro-inches)
For the TEC140APO prime focus f/7 DoF = 0".000 088 x 7^2 = 0".004312 (54 steps on the TCF-S focuser)
However in Seeing: Antoniadi I the wave-front is essentially plane, and the only significant wave-front error arises from
OG. In the case of the TEC140APO the P-V wave-front error is 1/28 wave, which is 7 times less than Conrady's
formula. (~±4 steps in the TCF-S focuser)
In achieving critical focus you are fighting three things: the depth-of-focus; the seeing; focus shift with falling
temperature.
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TRACKING FOCUS
A refractor's focus will shift inwards as the ambient temperature falls. For example my TEC140APO focus shifts ~-
0".100 during a night. The easiest way to track & follow this focus shift is by using the Optec TCF-S focuser piggy
backed on the TEC's StarLight Instruments FeatherTouch 3545 rack & pinion focuser.
It is possible to focus the Sony a900 using the Optec TCF-S to within ±4 steps (1 step = 80 micro-inches). This is the
limit, it is impossible to obtain a finer focus. The Exmor CMOS sensor is flat to about 5 micro-inches, so the focus
tolerance is approximately 64 times the sensor's flatness.
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SEEING
Seeing distorts the object wave-front & takes the image in & out of focus. Once the critical focus has been found, a
sequence of exposures will produce some frames more sharply focused than the rest. This is why it is worthwhile
shooting dozens of frames & selecting the sharpest.
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