The Dual speed focuser is a
focusing mechanism used in precision optics such as advanced
amateur astronomical telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...
s and
laboratory
A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratory services are provided in a variety of settings: physic ...
microscope
A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisi ...
s.
A dual speed focuser can provide two focusing speeds by using a set of co-axial knobs, one for fast focusing and another for fine focusing when the film or CCD is near the perfect focal plane. This is different from the two separate focusing knobs seen on low level microscopes. Another common reason to employ a dual-speed focuser is with the use of short
focal length eyepiece
An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks through the device. The ...
s, where the
depth of focus
Depth of focus is a lens optics concept that measures the tolerance of placement of the image plane (the film plane in a camera) in relation to the lens. In a camera, depth of focus indicates the tolerance of the film's displacement within the ca ...
is short, requiring critically accurate focusing.
A dual speed focusing mechanism is often combined with a
Crayford focuser
The Crayford focuser is a simplified focusing mechanism for amateur astronomical telescopes. Crayford focusers are considered superior to entry-level rack and pinion focusers, normally found in this type of device. Instead of the rack and pinio ...
, forming a Dual speed Crayford focuser in order to eliminate
backlash
Backlash may refer to:
Literature
* '' Backlash: The Undeclared War Against American Women'', a 1991 book by Susan Faludi
* ''Backlash'' (Star Wars novel), a 2010 novel by Aaron Allston
* Backlash (Marc Slayton), comic book character
* ''Backla ...
during fine focusing.
The concept
Dual speed focuser design borrows the
planetary gearing idea used in automobile
gearbox
Propulsion transmission is the mode of transmitting and controlling propulsion power of a machine. The term ''transmission'' properly refers to the whole drivetrain, including clutch, gearbox, prop shaft (for rear-wheel drive vehicles), differe ...
es. However, in order to eliminate backlash, balls and axles are used instead of gears. All the
transmission inside a dual speed focuser is achieved via
static friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of t ...
.
The cylinder case of a dual speed focuser is fixed on the telescope tube. Three metal balls are placed 120° apart inside the case in a holder shelf. An input axle penetrates through the hole on the center of the holder shelf and is pushed and tightened by three surrounding balls. The fine focusing knob connects to the input axle while the fast focusing knob connects to the holder shelf. On the other side, the axle from the Crayford focuser is connected to the holder shelf.
When one rotates the fine focusing knob, the three balls are driven by the input axle and rotate along the inner circumference of the case at a slower speed. The metal balls then push the holder shelf and drive the output axle. As a result, rotating either of the focusing knobs will cause the other to rotate in the same direction.
Calculation
The angular velocity of input and output axle can be described by the formula:
:
,
where the R and r are the radius of metal ball and input axle, and the
and
are the angular velocities of the input and output axles.
A 1:10 dual speed focuser has a
gear ratio
A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage.
Gear teeth are designed to ensure the Pitch circle diameter (gears), pitch circles of engaging gears roll on each other without slipping, pr ...
of 10 and R=4.5r. Thus the angular velocity ratio is actually 11. The gear ratio and velocity ratio should not be confused with each other.
External links
Shanghai Online Observatory Forum explaining how the dual speed focuser works{{Webarchive, url=https://web.archive.org/web/20110707012956/http://bbs.astron.ac.cn/thread-54988-1-3.html , date=2011-07-07
Optical devices