Comprehensive Insight into Telescope Focus Adjustment Mechanisms
Understanding Telescope Focusers in Amateur Astronomy
Telescope focusers play a crucial role in bringing distant celestial objects into sharp focus. In this article, we'll explore the various types of focusers, their functions, and common specifications found in amateur astronomy.
Focusers and Their Functions
A telescope focuser works by moving the eyepiece or camera closer to or farther from the telescope's main optical tube. This is achieved through sliding or rotating a drawtube or helical mechanism that changes the focal plane, allowing for fine adjustments to bring observed astronomical objects into clear focus. Fine focus knobs often control this precise movement.
Common focuser types in amateur astronomy include the Crayford, rack and pinion, and helical focusers. The Crayford focuser uses smooth, friction-based bearings for very precise and smooth focusing without backlash. The rack and pinion focuser consists of a geared mechanism where rotation of a knob moves the focuser tube via a gear and rack, providing solid, straightforward focus control. The helical focuser focuses by rotating the entire eyepiece holder tube in and out like a screw thread.
Common Sizes and Specifications
The drawtube diameter typically accepts 1.25-inch or 2-inch eyepieces or cameras, with 1.25-inch being standard and 2-inch providing a larger field of view at the eyepiece. Base plate diameters for focusers vary depending on the telescope tube size, with common base flange sizes mentioned being approximately 86.4 mm (3.40"), 96.6 mm, and 112.5 mm to fit different telescope tubes.
The focuser travel (the maximum drawtube movement) can be around 79 mm (3.11”) for dual-speed Crayford focusers, allowing fine and coarse focusing. Fine focus knobs often have a diameter near 40 mm, while micro-focus knobs (for very precise adjustments) might be around 26 mm in diameter.
Installing a New Focuser
If you're considering retrofitting a new focuser onto your telescope, the process is relatively straightforward for most refracting or reflecting telescopes. For refractors, you'll need an adapter flange to fit the new focuser body onto the back of the tube, with installation as easy as removing the old focuser and replacing it with the new flange. For reflectors, if the new and old focusers have different screw hole patterns, installation may require drilling additional holes in the tube.
Advanced Focusing Options
Some focusers offer dual-speed fine control for better precision, allowing smooth, precise adjustments by translating a lot of hand movement into very fine focuser movement. This feature is particularly beneficial during dynamic observing sessions where quick switching between coarse and fine focusing is needed.
In some Schmidt-Cassegrain and Maksutov-Cassegrain models, dual-speed focusers can be adapted, with Starlight Instruments selling Feather Touch dual-speed knobs for many commercial catadioptrics.
Large Format Focusers
Larger formats, such as 2.5-inch, 3-inch, or larger, are typically used in professional or highly specialized amateur telescopes. These larger focusers provide a wider field of view and increased light-gathering capabilities, making them ideal for observing faint, distant objects.
In conclusion, a focuser in amateur astronomy is a mechanical assembly that moves the eyepiece or imaging device precisely to achieve focus. By understanding the various types, sizes, and functions of focusers, you can make an informed decision when choosing the right focuser for your telescope.
- In the realm of amateur astronomy, telescope focusers are essential for bringing distant celestial objects into sharp focus.
- A Crayford focuser, a common type in amateur astronomy, utilizes smooth, friction-based bearings for precise and smooth focusing without backlash.
- For installation of a new focuser onto a refractor, an adapter flange is required to connect the new focuser body to the back of the tube.
- With dual-speed fine control, some focusers offer better precision, translating large hand movements into very fine focuser movement for dynamic observing sessions.
- Eyepieces or cameras with a drawtube diameter of either 1.25-inch or 2-inch are common specifications found in amateur astronomy, with the 2-inch providing a larger field of view.
- Maksutov-Cassegrain and Schmidt-Cassegrain models can be adapted to use dual-speed focusers, with Starlight Instruments offering Feather Touch dual-speed knobs for many commercial catadioptrics.
- Advanced telescope users, such as those interested in planetary imaging, astrophotography, or observing faint, distant objects, may benefit from using larger format eyepieces and focusers, like those with a 2.5-inch, 3-inch, or larger diameter.
