Dual Speed Focuser in Telescopes: A Comprehensive Guide

A dual speed focuser in a telescope is a type of focuser that has two different speeds for focusing, a coarse speed and a fine speed. The coarse speed is used for making large focus adjustments, while the fine speed is used for making small, precise focus adjustments. This dual speed mechanism allows for more accurate and easier focusing, especially when observing objects that require high magnification, such as planets.

Understanding the Dual Speed Mechanism

The dual speed mechanism in a telescope focuser is an example of a mechanical system that uses gear ratios to change the speed and torque of the focus mechanism. The gear ratio in a dual speed focuser is typically around 1:10, meaning that the fine focus speed is ten times slower than the coarse focus speed.

This gear ratio is achieved through the use of a helical thread created by the slight inclination of the ball bearing axis. As the focuser knob is turned, the helical thread causes the draw tube to move in or out, resulting in the focus adjustment. The fine focus speed is achieved by the slower rotation of the helical thread, while the coarse focus speed is achieved by the faster rotation of the helical thread.

The dual speed mechanism allows for precise focus adjustments, which is especially important in high magnification observations. This is because small changes in the focus can have a significant impact on the image quality, and the fine focus speed allows the observer to make these small adjustments with ease.

Examples of Dual Speed Focusers

One example of a dual speed focuser is the Dual Speed Helical Crayford Focuser (DSHC). The DSHC mechanism is simple to make and provides a fine focusing option similar to the one sold for Crayford Focusers. It uses a helical thread created by the slight inclination of the ball bearing axis to move the draw tube in or out for focusing.

Another example of a dual speed focuser is the GSO Crayford Focusers with 86.4mm Adapter (PAF-GS-M-CRF-86). This focuser has a dual speed mechanism that allows for precise focus adjustments, making it compatible with refractor telescopes.

Measuring a Telescope for a Dual Speed Focuser

When measuring a telescope for a clamp style electronic focuser, the following steps should be taken:

1. Remove the dual speed knobs
2. Measure the focuser shaft
3. Measure the collar/shoulder
4. Check the electronic focuser product listing or manual to ensure that the coupler size is close in size but larger than the measurement for the focuser shaft.

When measuring a telescope for a bracket style electronic focuser, the following steps should be taken:

1. Remove the coarse focus knob
2. Measure the focuser shaft
3. Check the electronic focuser product listing or manual to ensure that the coupler size is close in size but larger than the measurement for the focuser shaft.

It is important to ensure that the coupler size of the electronic focuser is close in size but larger than the measurement of the focuser shaft to ensure a secure and stable connection.

Physics of Dual Speed Focusers

The dual speed focuser is an example of a mechanical system that uses gear ratios to change the speed and torque of the focus mechanism. The gear ratio in a dual speed focuser is typically around 1:10, meaning that the fine focus speed is ten times slower than the coarse focus speed.

This gear ratio can be calculated using the following formula:

Gear Ratio = N1 / N2

Where:
– N1 is the number of teeth on the larger gear
– N2 is the number of teeth on the smaller gear

In the case of a dual speed focuser, the larger gear would be the coarse focus gear, and the smaller gear would be the fine focus gear.

For example, if the coarse focus gear has 100 teeth and the fine focus gear has 10 teeth, the gear ratio would be:

Gear Ratio = 100 / 10 = 10:1

This means that the fine focus speed is 10 times slower than the coarse focus speed, allowing for precise focus adjustments.

Numerical Example

Suppose a dual speed focuser has a gear ratio of 10:1, and the coarse focus knob requires 5 full rotations to move the draw tube by 1 mm. Calculate the number of rotations required for the fine focus knob to move the draw tube by the same 1 mm.

Given:
– Gear ratio = 10:1
– Coarse focus knob requires 5 full rotations to move the draw tube by 1 mm

To find the number of rotations required for the fine focus knob to move the draw tube by 1 mm, we can use the following formula:

Fine focus rotations = Coarse focus rotations × Gear ratio

Substituting the values, we get:

Fine focus rotations = 5 × 10 = 50 rotations

Therefore, the fine focus knob requires 50 rotations to move the draw tube by 1 mm, which is 10 times more than the coarse focus knob.

Conclusion

In summary, a dual speed focuser is a type of focuser that has two different speeds for focusing, a coarse speed and a fine speed. This dual speed mechanism allows for more accurate and easier focusing, especially when observing objects that require high magnification. Examples of dual speed focusers include the Dual Speed Helical Crayford Focuser and the GSO Crayford Focusers with 86.4mm Adapter. Measuring a telescope for a dual speed focuser involves removing the knobs, measuring the focuser shaft and collar/shoulder, and checking the electronic focuser product listing or manual to ensure that the coupler size is close in size but larger than the measurement for the focuser shaft. The dual speed mechanism is an example of a mechanical system that uses gear ratios to change the speed and torque of the focus mechanism, allowing for precise focus adjustments.

References:
– Baader Planetarium – 2″ BDS SC Baader Diamond Steeltrack
– Cloudy Nights – Introducing the Dual Speed Helical Crayford Focuser
– Reddit – Single Speed Focuser vs Dual Speed
– High Point Scientific – Measuring Your Telescope for an Electronic Focuser
– Agena Astro – GSO Crayford Focusers with Telescopes Compatibility