Feathertouch on a C8 ?Posted By Gary Carter |
Dave,
I've not used the feathertouch on a C8 but have heard good many things about them. The built-in focuser actually moves the mirror inward and outward to change the focal length. There are two issues you will face using any type of focuser that replaces or augments the knob on a C8. One is the inherent hysteresis that exists in the SCT focuser design, that is to say the "slack" that is evident in the focuser assembly when you reverse focus direction. (You may have noticed that the focus does not change during the initial rotation of the focuser when changing focus direction.) The second issue you will deal with is the fact that every time you physically touch the knob to focus, inevitably the scope/mount will shake during the focus operation. This makes the finer focusing rather difficult. Even though the Feathertouch will help make finer adjustments - you have to wait for the dampening to occur and thus the "shakes" to settle.
A very good solution to this is a motorized focuser such as the JMI NGFS. It comes with an electronic hand controller. This focuser attaches to the SCT threads on the rear cell in front of the diagonal. The design is a crayford focuser assembly that moves everything behind it (e.g. the diagonal, eyepiece or camera) instead of the mirror. No hysteresis exists in this design. You use the existing built-in focuser for the "rough" focus. Fine adjustments are then made with the NGFS without having to touch the OTA itself. With this very accurate focus adjustments can be made for visual or imaging use.
-Gary
I've not used the feathertouch on a C8 but have heard good many things about them. The built-in focuser actually moves the mirror inward and outward to change the focal length. There are two issues you will face using any type of focuser that replaces or augments the knob on a C8. One is the inherent hysteresis that exists in the SCT focuser design, that is to say the "slack" that is evident in the focuser assembly when you reverse focus direction. (You may have noticed that the focus does not change during the initial rotation of the focuser when changing focus direction.) The second issue you will deal with is the fact that every time you physically touch the knob to focus, inevitably the scope/mount will shake during the focus operation. This makes the finer focusing rather difficult. Even though the Feathertouch will help make finer adjustments - you have to wait for the dampening to occur and thus the "shakes" to settle.
A very good solution to this is a motorized focuser such as the JMI NGFS. It comes with an electronic hand controller. This focuser attaches to the SCT threads on the rear cell in front of the diagonal. The design is a crayford focuser assembly that moves everything behind it (e.g. the diagonal, eyepiece or camera) instead of the mirror. No hysteresis exists in this design. You use the existing built-in focuser for the "rough" focus. Fine adjustments are then made with the NGFS without having to touch the OTA itself. With this very accurate focus adjustments can be made for visual or imaging use.
-Gary
