Most DC motor controllers control the motor speed by with a potentiometer you have to turn to reduce the voltage to the motor, which also reduces the torque, often to the point where the motor will not start turning the focuser knob. So you have to use both hands, one to operate the button, one to adjust the potentiometer to high speed to get the motor started, then reduce it back down so it doesn’t turn too fast. And of course the motor continues to coast after you've lifted your thumb off of the button, so by the time you've reacted to what you see in the eyepiece, it's too late.
nFOCUS operating a Celestron CPC 1100 Focuser
nFOCUS provides pulse-width-modulation speed control. Which means the length of the pulse of current to the motor can be adjusted shorter or longer, to produce from a 0.5° to 1.5° rotation of the focuser knob per step. And it’s always a high-torque 12 volt pulse that provides plenty of oomph to reliably start the focuser shaft turning. Step by step, degree by degree. And at the end of each step, nFOCUS electronically brakes the motor to a quick stop.
nFOCUS also lets you select the length of the pauses between pulses. With nFOCUS the focuser steps then pauses while you check the discrete changes in the focus before nFOCUS steps to the next focus position. The pauses also give you time to lift your thumb off of the button so you don’t miss perfect focus.
nFOCUS can also be interfaced to your computer using a usb-nFOCUS adapter, for remote focus operation.Controlling the nFOCUS from your PC is excellent for the budget minded astroimager.
Why usb-nSTEP and a Stepper motor for serious astroimaging?
DC motors cannot be commanded to the exact same focus position every time -- stepper motors can. The step size is a tiny 0.06° rotation of the focus knob per step down to 0.03° in half-step mode.
Serious astroimagers autofocus their CCD cameras by letting the PC take a series of short exposure images of a bright star at multiple, exact, repeatable, focus positions on either side of perfect focus. A star in each image will be out of focus by a different amount, as measured by the autofocus software (e.g., focusmax) which will calculate the exact focal position of perfect focus and command the usb-nSTEP controller to step the stepper to that exact perfect focus position.
usb-nSTEP has an an optional external temperature sensor to support temperature compensation of the focus position. We’re talking about focusing within 1/100ths of a mm ! As the night outside cools, the telescope contracts, and the focus can change by multiple 100ths of a mm.
With usb-nSTEP, once you have everything initialized with your autofocus software, you can execute a refocus in 30-40 seconds to adapt to filter changes, temperature changes or telescope flexure as you aim your telescope to different part of the sky. In 30-40 seconds your autofocus software will take a pre-programmed set of images to recalculate the perfect focus setting without your intervention.
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