Quote:
Originally Posted by alistairsam
Hi,
I didn't really get the math, anyone able to do a quick calc for an F4?
As per Dave's data, the DoF is 48 microns. what resolution would I need with a geared stepper if i'm coupling to the main focuser shaft, not the 10:1 reduction shaft?
I'm using a moonlite and the reduction knob can't lift the load.
can a bipolar stepper be wired to work with this?
Cheers
Alistair
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The load "not lifting" (slipping) sounds like the crayford pinion is slipping on the DT, needs tension/friction adjustment to fix this.
A ML pinion is ~ 4.75mm dia (well mine is) thus
C=πD (D=4.75mm) thus each time you turn you pinion, 1 revolution, the DT will move the circumference of the pinion = 14.922 mm.
So you want to have a motor that moves this in small steps…a lot of small steps (aka a stepper motor) in a fashion that is finer than your finger can do and automatically!
So pick a stepper that has a lot of steps per revolution…the original Dave used is 2040 steps per revolution. Couple this to the pinion to move it. We know 1 full revolution = 14.922 mm now divide this by the number of steps = 14.922/2040 = .00731mm per step movement of the DT or 7.3 microns per step.
Your given CFZ for an F4 ~ 48 microns (ccdcalc states this as ~35 microns, so they are reasonably in agreement) 48/7.3 = 6.57 steps in the CFZ.
This is not enough, you need a stepper motor with more steps per revolution especially with filters etc.
Some people use reduction gearboxes like the McLennan gearbox (have I got that name right?) to achieve this, or use the 10:1 pinion or maybe even purchase a Hurst LSG35 stepper motor with a very high number steps per revolution.
http://www.hurst-motors.com/lsg35geared.html
If I was you I’d get the best stepper I could as you need high step resolution (or use a gearbox), maybe the HurstLSG35012E98P, 14400 steps per revolution, 12V. This would give you
14.922/14400 = .00103 mm per step = 1 microns per step! Now we are talking! Thus CFZ 48/1 = 48 steps in CFZ for your system.
I cannot tell you how many steps in the CFZ you need but more is better and I suggest at least ~50 or more maybe 100 to give filter change step accuracy.
Lastly I think you’ll need a unipolar stepper…the Hurst and fleaby are mostly these types.
Anyone else want to chime in?