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RedCat 71 WIFD and ZWO EAF step size?

William Optics Redcat 71 ZWO EAF Focuser Auto-Focusing
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Bill McLaughlin avatar
Bill McLaughlin avatar
Does anyone know what the step size in microns  is for the RedCat 71 WIFD (not the old 71)  and ZWO EAF (latest version) combination is? With a normal focuser one can measure this with a dial gauge but with the internal focus mechanism this is not possible. I would like to be able to calculate how many steps are within the depth of focus. The DOF in microns is an easy calculation but w/o knowing how big each step is in microns that info is not helpful.

I just set this scope up as a short focal length alternative along with my CDK 14 for those nights when the seeing does not justify use of the CDK.

Thanks
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Byron Miller avatar
Byron Miller avatar
On the 51 i use a step size of 55 and a backlash out of 170 and its a great focus.  Could be pretty similar since its WIFD
Bill McLaughlin avatar
Bill McLaughlin avatar
Byron Miller:
On the 51 i use a step size of 55 and a backlash out of 170 and its a great focus.  Could be pretty similar since its WIFD


Not what I meant. Not the step size in NINA or such, rather the physical size of a single focuser "tic" or increment in microns or the amount of physical distance the focuser moves for each motor "tic" or increment.

To be honest, NINA and such should NOT use "step size" like they do since it is misleading. It should be called "number of steps per move" or "number of steps between measurements" or "number of focuser tics between samples" or something like that.

Step size (origin is from "stepper motor") is really the physical distance size of each "tic" of the focuser.
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Christian Bennich avatar
Christian Bennich avatar
Good question @Bill McLaughlin - I am building a small rig with the 51, and I will be needing the same in a month 😁
Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar
Bill,

You'll probably need to do a little trial and error. The EAF has a pretty course 4um step size, but as you've said, this doesnt translate to focuser movement especially with how it moves a lens style focuser. 

I'm not sure why you actually need this number, but in practicality couldn't you just run a capture sequence where you move the focuser only a fee steps in between captures. 

By doing a high resolution curve you could see exactly how many steps correspond with the CFZ.   Keep in mind that corners will feel the impact of field curvature and/or defocus much "sooner" than the center. 

Can you measure the thread pitch for the focuser?  I don't know how easy it is to take a look on that scope. These petzval like designs though can be problematic for lens collimation, so be careful messing with anything. 

Why exactly do you need to know this?
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andrea tasselli avatar
andrea tasselli avatar
"Step" in a stepper motor such as the EAF is an angular measure not a linear one.
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Byron Miller avatar
Byron Miller avatar
Bill McLaughlin:
Byron Miller:
On the 51 i use a step size of 55 and a backlash out of 170 and its a great focus.  Could be pretty similar since its WIFD


Not what I meant. Not the step size in NINA or such, rather the physical size of a single focuser "tic" or increment in microns or the amount of physical distance the focuser moves for each motor "tic" or increment.

To be honest, NINA and such should NOT use "step size" like they do since it is misleading. It should be called "number of steps per move" or "number of steps between measurements" or "number of focuser tics between samples" or something like that.

Step size (origin is from "stepper motor") is really the physical distance size of each "tic" of the focuser.

It's the same number whether you use the zwo driver, nina, sgp or voyager. As others have said, not sure what knowing the step of the stepper motor is unless you're writing your own driver. The number that matters is how the software implements the driver which is the number i gave.

The focuser will NEVER move by the amount of the TIC, thus the huge backlash to start and the figuring of the curve using hocus focus or whatever tool you have. (and the weight of your imaging train and position of scope)
Bill McLaughlin avatar
Bill McLaughlin avatar
Chris White- Overcast Observatory:
Bill,

You'll probably need to do a little trial and error. The EAF has a pretty course 4um step size, but as you've said, this doesnt translate to focuser movement especially with how it moves a lens style focuser. 

I'm not sure why you actually need this number, but in practicality couldn't you just run a capture sequence where you move the focuser only a fee steps in between captures. 

By doing a high resolution curve you could see exactly how many steps correspond with the CFZ.   Keep in mind that corners will feel the impact of field curvature and/or defocus much "sooner" than the center. 

Can you measure the thread pitch for the focuser?  I don't know how easy it is to take a look on that scope. These petzval like designs though can be problematic for lens collimation, so be careful messing with anything. 

Why exactly do you need to know this?

Thanks Chris. I think you are the only reply that understood where I was going with this. What I was looking for is the physical amount that each motor tic/step moves the focal plane. Maybe I should have been clearer.  With focusers that move visibly this can be easily measured (and is usually a published spec by by the focuser maker) but not with, as you say, this "lens style" focuser.

The purpose of this is to do what I do on all of my systems and that is to establish a good number for "focus when the temp changes by (amount) ".  Decades of imaging have shown me that this is the most reliable and efficient way to make sure you are pretty much always well focused (barring any other mechanical things - but that is a different issue). You can use change of FWHM/HFD but that  "closes the door after the horse is out"  so is not ideal (but can be used in addition as a failsafe).

One can calculate the CFZ for the system  that is not a problem, but that will be in microns.. The data on how much the focus shifts with temperature that one can get in a number of ways will be in focuser movement units (aka motor "tics/steps").  One needs to be able to convert that to microns to know how many microns focus on that optical system shifts with temp. Then one can finally set the "focus when the temp changes by (amount) " that you wish to use so that temp shift does not put you outside the CFZ.

I suspect I may have to do this empirically with this system, as you suggest, but it would be much easier and faster if I had that number.

In addition, this number can be useful (but not essential) when running aberration tip/tilt adjustments in NINA.
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Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar
Ok, thanks for the additional information. 

I think mapping focus shift through temperature changes will be the most accurate. The good news, is that there is not much thermal mass on such a small scope so this should be a pretty easy process. Unlike a larger frac where you have a significant delay for the initial ambient acclimation as the temperature drops rapidly for the first few hours of night. This complicates things. 

I'll bet you could setup a script to autofocus every 10 minutes throughout the night and plot the results with temperature shifts and have a pretty reliable model after just one night.
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