Alfred, I have done that, and it takes care of the focuser's backlash. But what I was talking about is the slight primary mirror tilt I get when the focuser moves it outward. I have found that my images suffer from very small tilt when the focuser ends it's movement outward. Fortunately EKOS ends it's movement INWARD, which is great, but it overshoots the ideal calculated spot forcing me to backup and move the focuser outward to the best focuser position. That adds the tilt. So I have to first go outward quite a ways, and then back inward to straighten the primary.
iOptron CEM120 EC2 and CEM25P
Celestron C11 EdgeHD
William Optics Star71
ASI 1600Mm Pro, ASI 462MC
Moonlight motorfocus & Esatto focuser
LodeStar X2 and ZWO OAG
Matteo and Bernd, Both of you lost me on your comments about pixel size. Camera pixel size doesn't enter the equation. Microns per motor step is the derived quantity we're trying to find, and that is found by running the focuser some predefined number of counts, and then measuring the drawtube travel. An alternative method would be to know the pitch of the focuser screw thread (a linear distance per revolution) and then factor the counts per revolution of the focus motor. Given that the average Joe observer is going to most easily move the focus motor by some number of counts (say 1000), and then measure the millimeters of focuser drawtube travel, the GUI could allow for those fields and then convert to microns per motor count. The rest is just the formula as given by the critical focus zone web link (which includes f-ratio, aperture size, a user preference for focus tolerance, and an estimate of seeing). See here:
with backlash activated, the focuser never ends its movement outward. It always moves further out and back in, just like what you do manually. This should prevent your primary mirror from tilting.
If you find focuser position 60212 to be the point of best focus during the first run, just hit the stop button, set 60212 manually (in the "desired absolute focus position" field), hit the "go to an absolute focus position" button and you're fine.
At least this is how it works here. I can hear the motor "overshoot" and "come back in" on out movements.
Doug, my English is propbably bad. I didn't say pixel size is necessarily part of the equation: given that you can find CFZ equations that take pixel size into consideration when sensors are involved, I only said that, if needed we know pixel size; the whole point was to say that CFZ should be calculated automatically by code, not entered by the user (maybe you already said that and I missed it). If that equation is better than another one, I can't say and I won't debate about it.
About microns per motor step, we agree. Again, I was only saying that user should be encouraged to measure the focuser travel over the highest possible number of steps, to minimize the inevitable measure error.
Hi Matteo, ok, I understand now why you suggested max steps to reduce the measurement error. We agree that more is better in this regard. I think a tool-tip on the entry field could be useful for explaining that. The fields might be better named "focuser motor steps" and "focus travel (mm)" or similar. Then the alg should calculate microns/step, compare to calculated CFZ (based on the other params), and set the proper step size. CS Doug
Hi Ron, I debugged my ASI EAF through the hokey-cokey to see how it coped with the outward then immediately afterward, inward movement. The code is event driven with a user defined "polling" period (say 500ms). So an outward motion is started and every 500ms the focuser is queried to see what's happening. If the focuser is still moving then the system waits; if the focuser has completed the motion then the system starts the inward motion.
I don't have a Celestron focuser so I can't do the same thing with that. The only things I can suggest are:
1. Make sure you're on the latest version of Indi and the drivers to ensure you have the latest bug fixes.
2. If the Celestron driver works like the ZWO EAF, then there will be a polling period in the Indi tab that you can set. You could try upping the value and see if that fixes your problem.
3. If the above don't work you could raise an issue and hopefully someone with knowledge of the Celestron driver would be able to take a look.
I reached out to Jeff Winter at GoldAstro to see if he would be willing to collaborate. He's currently busy on a project but said he'd get back to me, so I'll let you know how things go. I'm sure you've seen the calculator on the GoldAstro website: www.goldastro.com/goldfocus/focus_calculator.php
So as we started off discussing, as a way to ensure Step Size is set to a "sensible" value this could have some merit. Don't forget though that we should be able to do better than just "somewhere in the NCFZ" by curve fitting the datapoints.
So for the inexperienced user it should provide some assistance during setup but I'm not sure it provides anything to the experienced user who has spent time working out his/her setup and has arrived at values that suit their equipment. Or am I missing something? What would your workflow be if this feature was available?
Thanks John. Yes, there is also a polling period setting in the Celestron focuser and it is set at 500ms. However, as I listen to the focuser move it is clearly not pausing between the move out and move in. Looks like I need to submit a ticket to Jasem!
Mounts: Sky-Watcher EQ6-R Pro, Meade LX85, Celestron NexStar Evolution Alt/Az
OTAs: Celestron 8" Edge HD w/Celestron Focus Motor, Meade 80mm APO Triplet Refractor w/ZWO EAF
Cameras: ASI533MC Pro, ASI183MC Pro, ASI224MC, ASI120MC-S, ZWO ASI290MM
Raspberry Pi 4 with Stellarmate OS, MacBook Pro
CFZ - Critical Focus Zone (micrometers)
θFWHM - total seeing (arc seconds)
τ - focus tolerance as a percentage of total seeing (unitless)
A - telescope aperture (millimeters)
f - effective imaging system f/ratio (unitless)
0.00225 - constant (micrometers per arc second per millimeter)
Given a user estimate of seeing, and their acceptable tolerance (both of which could be defaulted), and capturing system info (or having user provide) for aperture & f/ratio, the CFZ pops out. Step size is then just a fit of CFZ on the translation of motor counts/revolution and focuser travel (mm->microns) as discussed previously.