I kind of have a question and maybe a feature request if appropriate or maybe a little understanding instead with ref to a focusers backlash compensation.
It seems strange to me that you can only set backlash compensation to either ‘In’ or ‘Out’ but not both.
Recently I used a dial gauge to measure the backlash on my Moonlite with ref to working out step sizes etc. What dawned on me while doing this is that the measured backlash is the same in either direction, what I mean is, if my focuser tube is traveling outward and keeps going outwards the backlash is on the inward side and vice versa.
This got me thinking all because of the clouds LOLOL
When the tube it travelling outwards, If I were to suddenly change direction and go inward by 50 steps then my focuser has to first move the relevant amount of steps inwards taking up the backlash and then the tube will move the remaining amount, if your backlash step size was lets say 20 steps then the inward movement of the tube will only move 30 steps which is 20 steps out from what it thinks it’s at. Thereafter though, for every step inwards a move of 50 steps will move the tube 50 steps cos the backlash has been taken up.
Once again, if you then change direction to outward again and move 50 steps then the change in direction again needs to take up the 20 step backlash leaving you with a tube move of again…Only 30 steps.
I can see this kind of thing will have major effects on auto focusing. SGP when running it’s autofocusing routine begins by moving the tube outwards to the predetermined position based on half of data points * step size, if backlash comp was set to say 100 steps then it will move 100 further steps out and back again to take up the backlash for the inward direction, then SGP does it’s data points going inwards each time, past originally set focus point and ends inwards of the focus point and calculates best focus position. At this point SGP them moves the focuser out (change in direction) to send the tube to the position it calculated, but because it changed direction from inwards to outwards, if the tube was told to move out by 300 steps to hit that golden point then it will miss that point by 20 steps because 20 of the 300 steps were swallowed up by the backlash. Now 20 steps off might be fine for some but catastrophic for others based on what their CFZ is, it might end out of their CFZ.
If im talking complete rubbish then someone please correct me now !
So with the above in mind and no one makes me look like a clown, let me explain what I think should happen:
Backlash compensation options should contain a third option called ‘In & Out’ or ‘Both’, SGP will keep a note of which direction the tube is traveling every time there is a tube move. If the focuser changes direction and the move is 50 steps, SGP will in response to the direction change add the figure set in the backlash comp steps size box to the 50 steps move.
Backlash comp is set to 20 steps
If travel is inwards 50 steps at a time and the focuser tube changes direction then the signal sent to the motor will be 50 + 20 = 70 steps. This way, on direction change the 20 steps takes up the backlash and the 50 steps actually moves the tube by the required 50 steps, providing the next command is still outward by X amount then the backlash figure is not applied, again, if direction changes to inwards then SGP will again add the 20 steps compensation to the figure sent to the motor.
Surely a method like this gives a more ‘Absolute’ position to every move of the tube. Even it you cant pin your backlash step quantity perfectly, or there is a little bit of variation in the gears for example, you should at least get the tolerance to within a couple of steps from perfect, at least then every direction change move would only vary by a couple of steps from absolute position which has got to be better than 20 steps, When SGP does it’s final ‘Go to ‘X’ focus position’ it would also be only a couple of steps out instead of 20 steps.
So, again, unless I’m corrected I’d like to propose the following:
Add a new option to Focus backlash compensation dropdown box called ‘In & Out’ or ‘Both’
Have SGP monitor focuser tube direction and identify that a direction change is about to take place and to add the ‘step value’ in the compensation box to the figure sent to the focuser motor but only when the direction changes.
Well, im going to shut up now