Backlash is 50 steps…..i literally watched the micro feathertouch focuser on my SCT and saw it move after 50 steps
I am presuming that when the focuser actually turns tgen the mirror moves in an SCT?
Howard,
I have a FeatherTouch Microfocuser on my C11, and, whilst I have an additional reduction of 2.5:1 in the drive belts, I use a backlash compensation of 1500 steps! The logic is simple - whilst I might get away with a smaller value, the aim is to ensure that the mirror is moving against gravity to the position calculated by SharpCap’s focus routine, and I know from past experience that it’s better to move the mirror in a definite positive motion to the best focus position, and then give it at least several seconds to stabilise after it arrives, giving everything time to settle…
archer1960 wrote: Thu Jul 16, 2026 12:00 pm
Do you get better readings if you then move the focuser? Or is taht 7.5 actually the best you can get? On my system, I see the same thing using Voyager: the focus values I see in an actual image are never as good as what the autofocus run says it is giving me, but that is as good as they get. I'm not sure, but it might be that the AF run uses 2x binning, which will always have lower numbers than unbinned images.
No i get better fwhm number when i move the focuser after its settled at “best focus”
Backlash is 50 steps…..i literally watched the micro feathertouch focuser on my SCT and saw it move after 50 steps
I am presuming that when the focuser actually turns tgen the mirror moves in an SCT?
Howard,
I have a FeatherTouch Microfocuser on my C11, and, whilst I have an additional reduction of 2.5:1 in the drive belts, I use a backlash compensation of 1500 steps! The logic is simple - whilst I might get away with a smaller value, the aim is to ensure that the mirror is moving against gravity to the position calculated by SharpCap’s focus routine, and I know from past experience that it’s better to move the mirror in a definite positive motion to the best focus position, and then give it at least several seconds to stabilise after it arrives, giving everything time to settle…
Carl
Carl
The way my zwo Eaf focuser is setup is that a negative turn of the focuser in steps moves it counterclockwise which pushes the mirror up the tube…this is the way you are supposed to end focus so that mirror settles against gravity
I did a backlash test and saw the micro feathertouch focuser turn after 50 steps….there’s a rubber lid that comes with the zwo eaf that once removed allows you to see the actual focuser turn
But i do not know if the focuser turning actually moves the mirror
There could be backlash in how the focuser actually acts on the mirror
I will try with a bigger backlash setting in sharpcap and see if that fixes it
What kind of best fwhm figure do you guys get in average seeing ?
I get at best around 4.7 at focal length 1645mm and camera pixel size of 3.76
The way my zwo Eaf focuser is setup is that a negative turn of the focuser in steps moves it counterclockwise which pushes the mirror up the tube…this is the way you are supposed to end focus so that mirror settles against gravity
I did a backlash test and saw the micro feathertouch focuser turn after 50 steps….there’s a rubber lid that comes with the zwo eaf that once removed allows you to see the actual focuser turn
But i do not know if the focuser turning actually moves the mirror
There could be backlash in how the focuser actually acts on the mirror
I will try with a bigger backlash setting in sharpcap and see if that fixes it
What kind of best fwhm figure do you guys get in average seeing ?
I get at best around 4.7 at focal length 1645mm and camera pixel size of 3.76
Your FWHM is 2.2 arcsec on the image. Now you need to compare/estimate with your average seeing condition.
If you are in the Atacama desert or similar with 0.5" seeing ... then you have a "poor" image
If your location allows only 2 to 3 arcsec seeing ... then you are good.
just to explain perhaps what was going on... The backlash that you need is the backlash of the whole system from the motor to the mirror - your initial low value was (I think) just the backlash from the motor to move the focus knob visibly in one direction or another. However, with an SCT, you tend to have to turn the focus knob quite some distance to take up the backlash in the mirror movement mechanism (more than half a turn of the knob on mine). That's the backlash that you need to measure and use , with a procedure like this...
* Remove any existing backlash corrections in SharpCap or the ASCOM focuser driver
* Move the focuser +ve until you have seen the image focus shift and continue to shift for each small extra move
* note down the first focuser position
* Move the focuser -ve and keep moving it until you see the on-screen image start to change in focus in the other direction
* note down the second focuser position
Calculate the backlash as position 1 minus position 2, then add 10 to 20% for good measure and that is your backlash correction amount for 'overshoot' focus handling.
There is backlash in the eaf motor and the focuser knob AND in the focuser knob and the actual SCT mirror
I set backlash to 500 and all seems to be good with focus assist now
Measuring the backlash with the method you detailed would be tricky as it is hard to see focus movement on screen with small movements
