Guiding - backlash in RA

[Guest]

When DEC is adjusted by a guiding pulse I can see that the pulse might be to increase or decrease the DEC and backlash comes into effect. But what happens with RA? Since the RA of the mount is normally tracking forward all the time I wondered what happens when a reverse pulse is given by the guiding software. Does the RA actually reverse or does it just kind of skip a few normal forward driving pulses? And does that mean that backlash in RA doesn't really matter too much? Obviously when the mount is slewing to a new RA then backlash matters depending on which direction it's coming from. But if you are already on target and tracking forward at the sidereal rate then does RA backlash matter?

I thought of this because it seemed that it might be better to allow some backlash in RA in order to avoid binding.

Cheers

Steve

 

[newbie alert]

I'd assume on your calibration the backlash is measured, calculated and then used when you're actually guiding..

[ollypenrice]

I think it just stops driving for corrections in that direction, but I'm not certain. Backlash is never a good thing but, arguably, it cannot be fully eliminated from a worm and wheel since a few microns of grease will always be needed between the gears.  The more persistent problem is eccentricity, meaning the clearance in the mesh will be inconsistent during a cycle. A fairly common solution involves spring-loading the worm side into mesh with the wheel, allowing eccentricity to be compensated for.  Without this there will be a tight spot in the cycle which will experience binding before the rest, so the best you can do is get that part to have as little backlash as possible without binding. This will mean there's a potential for backlash at other points in the cycle. So find the tight spot, adjust the mesh there and that's about it.

The standard way to minimize the effects of RA backlash is to run the mount east-heavy so that it tends to rest against the driven side. The motor is always pushing against the wheel. In perfect balance it will tend to oscillate across the backlash.  (In Dec you can try running the guide corrections in just one direction after trial and error to see which direction should be disabled. Again, this lets the payload settle against one side of mesh.)

Olly

[Guest]

Yes. For DEC guiding backlash is important and you want to minimize it as far as possible. This is because DEC is normally stopped while guiding but guide pulses will step it forward or reverse so it's changing direction.

As far as I understand backlash in RA is not important - in guiding anyway.  The reason is that RA is always tracking forward. I had some answers on another forum.   There is a guiding speed parameter which might be set to say 0.5 which I think is the default in PHD2. I didn't know what the 0.5 meant but it's the adjustment it makes to the RA rate. The normal sidereal rate is 1.0. When PHD2 wants to make a forward adjustment the speed goes to 1.5 sidereal for a short time. When it wants to make a backward adjustment the speed goes to 0.5 sidereal for a short time. This means that the RA is always going forward just sometimes faster or slower. So backlash doesn't have much effect if any.