Synscan + EQ5 backlash matters..

[GeoffW1 (Geoff)]

Hi,

I sent the following to Skywatcher Tech Support. Comment would also be most welcome from you Synscan gurus out there.

"
Backlash in EQ5 with Synscan upgrade......


I have the above kit, and quite like it, with a few minor problems. It carries a Skywatcher ED100.

The Synscan unit is V3.25, and offers backlash compensation entry. However I am puzzled as to how to reliably assess the total backlash in the drive train of an axis. I have done a lot of searching on the Net but advice tends to be non-specific and even contradictory.

I found a maintenance resource, disassembled the Dec axis, and found the worm shaft housing screws were loose. Having fixed that I tried to reduce the backlash overall and found that it could be composed of:

- motor hysteresis
- motor pinion to reduction gear backlash
- reduction gear to axis worm drive gear backlash
- axis worm drive gear to worm ring gear backlash
- worm ring gear to support shaft clearance (this was a size-for-size fit, so good there..)
- locking lever tip slip

I was not surprised to find I could not eliminate all backlash (as assessed optically). My questions are, because the Synscan user manual gives no clue:

- what is a good procedure to derive a value for entering into the Synscan setup?

- when and how does this value take effect ?
ie is it a sort of preload, applied by firmware?

Thanks"

[pjphilli (Peter)]

Hi Geoff

When I first got my EQ5 I also fiddled around with the DEC backlash - adjusting the gears etc and then finding by observation the minimum backlash setting in Synscan to just eliminate the problem (which incidentally was much less than the default value of 10arcmin of Synscan manual). I did this with the intention that it would improve my DEC guiding. However, I discovered that when used in conjuction with my PHD guiding software that the auto backlash seemed to upset PHD. So I have now switched it off and PHD guiding works very well. Incidentally I set the polar alignment so that there is always a slow drift to the North so that the guiding does not have to contend with the backlash region. Also I have the Dec Algorithm in PHD set at Resist Switching as I found that the Lowpass Filter mode caused occasional large excursions in the PHD guiding.
Cheers Peter