Shoestring FCUSB and SGP

[NeilW]

Hi all,

I have a Skywatcher focus motor attached to a shoestring FCUSB unit. This works fine with BackyardEOS, but I'd really like to get Sequence Generator Pro to work with it. SGP only recognises "absolute" focusers, not "relative", but the latest FCUSB Combined driver has the capacity to "fool" software into thinking it's looking at an absolute focuser.

SGP will connect with the FCUSB, but no matter what I try it will only drive the motor in one direction, regardless of which direction arrow I click on. I've tried sending it to a particular position in relation to the starting position, (eg. if you tell it that your starting position is 5000 and you tell it to go to 6000) but to no avail.

Any suggestions (apart from buying a decent stepper motor!)?

Regards,

[RobF (Rob)]

Hi Neil

Having used a relative focuser (Skywatcher motor on crayford focuser) on my Newt and an absolute stepper driven rack and pinion on a refractor, I'd be concerned if SGP could do this reliably for you.

Not sure if you've seen SGP in operation, but effectively:

1. Do a rough manual focus
2. Run auto-focus
3. SGP moves focus n/2 defined focuser steps away from focus point
4. Then iteratively moves focuser back through focus point 1 until n/2 x steps out the other side
5. Plots slope of calculated star HFRs either side of focus, then commands focuser to move back to the calculated central point


Bottom line - you need reliable movement, with negligible backlash.
My old Newt rig wouldn't have done this reliably, particularly if pulling a camera up against gravity at some angles......

Hope all that guff makes sense

Presumably you're already getting some sort of reliability from Backyard EOS though.

[NeilW]

Thanks Rob,

I haven't had this out under a night sky yet....just experimenting indoors. Having said the above, I now seem to have managed bi-directional travel by giving SGP a starting point, and telling it to "go to" plus or minus that figure. Not sure why that didn't work before.

I have this on the original SW R&P focuser that came with this ED80 Pro (gold scope). I have no idea as to its precision though. Can you suggest an alternative focuser (Feathertouch is out of my price range)? I'd consider adding something like a Sharpsky Pro focus motor if the cheap SW motor isn't up to it.

[RobF (Rob)]

I've never upgraded a focuser, but I'm sure there will be gurus here than can help. If you have it accepting commands suggest try to follow the SGP instructions for auto-focus and see if you can get parameters that work.

If you're not desperate to have automatic unsupervised operation probably no reason you couldn't use the "calculate HFR" option to focus manually to very good precision. Would be surprised if your critical focus zone on the ED80 is overly tiny, which helps a lot.