Saw toothing with PHD2

[Tommohawk]

As I understand it, backlash in RA is easily sorted by ensuring east heavy. Backlash in Dec is more tricky cos need to drive in both directions. Isnt there some trick of deliberately misaligning polar alignement so that the dec only drives one way?

I've never tried this, cos I cant work out which way to misalign, but I'm sure someone will know!

Worth considering?

[swag72]

This shouldn't be backlash as the Avalon, being belt driven is backlash free.

[libraryman]

i just did a quick calc on your arc sec ratio and its fine, i assumed 9um pixels for the Loadstar and 5.4um for the QSI so 5.2 arcsec/ pixel for guiding and 3.1 arcsec pixel/ for imaging.

everything looks well mounted n tidy ish, i would run those cables along the scope and down through the mount centre line if i could, just to negate the possibility of drag.

nice set up

Ray

[Tommohawk]

This shouldn't be backlash as the Avalon, being belt driven is backlash free.

Ahhhh... well, not that then!

[libraryman]

Just out of interest, this is how i ran my cables along the scope side, then out and down the mount centre!!

just ideas for cables.

Ray

[Phillyo]

Once I'm permanently set up in my own little obsy then I will be making very similar tweaks. But having to set up and take down every time makes it a faff to do this right now. I would make a permanent set up now but I'm trying to sell my house and move somewhere else so it seems kinda pointless really

But thank you for the ideas, I like what you've done!!

[libraryman]

i set up every night its clear, i don't have an obsy but that's the reason to make it so much easier, i do it with an eq8....heavy man!! but it takes me min's because of the work iv'e done on the set up.

Ray

[Phillyo]

Additionally, I have been in fairly close correspondence with Luciano from Avalon and he has been sending my hints, tips and ideas. He and I are hopefully going to have a talk about it over skype one evening to try and get to the bottom of it, but more importantly he has offered to replace the mount free of charge if it turns out that the mount is faulty.

Couldn't ask for more than that!!

Phil

[libraryman]

wow good service.

Ray

[swag72]

You can't knock that for customer service

[ollypenrice]

I just think the mount is getting spurious commands in Dec. It doesn't look at all  like flexure or imbalance to me. You have a sudden and very violent lurch in Dec fairly systematically. In my view the guider will be driving this.

Firstly are you using a slowish guide speed, less than 0.5x siderreal? If you're talking to Luciano I suspect he'll already have covered this.

The big thing I'd try would be disabling correction in one of the two directions possible in Dec. Trial and error. Disabling one of the directions will result in a slow drift off course. You can't have that, so restore that direction and disable the other. That might well fix it.

Olly

[Phillyo]

Great minds think alike I was thinking of doing just that this evening Olly. I'm also going to try using an ST4 cable rather than pulse guiding to see if that's having an effect for some unknown reason.

Thanks for the suggestions, I'll post back with an update when I have one.

Phil

[ollypenrice]

I've never tried pulse guiding. I only ever use the ST4 cable. Certainly worth a try! These are very good mounts, really well engineered and not prone to anything sudden. I'm sure you'll crack it...

Olly

[strutsinaction]

Hi Phil

My setup is very similar to yours - Avalon Linear + Hercules tripod + Extension. I do use the Avalon 3kg c/w though and a small c/w on the front of the Losmandy plate in order to achieve balance.

Is your tripod on a solid surface or turf?

For PHD2, I have the RA and Dec minimum move distance set to 0.19 (PHD2 set these automatically when I first created the profile for the FSQ85+Lodestar X2). I have Hysteresis set to 5 and Aggression set to 90 but often tweak these settings to improve the trace.

My exposure is usually 3s.

In EQMOD the pulse guide speed is set to x0.1 for both RA and DEC, minimum pulse width 20ms and DEC backlash 0ms (note - when I start EQMOD this backlash setting is often shown as 2000ms in the UI so I normally move the scrollbar forward and backwards a bit to make the UI show 0ms).

Regards

John

[Phillyo]

Ok, it's not all that clear this evening but it's high thin clouds so should be ok for messing about with settings etc. Spent 20min or so doing Polar Alignment in PHD2 and this is where I'm at for now.

I don't think I'm going to get much better than that. So that should rule out PA as much as possible.

Going to try guiding for a bit with just the ST4 cable and see how that goes. Hopefully clouds wont ruin play too much.

Phil

[libraryman]

Pa looks fine phil,are you sure that you need to guide that tightly 0.03 min movement is very low!!

if my assumtions are correct for your arcsec pixel ratio, perhaps you should be around 0.15 for you min motion.

however i am more than happy to be corrected.

Ray

[Phillyo]

Hi Ray,

Probably not! I'm still messing around with the settings until I'm happy. I shall move it up to what you suggest and see how it goes

Phil

[Phillyo]

Well I thought I'd finally cracked it with using the ST4 cable. Guiding was looking peachy for 10min or so...

But then when I slewed to my target and began an imaging run...

DOH!

Back to the drawing board.

[frugal]

How far apart were the test target and the imaging target?

[msh1]

i always goto my target then calibrate phd, as soon as its calibrated i press stop ,recentre the object then start guiding wait for about 10 mins for it to settle down then start imaging run 

mark

[Phillyo]

I've just done both of these and left it to go to bed. I'll check the log in the morning and see how we get on.

Thanks guys.

[Carina Lass]

Balance.

Always have a small bias to the East.it keeps the weight on the gears rather than bouncing over to the opposite end of the backlash every time it overcomes 'stiction'

[Phillyo]

Nope it's not balance. Checked that already, plus avalon needs no bias east as it has no backlash to deal with.

Thanks,

Phil.

[strutsinaction]

Interesting. That last trace clearly shows that PHD2 did not issue any guiding corrections when DEC first throws a wobbly, then recovers a short while later.

Could this be a USB or cable problem? Are you using a USB hub? How long are your USB cables? Do you only have guiding issues when you are imaging?

[libraryman]

Phil, that does look different, actually, the wind has been quite high just lately, is it possible that this is wind moving the scope and PHD is pulling it back?

In my view, i still see Dec over correcting, whilst RA is better, i would say that you can still raise that min motion....3 secs is a better exposure time for that mount.

Ray