Finding out your DEC clutch wasn't on tight enough!

[g__day]

Well after a long time scince I last did maintenance on my gear - I went through a routine check to confirm everything was nice and tight. For some reason I was particularly interested in how much - if any - play I could find in my mount's DEC axis. I tugged the end of the OTA gently back and forth along the DEC axis - to confirm there was no wiggle at all. On the fourth or fifth push / pull things moved about a quarter on an inch - I was stunned!

I double checked my DEC clutch and it turns out it could be a whole lot tighter. So it turned about an tenth of a turn more without applying excess force. I repeated trying to move the end of the OTA in DEC - now it was rock solid.

So what difference has that made to my set up I pondered. Well tonight I aligned carefully on 3 stars then used MaxPoint to do an 70 star alignment check. Last time I did this my all over sky pointing was (MaxPoint corrected) 45 arc seconds. Tonight pointing improved to 20 arc seconds in the East and all over sky around 30 arc seconds - which means accurate to +/- 15 arc seonds I just realised! Pretty sensational in my book for this price point!

I was blown away that pointing improved that much from correcting such a simple error. So on a permanently mounted rig - my DEC clutch wasn't on enough.

I am delighted with the improvement in pointing. First cut analysis shows that my polar alignment is 25 arc seconds low in Alt and about 3 arc minutes out in Az.

So after the first six or so stars where modelled in MaxPoint - every subsequent star fell only a smidgeon off the dead centre of target - it was beautiful to watch!

Next I'll be interested in how alignment varies from one day to the next - a check of my gear's power on and remembering its calibration settings. Then finally I'll try a longer Tpoint run to see what its analysis reveals.

I expect with improving DEC position - all other related calibration activities will also improve. It's a delight to find such an easy win.

So folk I guess my learning is triple check your gear for rigidity! I don't know if its just seasonal temperature variation or gremlins - but everytime I find a way to remove any flexure - I'm delighted!

Cheers all,

Matt

[leon]

Excellent Matt, I suppose you wonder why you did not do this earlier, however it has all work in your favor.

Leon

[g__day]

Leon,

It felt plenty tight to me - and pushing the end a few times didn't move the scopes much at all (say originally pushing the end flexed things maybe a mm or less). That's applying say enough force to lift say 3-4 kgs - I put a bit more force on it - say enough to lift 6-8 kgs and suddenly things shifted a few inches.

The clutches were both on - but I didn't realise the DEC could go on a whole lot farther on without causing too much stress on the gear. It was like there was a sticking point on the DEC clutch - where everything went on tight - but push just a tad harder and then suddenly the clutch turned easier and tightened a whole lot more with the same base tighening force.

About two weeks ago when I did a 80 star run - the pointing errors followed a kind of H pattern - with most of the error in DEC. I figured if its not DEC backlash its most likely the DEC clutch - which seemed plenty tight.

So as I said - now I'm smiling - just wish I found all the stuff I've learned a few years earlier! At least now I'm a stickler for checking everything is very firmly engaged.

Matt

PS

I'd be very interest to know what folk are achieving in terms of pointing accuracy for permanent rigs. I ponder if I set backlash compensation in the ASCOM driver settings - so I always approach a target from the same direction in the final degree or two of movement - whether I will see these settings improve significantly again...

[Omaroo]

Interesting findings Matt. One thing that constantly crosses my mind is why these mounts need to have their clutches (metaphorically) spanner-tight whereas a G-11's can be left loose and still function smoothly and reliable as long as the payload is in perfect balance.

Is is just that your payload isn't in perfect balance on the DEC axis and you need the clutches locked so tight as to carry any imbalance? Can the backlash be effectively tuned out in them - or decreased to inconsequential amounts?

[g__day]

Chris,

The balance is pretty good - not perfect - but I'd say there is some DEC backlash - not a huge amount. I haven't set any backlash settings on the hand controller - and the ASCOM driver isn't set to compensate for backlash (ensure it always arrives at a target from a consistent direction) - will try that next.

What sort of pointing do you guys see on your mounts out of interest?