ZWO EAF - small moves fail when pointing straight up about once per night?

[g__day]

This is a strange one - my ZWO EFF fails on small filter change positioning moves - normally when the scope is pointing straight up (so maximum weight on imaging train) - but only about once per night.

A remove the USB cable from the motor and immediately reinsert it fixes the problem immediately. When the issue occurs I was present luckily and I could hear the motor whirring and whirring - and I know these motors have a lot of torque.

The unit connects to a Williams Optics 132mm FLT and it has a extension tube, ZWO filter wheel and ZWO 1600MM-m attached to it - so not what I would call excessive weight.

Wondering from the symptoms what is going on? It happened around midnight last night when I shifted from M83 to M16 as an imaging target and was about to refocus. I stopped the imaging run on APT and the filter went from Green back to Luminance - triggering about a 43 step move that sent the motor racing?

Any ideas folks - I am unsure what is happening here - noting that a simple USB remove and reinsert addressed the issue. The firmware on the EFF is about 6 - 8 months old - last two flashes failed - may need to do the firmware flash from a higher powered USB hub.

Wondering what causes 1 fail per night - hours and hours into an imaging run that causes the motor to rev but not move and a connection cable reset to immediately resolve?

[The_bluester]

I have had one of the 12V ZWO focus motors for a coup[le of years and it has just worked night after night. The only issues I have ever had with it were related to the rubbish power cables you can get in most places (The ones with a solid inner contact, not a spring loaded one)

It will happily make moves of only a couple of steps even with the scope pointed straight up.

[g__day]

My verion is USB powered so no external 12V power supply. The USB cable plug into a powered, combo USB2 / 3 hub. Thinking to swapping the USB cable to another brand - to see if that has any change to its at time strange behaviour!

[ronson]

Try bypassing the USB hub and use a different power source.

[oska]

I'd be really concerned about the failed firmware update. That can leave the unit in an unknown state and then all bets are off. Reinserting USB will cause a reboot of the onboard MCU so it's not necessarily an indicator of a dicky cable/connection.

Replacing the cable should be part of your normal debugging procedures. It still could be stiction in the mechanism from say old grease or an over tight tensioner.

[g__day]

Me too about the firmware update fail - let the device in a unusable state until a power reset (twice)! Luckily it failed at clear EPROM stage and not later - so no updates occurred - but a heart stopping moment or two gear wise!

Last night it ran for 8 hours straight with zero issues - we small focus shifts every 6 minutes - that is how intermittent it is!

[g__day]

Happened again last night - same position - and its been about 3 days since the last occurence. About 6 hours into an 8 hour imaing run. The telescope pointed straight up, focuser needs to do a tiny move - as it has every 5 minutes for the last few hours and then motor is continually whirring - remove USB and reinsert and problem goes away for the rest of the night.

Still no comment from ZWO on this one. It's pretty much my last remaining software / mechnical factor to get sorted...

[The_bluester]

Sam position as in same step count? I am assuming it is an R&P focuser? I did have one get a bit unreliable at one point which turned out to be a tiny bit of grit which had found it's way into the rack. I ran it out and brushed the grit away and it was back to reliable operation.

The other possibility would obviously be a mechanical one in the focus motor itself, could you release the shaft, move the motor position by some amount of steps ivia the software and see what happens? If you move the motor by 200 steps inward for the same rack position and the problem then occurs 200 reported steps "out" from where it did before then the focus rack might be sticking, likewise if it happens again at the same step count on the motor, then the motor would likely be to blame.

[g__day]

Something blocking the track would be the obivous answer (not sure how this would occur though as its all enclosed) but the fact that taking the USB out and putting it immediately back in and then it can do any move leaves me stumped - as a blockage issue would never resolve this way.

Be interesting to see what ZWO come back with when they eventually respond!

[Drac0]

Hi Matthew,

Can't help with your current issue but I am curious as to why you are refocussing so often? Is it that you're cycling through your filters rather than doing them in batches? Just curious. :)

Cheers,
Mark

[g__day]

Hi Mark,

My light frames on the Mono camera often go 5 - 10 loops of

Luminance 2 mins
Luminance 3 mins
Red 2 mins
Red 3 mins
Green 2 mins
Green 3 mins
Blue 2 mins
Blue 3 mins

So every 5 minutes there is a filter change which triggers a small focuser offset change. This way collect my LRGB data at the same time - and any issue or collateral benefit in the seeing is seen by all. So a risk minisation approach - even though instead of changing filters and focus position only 4 times in a plan (the top down approach) I use the cycle approach meaning I change filters 20 - 40 times in a plan!

Every hour or every 1 to 2 degree temperature drop I tend to halt the plan and manually trigger an automated refocus. I haven't got the automated trigger of a automated refocus to work in either SGP or APT yet!

[JFWall54]

I had the same issue a few nights ago. I was able to determine that when the scope was pointing straight up, the focuser on the scope was actually slipping when the EAF tried to pull the weight of my camera and EFW up. All I needed to do was increase the friction on the scopes focuser and "no more slip".

I figured that the scope was getting rather cold and that caused the metal to contract slightly... hence the focuser slipping.

I hope it's that easy for you.

[g__day]

Hi John, that sounds rather plausible! I just have to figure out now what sets the friction on the scopes focuser as you say as its a rack and pinion set up with a funny spiral (metal cylinder - that has a groove cut into it like a spring.

I will research how the focuser it meant to be installed for Williams Optics scope and see if the friction is just a tad off as you say!

[g__day]

So watching how to fit this to a smaller ZWO scope on youtube - I don't see any friction adjustment on the focuser in two videos - so I will research more.

What I did notice was advice on this video not to mount the motor hard up against the coupler but to give it half a millimeter of play to allow for expansion and contraction - that will be something I check too.

I wonder if it can be stiction? When I heard the motor whirring I didn't shine a light on the coupler to see if it was in fact turning (I strongly suspect not) nor try to help it move one way or the other.

https://www.youtube.com/watch?v=eqAiycIG7rs

[g__day]

I am wonder if the coupler is installed set too close to the telescope and not close enough to the motor. Does anyone know for this specific scope - a Williams Optics 132mm FLT?

I am suspecting if the coupler is too close to the OTA bracket (and it was likely installed during the day by the original owner - then when it gets very cold the shrinkate of the parts might be causing it to become stuck on the OTA bracket itself!

The instruction manual doesn't specify the recommended play needed between coupler on either the motor side or the scope side as far as I can tell!

I am thinking to maybe move the motor and coupler say 0.5 - 1 mm further away from the OTA (by loosening coupler to OTX two hex screws - then loosening the mounting brackets two screws and simply sliding the motor and coupler out a fraction and locking it all together again).

[AstroViking]

Hi Matthew,

From the images, it looks to me like the flexible coupling is a very tight fit inside that black 'box' part of the focuser. How much clearance do you have in there and is it possible to get more?

Regards,
V.

[oska]

I think you're right V, there's a shiny spot on the coupler like it's been rubbing. That would definitely cause you grief.

Mathew, if you can mount the coupler on a rod (to keep it rigid) in a drill (preferably a pedestal) and whittle it down with a file or sand paper. I've done a couple of mine in the lathe for this reason.

[The_bluester]

Come to say that, on my original small frac (Evostar 72) I had to turn the coupler down as it was fouling in the focuser housing.

If you are going to do that they are a quite flexible beast, you really need to find a suitable shaft to put through it, lock one end with the grub screw, twist it to close up the gaps and lock the other end or they can really dance around and be hard to do a neat job on.

[g__day]

Another Williams Optics FLT 132mm owner showed me a closer up of his coupler in the focus well - it is inserted the same distance as mine - but it has about a 2mm gap between the coupler and the focuser bracket.

Mine has about a 0.2mm gap if that!

His focuser bracket is round mine is square! I would guess his focuser bracket's well is about 23mm diameter and mine is likely 19.5 to maybe 20mm!

An image of his attached. So I am wondering did Williams Optics change the dimensions of the inner well in their focuser bracket in the last 10 years - and is my older scope just a tad too tight!

My gear has the coupler well centred but 0.2mm of gap (guess) is rather too snug. I added lithium grease as a help - but I may have to remove my coupler and get a professional machinist with a lathe to shim it down 1-2 mm!

[oska]

It's only just rubbing by the look of it. You could remove it and take to the shiny spot (plus a bit) with a file, it wont be pretty but it should work? What size is the focuser shaft?

[g__day]

The coupler is 19mm long - the bracket I would guess is 20mm diameter max!

I am thinking if lithium grease doesn't solve it then take it out and take 1mm off where it is would inside the bracket.

I ran in a few times today in and out 20,000 steps with lithium grease - couldn't hear any rubbing - so I think it might just be a combination of cold weather and differential flexure at the wrong angle is exposing it is just a tad too tight.

Though not sure why removing and reinserting the focusers USB cable clears the problem for a few days. If it were sticking I wouldn't think stopping and restarting the motor would do anything...

[The_bluester]

Going back a few points to where the tension setting on the focuser was mentioned. Do you have any focus lock tension applied via whatever method the WO focuser uses? If so, release that, they are meant to keep focus from moving when you are focusing manually, the focus motor itself will keep it from moving and with a rack and pinion focuser you don't have "Slip" to worry about as you do with a Crayford style. The focus motor will apply a fair bit of resistance when powered off and assuming it is a stepper motor, a stepper actually has a holding torque of it's own when it is powered on but not being stepped.

Edited to add, I had a look at the ZWO blurb and they are a stepper motor, so unless the firmware takes power off the motor when it is not being stepped, they will add holding torque when powered on. I know my old 12V one is quite resistant to moving when when not powered. I had a look at that as the Celestron focus motor I had for my old SCT was a DC motor with an encoder for positioning rather than a stepper.

[g__day]

Hi Paul,


Hard to find technical specs on the Willaims Optics FLT 132mm focus tension adjustments for its 3.5" diameter focus tube. I think they are present (via 6 hex lock bolts) and I think they are adjusted well - as there is no slop I can see in the rack and pinion focuser. And for 99.5% of moves everything is silky smooth and no noise and no issues - it comes down to a midnight at vertical at below 10 degrees Celcius occassional issue to resolve!

I am thinking - take out the coupler - place it on a snug drill - maybe a 4.5 - 5mm one - and then bolt this close to the end of the drill that isn't threaded. Put this drill and attached coupler into my drop drill and then switch it on at low speed and use a metal file to shave 1mm off the diameter of the last 5mm of the coupler. Finally polish this with very fine sandpaper - then reinsert this back into my scope.

Does that sound like a workable plan folks?

[oska]

As long as the drill bit (or whatever) goes all the way through so the coupler cant 'wiggle' and you don't go crazy fast you'll be fine.

[The_bluester]

What I was meaning was any focus position lock screws, rather than tension adjustment on the drawtube or rack, most have some arrangment to lock the drawtube in a fixed positon but when you are using a focus motor you want that backed off completely (I removed it from my focuser)

Regards using a drill, anything like that should work, but it will squirm and dance around a bit due to the way the coupler is designed, it is doable but unless you spend ages on it doing it very gently it probably won't look very pretty. It is hard to keep whatever tool you are using from chattering.

[g__day]

The 4 Lock nuts on the coupler might be sufficient to align the couple to a central axis that is rigid sufficiently.

I found a CBC milling place 1km from where I live - so if need arises this is likely a trival ask for them with their gear!