squeaky

Maybe you could try doing a reset to factory defaults from the handset menu.

Then look up "re-setting encoders" in your handbook and see how it goes.

Got a phone call from my supplier.

Remember that I ordered replacement base boards back in August and was told three to four months?

So that would be Nov or Dec, yes?

Delays now mean January!!

"Hardly been used" is true of every telescope in the country this year!

That's very odd.

Have you tried resetting to factory defaults?

After that, I'd say it's time to have a word with your retailer.

The clutch isn't likely to be the problem because the encoder is on the shaft - so it only records a change when the shaft moves. If the clutch slips - the shaft doesn't move and so the encoder doesn't change.

That said - it may be that you have a problem with the encoder itself. Or the connection between that and the handset. Possibly the little board with the IR sensor on it, or even the little motherboard in the motor/encoder enclosure (or one of the connectors to it). We're dealing with low voltage DC here which is highly susceptible to even the smallest amount of corrosion, dirt or moisture. Sometimes just unplugging each connector and remaking it can clear faults.

I suspect that this problem is going to be difficult to track down.

I've been sitting here trying to think of a mechanical reason why you should miss by so much, especially an intermittent one; and am coming up blank.

For such an error I'd have thought that one or more of the following would be the culprit:-

• Incorrectly identified first alignment star. Yeah, I know.... granny, eggs, suck... but I've known an experienced astronomer convinced her was on Arcturus actually be on Altair. Convinced me of it too and my goto was miles off. It wasn't until I checked myself against Stellarium that I realised what the problem was.
  

• Wrong date. Inadvertently skipping past putting in the right date, OR... forgetting that the date is input using the American system which is MONTH first, then the day and then the year. This can put you out by a long way.
  

• Wrong lat/long. Most sites give you your location in decimal - as in 52.5 deg. The input needs to be in deg and minutes... so this would be 52deg and 30 arc mins. An error that can put you out by different amounts in different parts of the sky and so "appear" intermittent.
  

Only when I've checked all three of the above do I then start looking for other possible reasons for errors. I sometimes amaze myself at how I can see what I want to see instead of what is there. Finagle's 3rd Law applies here

"In any collection of data, the figure most obviously correct, beyond all need of checking; is the mistake."

Catches me every time

Sorry, but apart from the above I can't think of any reason for your problem.

Yep. The feet I have start at a minimum of 10cm (4") and max at 16cm (6").

Doesn't bother me I have a "step chair" wot I made. And it's of no consequence when I'm imaging because I'm watching my laptop.

There are plenty of other types of adjustable feet available that may suit you better - these work for me because they split so that I can move on castors then stand above the castors on my feet. They're push fit - so not exactly a pain to set up.

Those kitchen cabinet feet work a treat. Honest

Anywhere.

Everywhere.

Another test tonight - but so far so good. I have a usable scope once again.

A few days ago I had some test time and discovered that I could pretty much eliminate drift in ALT by moving my balance weights to slightly the "wrong" position. I.e. The subs were drifting down and right. In essence - if the image is drifting down then the front of the scope is "apparently" going up. So moving my balance weight towards the front effectively pulls the front end back down again. This probably only works because I have my friction set quite low given that I'm about a quarter of a turn past "can just pick up the weight" of OTA plus camera at all angles.

I'd started to test adding in some backlash to see if I could offset for the rightward drift but increasing haze put an end to things.

I've also yet to determine if this will work on targets in all parts of the sky and how different the weight or position might have to be.

Plus I think I need to re-register the encoder again using different "mid sky" stars.

New rubber wheel castors should make rolling the base from the shed to my viewing position a much smoother ride so less chance of disturbing my collimation and, if I get time today I'll be making and fitting a 1W dew heater for the secondary since that suffers most as I cross the dew point every night. But first I want to smooth off the filler in old holes in the lower base where my original castors and adjustable feet were and get a coat of paint over it. I'd knocked up "proof of concept" adjustable feet until I acquired some kitchen cabinet plinth feet (dirt cheap if you want to google them) which are much easier to use. The mounting bases are fixed in position and have a lower profile than the castor wheels - then the actual "feet" are a push fit into the bases and lift the beast clear of the castors. It's been a fair bit of work but pays off now because I can move and level the thing in two minutes flat without pernackerating myself

For sure

I've used a router to level out all the swollen bumps in the base and done a test indoors which looks OK.

Surprise surprise I CAN SEE STARS!

So I'll be off out to set up and test alignment and targeting. The sky is nowhere near good enough for imaging - but as long as I can see stars I can at least check things.

One thing I noticed in all of this, by the way, was that I could move my OTA by hand in AZ about 2-3 degrees before I felt it "bite". In other words - if I moved it clockwise a fair bit, then reversed direction - that's when I noticed the slack.

And it wasn't the backlash on the gears.

When stripped right down with the upper base plate removed and the spring cup washer taken off you can see that the upper clutch plate fits over a flat on the shaft. There's a gap between the two flats which is what allows this free rotation.

It's hard to see in this image because I don't have macro available on this camera, but it looks as though there are two square punch marks which are meant to squeeze the gap closed at each end of the clutch plate's flat:-

I tried reassembling with some shim material in there but it didn't work. In the end I got my own trusty punch out and redid the punching. That slack has now gone

I'm not going to be able to do any testing soon either!

Just heard back from my retailer. OVL say that the parts I need - primarily the top base board, (but for safety both the top and bottom boards) will take months to supply. Possibly three or even four.

MONTHS !!

It's a Bank Holiday weekend so the next stage of talks will begin on Tuesday.

It sounds like you ran into backlash.

First thing to try next time would be just to hit ENTER a couple of time to re-acquire. That may be all that you need to do.

I'm sorry I can't help you out with more definite information here - but I just haven't had the chance to experiment.

Apologies if this has been mentioned or I'm stating the obvious but am I correct in assuming that whenever the weight balance is changed on the OTA, i.e a camera attached or counter weight removed whilst tracking is activated that in order to avoid considerable back lash issues the tracking has to be re-set?

I'm saying this because my scope needs a counter weight for tracking in the east but not the west so one night whilst imaging an object that had moved into the west i removed the counter weight and experienced massive back lash issues in alt.

I haven't got that far yet!

I know for sure that since the pivot point axis is fixed on my dob - changes of equipment mean a change of weight so a change of balance weight is needed.

I ALSO know for sure that a change of viewing ALT angle can mean needing a different balance weight, and/or that balance weight to be moved up or down the tube as I described in post #32 (I could put together some diagrams if you can't visualise it?)

I've got some alignment and tracking issues from poor clutch friction caused by my mount warping - so until I get that sorted out I cannot say for sure - but an early test showed that moving or changing the balance weight after acquiring a target star meant that my target "moved" in my FOV - but also that simply pressing ENTER a couple of times would re-acquire the target properly. That's as far as I've been able to go to date.

After I've got my mount fixed I'll be starting all of the above from scratch again and then will be going on to test responses and accuracy in different areas of the sky and what I have to do (or not do) to keep the tracking stable.

I'd guess its because low stars give you the greatest arc length to traverse so there are more steps on the encoder between the two alignment points.

As for replacement parts... no answer yet....

Oh... and THEN...

There's backlash and encoder registration to look at, plus the value of using PAE...

..and I'm also wondering what value there would be in doing a two star alignment and THEN switching on the Auto Guide function...

Indeed it does!

It means that you have to adjust the amount of balance weight you use (and/or its position) depending on whether or not you have finderscope and/or camera and/or light shroud and/or dew shield....

AND....

That this weight, and/or its position will change depending on what ALT angle your OTA is at.

Seemples

This probably only really applies if you've got the friction pressure set low enough to only just allow the drive to pick the tube up (or down) without slipping or overshooting. I.e. I could set like that for the finder and camera - plus a little bit extra for safety... but if I then fitted a light shroud and a dew shield the extra weight "might" be more than my safety allowance could cope with and the tube would move down on its own, or the drive would slip a little when trying to lift the tube from horizontal. In practice, after setting up in my kitchen to minimums plus a small safety factor - out in the real world I ended up adding a full quarter turn of extra friction. I guess I'm trying to say that it will save a lot of messing about if you balance for your full set up and always set up that way., and that minimums plus a quarter turn will get you close to "optimum". WITH balance weights in the right position. If you don't want to mess about with balance weights but just keep the OTA stable and responsive then you'd have to increase the friction by some further amount.

As I said in an earlier post, "the book" doesn't give any actual values. And as for balance - not a word.

Once I get my mount sorted I'll be doing all this again and I WILL check how much effort it takes to move the OTA against my friction setting by using a spring balance as a strain gauge. ( I'll record my results for up, down, left, right so that I can check every now and then and, especially if my tracking seems to be off - reset to those values if they show a change.)

Even so, after doing all that it is still likely to need a tweak to get the tracking spot on.

The aim is to get to the point where I can consistently get 30 sec subs and to keep 80% of them. Erm... over a reasonable amount of sky. Because that's the other thing - the sky "moves faster" at the celestial equator than it does at the pole so the tracking has to work harder.

Oh forgot to say... the balance point is a geometry problem

The bearing points on the OTA are probably not at the centre of gravity. Or perhaps they ARE at the C.o.G. for a bare OTA which isn't the same as one with a finderscope and optics fitted, or a camera, and would also change if your primary was collimated but as far back down the tube as it will go, or as far up the tube as it will go. A twelve inch mirror plus its mounting is not exactly lightweight.

Since the mounting pivot points are fixed - any change in weight anywhere is going to change the C.o.G.

I "think"

The odds are that the bearing centre point is either above or below the actual C.o.G of a bare OTA when horizontal, and, for a bet - it's below

So imagine your OTA in front of you and horizontal with the secondary and finderscope fixing point to the left hand side, and the primary is to the right.

So picture two dots near the middle, one above the other. The top one is CoG and the bottom one is PPC. (Pivot Point Centre) If you fit a finder the CoG (top one) is going to move left - so the scope will be nose heavy.

As you raise the tube the CoG will rotate around the PPC...

At some point it will be directly above PPC again and so the tube will be balanced.

Any further raising of the tube will rotate the CoG to the right of the PPC - so now it will be tail heavy.

I "think"

I took the AZ apart so that I could increase the friction only to find that the base has warped and there's absolutely no point in trying an adjustment. Only found out at 17:36 and the shop is shut. <sigh>

BTW - I don't have the OTA fitted so there's not a lot of momentum to swing the mount past it's "stop point" as I release the button when doing speed #2. So I don't think that the delay on speed #1 is it trying to catch up with an over-run.

To make sure I changed the speed to #3 moved right, changed to speed #2, and it moved straight away. So no over-run on speed #3 that means that speed #2 has to play catch up before moving the mount. So I'm fairly sure that the lag I'm seeing when I change from speed #2 to speed #1 is to do with friction.

Sure

YEP!!

Going right at speed #2 once the backlash is taken up - if I then press the right arrow it moves straight away. If I release the arrow button, change to speed #1 - there's a definite lag of about a second before it moves.

So when "tracking" I assume that the Synscan sends little increments to the assembly at low speeds which are falling inside this "lag".

Right, off to strip the AZ down again and increase the friction on those cup springs...

FINALLY got a pdf from OVL (via my retailer) thank you both

It only covers the ALT side of things and there isn't, apparently, a similar one for AZ.

The only thing I didn't get quite right is that tightening or loosening should be done on BOTH sides of the axis shaft. It recommends about a quarter of a turn per adjustment and then to test and repeat if necessary until the proper adjustment is reached.

It doesn't say what the proper adjustment is - but what I've gone for is that with my camera attached the OTA stays put when placed anywhere by hand. Testing in steps from 0 deg through to 90 deg the tension is sufficient to ensure that when using the up or down arrows at all speeds the mount moves without there being any evident drive slippage and that it doesn't overshoot or run away.

I've gone for much the same effect in AZ though it doesn't seem to be as straightforward or as clear cut.

I now get a straight line "drift" of star positions between each sub - but I STILL can't get decent tracking.

Alignment and targeting are fine - but it just won't track.

So while once upon a time I was getting 30 second subs and only having to bin two of them because of the dobsonian tracking adjustments - now I'm down to 10 second subs and have to bin eight of them because of tracking. <sigh>

I "think" I need a tad more friction in AZ.... it's really hard to tell at the low speed used for tracking if there's any initial slippage. I KNOW it runs OK at speed One because I've run it and watched it turn - but it's so s..l..o..w... that I can't for the life of me tell if there's any hesitation right at the start.

So I'll be adding a bit more friction. I think what I might end up doing for the AZ is pretty much the opposite of what I did in ALT. In ALT I started from too little and gradually tightened up until it behaved - then gave it a tiny bit more for safety. In AZ I think I'm going to have to go for fairly high friction and then ease it back between tests until I start to lose it - then go back up a bit. Maybe!

Hmm... I wonder if I can set my DTI onto the side wall. Might be able to spot hesitation or slippage then.

Ain't all this fun!?

<sigh>

Too light on the Alt clutch. I had to put the right balance weights at the right place for my intended target and then put the scope somewhere close in Alt by hand before the motor would take charge. So I'll need to put more clutch friction on. I had about an hour last night before the cloud rolled in and couldn't get usable 30 sec subs. 15 secs were OK (DSS stacked 22 out of 34 which is the best ratio I've had so far) - but I really want thirties

I've contacted Celestron to see if they have a technical page somewhere to set up optimums.