Stripping a skywatcher dob (Auto) mount

[squeaky]

Hi all,

So there was me thinking I just have to undo the four nuts on the plate underneath the centre of the mount, then maybe undo a central locking bolt... and lift... to separate them.

Nah... course it ain't that easy.

The nuts on those blots are stiff-nuts so the bolts turn with the nuts.

So I thought, OK, I must need to work from both sides. Removed the motor/encoder cover plate, undid the four machine screws holding that assembly to the base and... nope, the motor/encoder doesn't lift out of the way.

I'm currently searching for instructions, pref with images, but so far I'm swamped with non relevant bumph.

Does anybody know of a good site, please?

Edited July 24, 2012 by squeaky

[Rossco72]

Sorry I can't help. I was similarly looking for info for a strip down and fettle to try and improve my Goto Dob accuracy and holding but despite numerous searches and trawls wasn't able to come up with much. I'll keep an eye on this thread just incase someone else found something tucked away.

[Cjg]

There was a very detailed series of posts on this forum by someone; a Dr. if I recall. Think the title was "improving the go to accuracy" of his Skywatcher dob...might be worth a search to see if that thread (was quite lengthy and detailed) has any pointers for you?

Chris

[squeaky]

That'd be this one then?

[Steve Ward]

Why you wait for clear skies to discombobulate your scope ?

I can hear you swearing from here....

[squeaky]

Because, My DEAR Sir, I didn't know that the movement had stiffened up UNTIL we had clear skies and I tried using it.

I'm getting fairly convinced that those four screws on the underside plate shouldn't be spinning. I took the white foam off (the stuff that sits between the main base disks) and had a look. The heads of those bolts are underneath plates of sheet metal. No way that I can see to get at them. I'll do photos later...

Still have more reading to do but I'm shattered. Need a nap.

EDIT: Your tag line reminds me of this...

DAS MACHINEN NICHT BERVERKEN, EIS IST GERFUNKED GROSSEN.

A sign we used to use at work. (This machine isn't working, it's "broken" bigtime. Loosely translated )

Edited July 24, 2012 by squeaky

[Steve Ward]

Could be worse , I put the kit out last night ready for a long one , popped in to watch the news at 10 .............woken by birdsong at 03:50............

Been out in sun all day , that's my excuse....

[Cjg]

That'd be this one then?

http://stargazerslou...__hl__+accuracy

Spot on, well done - hope it's useful to the OP?

[squeaky]

So so. Not enough detail for me.

I certainly have the "move away and don't get back to the mark" thing - but I'm more concerned about how stiff everything is and that my images are doing a "square dance" around the frame and every single star is elongated or doubled.

Seeing this last night I had to drop from 30 to 20 sec subs and took 148 of them. Of these only 25 were half way decent and DSS stacked 11 of them.

So 11 out of 149 is not exactly good.

I had round stars at 30 secs last time out with only 1 or 2 in ten dropped by me - and these more for framing after correcting drift than for star trails.

The thing is I've gone from keeping the scope in the house to putting it in my brand new (bought and built for the purpose) shed. I can only think that all this wet and humid weather, plus the cumulative effect of so many heavy dew falls (pretty much every time I've been out) have done something nasty. I got a bit of lubricant between the main sliding surfaces of the3 dob base yesterday and last night was a very slight improvement. I'm going to have to work out this clutch thing and completely release the Azi one to see what the friction level really is.

As it is - I left the beast out in the sun all day to have a nice bake and dry out. Still stiff right now but may be better as it cools for the night.

[squeaky]

Right...

Got my Alt friction sorted out and the scope balanced.

One thing I found on doing the balancing was that there doesn't seem to be a single balance point. What I was seeing was that when I balanced the OTA at 0 deg - as I then tilted it to "view" higher in the sky it got more and more tail heavy. So for the moment I've balanced it for 45 deg - but if a test tonight shows an improvement in tracking I'll re-do the balance weight and sort out a slider and a scale so that I can move the weight to balance whatever my Alt is.

Since the clutch was freed completely for this - when I tightened it I did so just enough to allow the drive to pick up the scope from almost vertical. Again - the higher I pointed the scope the more slippage there was.

Balancing for the camera was even worse for not having a single balance point - so the weights for that will have to be on a slider and scaled too.

Even so, it's much easier to move in Alt than it has been lately. Much more like the day it first arrived here.

As for the Az friction - I got the base mostly disassembled and tweaked a few things but it doesn't feel any easier or smoother at the moment. But then, I wanted it all back together to try for viewing tonight. At least I'll find out if I've made things better or worse.

The rubbing plates (bearing plates?) between the dob's foot base and the rotating platform are fine btw. I didn't need to disassemble that bit because once I lifted the motor drive out it turned free as a bird. So I need to sort out the motor friction (or something).

Anyway, I've got photo's and IF it works I'll post annotated step by step things.

At the moment I'm only working on getting rid of the increasing stiffness of the mount (and balance). Backlash etc will follow, and I'll be relying a lot on the thread in that earlier post.

[squeaky]

Here's a before and after diagram of how my subs were moving before and after sorting out the balance and friction pressure in Alt...

Doing the job was quite a fiddle - and AFTER I'd done it I noticed another problem...

THIS:-

Those are witness marks which show that the OTA's mounting boss is rubbing on the side of the mount. I'm assuming now that the change from living indoors to moving into its very own new super duper shed, along with the damp weather, high humidity, and chronic dew fall that I get here; possibly including moving and transporting, has all contributed to the gap that should be there closing up over time.

To make sure that the fault was current and not something old I cleaned the marks up and then got some engineer's blue and applied a thin film to the mounting boss - put the OTA back in place and moved it through its full range a few times. After taking it off again I had this...

Ignore the stuff in the middle and top - that was me setting it in place. But as you can see (apart from the fact that my blue film was a bit too thick) that rubbing mark is still there.

To check, and in fact to help with the remedy, I cut a length of scrap timber to about 1mm wider than the current gap between the side mounting walls and wedged it in place below the OTA...

And sure enough I now have a gap between the OTA mounting boss and the side wall...

The solution is nice and straightforward.

Slacken the two lower screws that hold that side wall to the front/centre wall and remove the top screw completely. (With the timber still in place.

A gap will open up.

Ease a washer into the gap. The easy (..er) way is to stick it to a piece of tape that will easily break later...

Then slide it into the gap, align it over the hole and start the screw straight through the washer and the tape...

Now break and remove the tape, remove the timber stretcher, and tighten all three screws fully.

And yep, I now have a gap between the OTA mounting boss and the dob's side wall. Probably bigger than it needs to be, but I'm going to leave it alone for now.

So I'm going to have to check the balance and friction again because this rubbing must have had some effect. I won't post my step by step images for this until I've checked, just in case some new wrinkles or problems turn up.

[Space Cowboy]

Very interesting thread Squeaky. As you say dampness will play havoc with the wooden mount. I always find my mount tracks better in the western sky this being my 2nd mount, the previous one was the same. I only use a counter weight for tracking in the east. AZ tracking has always been fine. Whether there is a logical reason for this I don't know.

Edited July 28, 2012 by Space Cowboy

[squeaky]

I found tracking to be different too.

Mind you - I haven't even started on the AZ bit of my mounting yet and it needs some serious work too.

And after that I'll need to take a good look at the thread link posted earlier.

[squeaky]

OK... the basics of easing friction if your dob auto mount is tight.

If you haven't read post eleven here just scroll up to check that your mounting hasn't moved/warped and is too tight on the OTA's bearing bosses.

Tool list:-

• Phillips screwdriver
  
• 2" sticky tape (NOT parcel tape - it's a real pain to clean off later)
  
• 2mm allen key
  
• 2.5mm allen key
  
• 17mm spanner
  

First things first. You are going to be completely releasing the friction clutch in Altimuth - and that means that your OTA can swing very freely. You will need to take care throughout this procedure that it doesn't get away from you!

Set your telescope up in your normal viewing configuration... blanking plates off (WARNING - you are going to using lots of tools so be careful), finder fitted, dew shield and shroud fitted if you use them.

I already have a high angle (90°) bumper foam piece glued to the back of my mount's front board.

For the moment I put a folded towel over the top of the front board - but I will be fitting bumper foam pads later.

There's a nut on the inside, locking the main drive shaft in place, but I shan't be working on that one in this basic set up. (See this thread for more detail: http://stargazerslou...__hl__+accuracy)

The drive mechanism and the friction clutch are under this cover.

Have a piece of sticky tape ready; undo the four phillips screws and then support the cover with tape so that the weight isn't hanging on the internal wires.

On the outside end of the shaft is the encoder which I shan't be removing here. In from that is the lock nut (with two grub screws) and then the friction clutch and bearing bits.

Back off each of the two grub screws a couple or three turns with a 2mm allen key

More to come - please don't post in until I'm done

Edited July 30, 2012 by squeaky

[squeaky]

Have your OTA horizontal and steady it in place with one hand because it will swing on its own as the friction reduces... Turn the lock nut back on the shaft until it is close to the encoder's lock nut and the bearing and friction clutch bits are loose. (I'm pretty sure that most bolts on this thing are not metric - but the nearest fit is 17mm spanner)

With my OTA at 0° (ish) I found that it was nose heavy and would dip to the front of the base.

Going up in five degree steps I found that the OTA was nose heavy until about 20° and it held steady until about 40°... after which it was tail heavy all the rest of the way.

Oh - ignore the yellow tape on the scale - that's one of my markers for the Alt for Polaris. Helps me find it in the viewer before I can actually see it with my naked eye.

Set your OTA to exactly zero (supported by you)...

..and add weights to the front of your tube until you are balanced. I.e. the OTA stays put when you let go.

Although I've shown a weight on the top here - I found that putting them underneath was more effective. Plus there's that much less chance of them going down my tube between the trusses if they fall off.

I've got two speaker magnets and what I did was keep adding bits of metal to just one of them until (while it was at the lowest point of the front part of my tube) I got a balance. THEN I glued those bits to the magnet to be safe!

Now when you check, as you lift five degrees per time, you'll probably find that the tube stays balanced until about 40°. (Well, my 300P flex does - your dob is likely to be different)

I found that once past 40° I couldn't get a balance with one weight even if I moved it as far forward on the tube as I could get. Which is where my second speaker magnet came in. That and two very large nuts taped securely together.

From 40° to around 70° the other magnet with the two large nuts on it (at the low end of the front part of my OTA) kept a balance.

Beyond 70° I needed both magnets sited fully forward to keep a balance.

I've got a list taped to the front end of my tube stating which weights I need where for whichever Alt angle I will be viewing at.

So that's viewing balance sorted. The next post deals with balancing for a camera.

More to come - please don't post in until I'm done

Edited July 30, 2012 by squeaky

[squeaky]

OK, if you are doing imaging read on, otherwise you can skip to the next post.

It should come as no surprise that a DSLR added to the OTA makes the beast nose heavy

.

Which is why I'm going to be replacing that towel with foam bumper bits...

Take your weights off the front - you don't need any extra weight there - the camera is plenty.

I found that trying to use those weights, even right at the very bottom of the OTA were nothing like enough to balance the camera when at 0°

What I did was tape a small cardboard box to the bottom of my tube and evn offset it a little; a) to keep it out of the way of my flex trusses and to help counter the one sided weight of my camera.

Then I raided the pantry for assorted tins since I have no other weights available. It took one can of beans and one can of peas (824g) to balance at 0°

Going up in five degree steps I found I had to change the weight a lot. I'm going to have to sort out some decent weights and a slider to make adjustments a lot easier but here's the table I finished up with:-

• 00 - 10 ... 824g
  
• 15 +/-5 ... 699
  
• 20 - 25 ... 610
  
• 25 - 35 ... 467
  
• 35 - 40 ... 357
  
• 45 +/-5 ... 143
  
• 45 - 55 ... ZERO !!
  

Above 55° instead of being nose heavy the OTA went tail heavy - so now I had to start adding weights at the front. Luckily the magnets were enough so my table for a camera continues...

• 55 - 60 ... plain magnet low
  
• 65 +/-5 ... plain magnet high
  
• 70 - 75 ... mag plus nuts low
  
• 80 +++ ... both magnets plus nuts high
  

I'm going to have to sort out dovetails to sit weights on to make doing all this a lot easier - but at least I have proof of concept and I know where my balance points are.

Next step - putting the friction back on.

More to come - please don't post in until I'm done

[squeaky]

Putting the friction back on...

Oops! I carelessly managed to lose this set. I'll do them again tomorrow, but in plain text....

If you use a camera - have your tube set up for this, Set your weights for 0°. Set and hold your OTA at 0°

Tighten the locking nut you loosened earlier until you feel it just bite up against the bearing plate.

Move (AND HOLD) the OTA to about -5° and relax, but do not let go completely, your grip. You'll probably find that the tube is nose heavy and moves. Tighten the locking nut in small amounts - as in the end of the spanner you are holding moves about 1mm per time until the OTA stays put when you let go. (IF you have a viewing horizon that goes lower than this - then do this setting for five degrees below that horizon).

Now we need to check that the motor drive picks up the tube. So connect up your handset and power, turn it on.

Press the UP button and your tube should lift straight away with no sign of slipping. It probably won't. So carry on tightening the lock nut in small increments, trying the UP drive each time, until the motor cleanly picks up the tube and lifts it.

So, that's your zero point sorted.

Now set your weights for 80+ degrees and move the tube to 80° by hand.

Press the DOWN button and the motor should lower the OTA cleanly. If it doesn't, tighten the lock nut in small increments until it does.

Now use the UP button, drive to 85°.... the tube should stop quickly. If you press the DOWN button the OTA should lower straight away. If not, tighten a touch more.

Repeat at 87/88 and then 90°

Now, throughout the full range of movement, with the correct weights fitted for whatever angle you are at - check that the motor correctly moves the OTA without any slippage, and without running on too far, or even allowing the tube to just run on its own.

Tighten up the two grub screws on the lock nut and replace the cover.

Right. that's that bit done. Proof of the pudding will come when you have chance (unlike me at the moment) to do some imaging and see how much better your tracking is and how much your star trails have reduced by. Remember, this is only the Alt balance and friction. I'll post stripping Azimuth pictures tomorrow.

THEN... you'll likely need to read that more detailed thread to do any fine tuning needed, or if you find that you need to strip things further than I have here.

OK... you can tell em where I've gone wrong now

Edited July 30, 2012 by squeaky

[squeaky]

Putting the friction back on... repeated with images because I can't edit the previous post.

My improvised balance weights....

My camera balance weight holder and balance weights for zero deg inserted...

Note that the holder is offset from the centre line. It's enough to help counter balance the camera's off axis weight and to clear the way for my truss tubes to move down; but not so much that the box touches the side of the dob mount.

If you use a camera - have your tube set up for this, Set your weights for 0°. Set and hold your OTA at 0°

Tighten the locking nut you loosened earlier until you feel it just bite up against the bearing plate.

Move (AND HOLD) the OTA to about -5° and relax, but do not let go completely, your grip. You'll probably find that the tube is nose heavy and moves. Tighten the locking nut in small amounts - as in the end of the spanner you are holding moves about 1mm per time until the OTA stays put when you let go. (IF you have a viewing horizon that goes lower than zero - then do this setting for five degrees below that horizon).

Now we need to check that the motor drive picks up the tube. So connect up your handset and power, turn it on.

Press the UP button and your tube should lift straight away with no sign of slipping. It probably won't. So carry on tightening the lock nut in small increments, trying the UP drive each time, until the motor cleanly picks up the tube and lifts it.

So, that's your zero point sorted.

Now set your weights for 80+ degrees and move the tube to 80° by hand.

Press the DOWN button and the motor should lower the OTA cleanly. If it doesn't, tighten the lock nut in small increments until it does.

Now use the UP button, drive to 85°.... the tube should stop quickly. If you press the DOWN button the OTA should lower straight away. If not, tighten a touch more.

Repeat at 87/88 and then 90°

Now, throughout the full range of movement, with the correct weights fitted for whatever angle you are at - check that the motor correctly moves the OTA without any slippage, and without running on too far, or even allowing the tube to just run on its own.

Tighten up the two grub screws on the lock nut and replace the cover.

Right. that's that bit done. Proof of the pudding will come when you have chance (unlike me at the moment) to do some imaging and see how much better your tracking is and how much your star trails have reduced by. Remember, this is only the Alt balance and friction. I'll post stripping Azimuth pictures tomorrow.

THEN... you'll likely need to read that more detailed thread to do any fine tuning needed, or if you find that you need to strip things further than I have here.

My two balance charts...

And the reason why using parcel tape is not a good idea. Especially if it's old like mine. That's going to be a pain to clean off:-

OK... you can tell em where I've gone wrong now

Edited July 31, 2012 by squeaky

[squeaky]

This has yet to be tested on an image run but since I saw an improvement of Alt tracking on my first tweak I'm hoping that I'll now do even better.

PLEASE NOTE:-

I have set my friction here close to minimums. What this means on my 300P with my DSLR fitted is that the tube will NOT stay where it is put unless I have the right balance weights for the viewing angle loaded onto the OTA. So it is really REALLY REALLY a good idea to fit foam bumper bits.

Okay... next bit... the Azimuth setting

[squeaky]

Stripping Azimuth (the base plate).

Mine has gone really stiff and I can hear the motor straining to turn it.

The motor drive and the friction stuff are all under this cover and you have to start the strip from here to separate the moveable part from the fixed base.

So unscrew the four phillips screws and set the cover aside. (taping it to the base can help later)

The encoder and the radial pillar are now visible

You'll need to get down to floor level to see that the encoder has two fixing points. If you can't see them you'll need to rotate the top against the bottom until they come into view.

There's a grub screw

And a socket head screw

First back off each of the two grub screws on the radial support pillar by two or three turns

Then back off the grub screw

And finally remove the socket head screw completely. Make sure that you keep it safe because it's difficult to find again if you lose it. (Don't bother asking me how I know that )

Gently ease the encoder off the shafts (you may need to rock it a little) and lay it into the cover assembly

Now you can see that the grub screw was sitting on a flat, and the socket head screw fits directly into the shaft which is why we had to remove it completely

Edited July 31, 2012 by squeaky

[squeaky]

Next - remove the motor drive assembly...

Undo the stiff nut as shown. If I remember rightly it needs a 19mm spanner

Lift off the bearing set

Note that in case you disturb them - the plate with the larger hole is at the bottom of the pile. Keep these in a clean safe place.

Remove the four machine screws holding the assembly in place

Use a small pair of pliers to rotate the worm screw free of the drive gear and at the same time lift the motor assembly clear

You may need to slip a small screwdriver under it at the far end to free it from the shaft

Now you can see the worm screw and how its spring tensioner works

You can also see the main drive gear

Make sure that your motor cover and the encoder and the motor assembly are all secured with tape.

Make sure that you have left no tools bolts nuts or screws in the base.

Lift the mounting clear of the fixed base and lay it on its "front". You'll see the three main support points that the mounting sits on.

By the way - you may also have noticed that the white foam strip around the outside edge of my base is missing. That was me working out how I got into this thing. You can leave yours in place.

On the fixed base now you'll see that there's a large circular metal sheet. (If you look from the side you'll see that below it is a black "plastic" layer and then another metal sheet.)

The top sheet should rotate freely even if you press firmly on it.

Edited July 31, 2012 by squeaky

[squeaky]

All that stuff in the centre contains the friction clutch for the azimuth rotation

There are two grub screws in that locking nut, so release them a couple or three turns

Then use a 25mm spanner to undo the nut

Below this are two cupped spring washers - lift them both clear

Now lift the gear and clutch plate assembly. You may need to slip the spanner under it to gently ease it up

You can now see one of the two clutch plates in the centre

The other is underneath the silvery disk sitting in the centre of the gear wheel

They should be dry, clean, free of grease and have no discernible rough areas or score marks.

Be very careful handling them because you are pretty much guaranteed to have grease on your hands by now.

If you need to strip further to clean up between those main circular disks and the black "plastic" thing then the four screws in the centre here have stiff nuts (on a cover disk) underneath the base.

From here on in - reassembly is the reverse of this disassembly so you just need to follow the pictures back up.

When I tightened up the locking nut above the two cupped spring washers I did it finger tight and then gave it an extra quarter of a turn with the spanner. It's quite difficult trying to asses the right amount needed here. It felt easier than it was before stripping - but now that it's ALL back together I think I could possibly have got away with adding less pressure when using the spanner. I'm going to leave it as is though until I do some imaging tests.

So that's the basics done if your tracking is really bad and jumpy like mine was, or if your dob feels as though it's stiffer to move around by hand than it used to be.

After image testing, and possibly a quick tweak of that spring cup tension, I shall be heading for the accuracy thread to get some fine tuning done. At least now I'm familiar with all the bits and what they do

Edited July 31, 2012 by squeaky

[squeaky]

<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.

[squeaky]

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!?

[squeaky]

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...