Stripping a skywatcher dob (Auto) mount

[gary1968]

Any chance you could share the PDF Squeaky? It would no doubt be a useful to read it as I do intend to attempt to improve my dob soon.

Cheers,

Gary

[squeaky]

Sure

[gary1968]

Sure

Thanks........

[squeaky]

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.

[squeaky]

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>

[gary1968]

A little bit of chat Richard (squeaky) and I have been having by PM, he thought it would have been better here so I have copied and pasted it over.... a bit of a read but there is some good info in it.

Gary..

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Richards link to the PDF for the 'how to' on clutch adjustment,

Here ya go... http://www.squeaky.org.uk/temp/AZ.pdf

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Thanks a lot for that Richard, much appriciated mate...........

Cheers,

gary

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You're welcome

Um... tell me....

When you do your friction adjustment - if you want to check the OTA's balance as I did in the thread - could you let me know what angles the basic tube "stayed put" in with no friction applied?

Mine, from memory, was...

Below 20 deg - nose heavy

20 - 25 deg = balanced

Above 25 deg - tail heavy

And the balance point with my 630g DSLR weight added moved to 45 - 50 deg

I've also got some fisherman's spring balances somewhere about the place. I'm thinking that once done I'll "pull" the OTA round in AZ and see how much "weight" it takes to move against the friction.

That should make it an easy check for the future and if my tracking goes off again I would be able to check that weight and see which way the clutch friction needed to go. Plus this could be checked without having to wait for a clear sky and do trial imaging.

Richard

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Ok, When I get round to doing it I will do a little study of the balance points and let you know what my findings are.

However, all this talk of balance point changing at different angles has me a bit perplexed?? I really cant get my head round this phenomenom, I would have thought that if it was, for example, tail heavy then regardless of alt it would remain tail heavy...... SO, this alone makes me want to take it to bits and investigate, lol, I am a compulsive tinkerer, hehehe.

Yeah, using the fishermans scale as a strain guage is a good idea, I may use a luggage scale as thats what I have lying about.

I am not intending doing this right away though. I think I'll wait until the tracking starts to cause me image quality issues, my ability is the main limiting factor just mow as I have a lot to learn about planetary imaging and processing.

Cheers,

Gaz

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I have lots to learn about imaging too - but it's kinda difficult when all I get is double stars and trails....

Not that I'm going to be getting any for a while. 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>

Richard

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The base has warped? I would have thought that was unlikely being MDF, where / how do you store it when not in use? Mine either lives in the (very damp humid) summerhouse or in my garage which is nice and dry, although I am starting to favour the garage because of the dampness in the summerhouse..

Gaz

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It's not mdf it's chip board. Untreated.

It's stored in my brand new GRP shed, bought and built expressly to house my scope

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Chipboard? Not impressed by that, I would have thought MDF would have been more stable. Looks like mine will live in the garage now, with the scope in the summerhouse packed in its wooden box along with bags of silica gel.

Gaz

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And BEFORE you even do that....

Sort out some bags or cartridges of silica gel and insert as many as you can under the covers of the two motor/encoder assemblies.

I wiped my beast dry of dew after every use and left the shed door open on every warm dry day and it has still gone! There was even evidence of corrosion on the small motherboard in the AZ cover. (The board that the drive cable plugs into.)

We should really be having this conversation in the thread - it may be of help to others.

Richard

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Agreed, it would be of use to others...... I attempt to copy n paste it.

Gaz

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[squeaky]

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"

[gary1968]

Ah, that makes sense I suppose. It does however make balancing a bit tricky........

Gary

[squeaky]

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.

[squeaky]

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...

[gary1968]

Its a bloomin mine field!! lol.

Mine is as it was out of the box. It tracks 'fairly' well, but could be better. It keeps objects in a 25mm ep for ages. It keeps Jupiter on the camera chip for a good few minutes at F10, I have recently bought a 4x Imagemate and yet to try that, so this might be the push I need to make it better.

Goto has been a problem a few times. I have found that the 1st stars selected by the hand set during alignment are not usually the best. I have found that stars reasonably low in the sky, & at a similar ALT, and at least 90 degrees apart in AZ are best. (This confirms Dr. Robins findings in another post).

I think a 3 star alignment routine would be much more accurate, hopefully SW will include this in the next firmware update.

How are you going to replair your mount? Is the supplier (OVL) going to replace the warped board?

Gary

[squeaky]

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....

[Space Cowboy]

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.

[squeaky]

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.

[Space Cowboy]

No worries.

The theory only occured to me recently and I've not yet been able to test it out. I was imaging Saturn earlier in the year in the western sky and the planet was drifting upwards on screen (scope tail heavy) which does not happen without a counter weight so I removed the weight and it became impossible to keep Saturn on screen.

Each time I needed to make an adjustment in alt to keep the planet on screen nothing would happen for ages and then WHAM! It would suddenly fly past like a shooting star. At the time I thought it was something to do with the cold air on the electrics but I think it must have been the change in balance and that the tracking has to be re-set whenever the weight balance is changed.

I think it would make sense for me to re-set the tracking when I replace the eyepiece with my DFK at the start of a imaging session. Might improve tracking not that my tracking is terrible as I was imaging Mars at over f50 but it did need constant adjustment with the handset.

[squeaky]

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.

[squeaky]

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.

[squeaky]

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

[gary1968]

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.

MONTHS????? That is shocking. I am surprised that a distributor, such as OVL, or any distributor of any device, is allowed to not carry spare parts. Or at least have a reasonable time scale to supply such parts.

We all know that SW scopes a pretty good for the money we pay but I have read a good few posts over the years talking about problems in getting spare parts. They (SW & OVL) should realise that these things get used, are gonna break down for various reasons and folks are going to require parts.

Anyway, rant over, hope you gets the parts MUCH sooner than that.

Gary

[squeaky]

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.

[gary1968]

Keep us posted mate..........

Gary

[squeaky]

For sure

[gary1968]

How did it go then dude?

Gary

[squeaky]

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

[gary1968]

Sounds like you are making progress...... I am interested in making a levelling plaatform, I regularly take mine to see dark site and it's always a pita to level.

Gaz