Stripping a skywatcher dob (Auto) mount

[squeaky]

Those kitchen cabinet feet work a treat. Honest

Anywhere.

Everywhere.

[gary1968]

Do they not make the base too high? The ones I know of are another 6 inches high. .... I am already on tiptoes at the zenith.

Gaz

Sent from my U8815 using Tapatalk 2

[squeaky]

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.

[Rossco72]

Hopefully next week I will finally get to grips with my Goto Dob as it's never consistently worked since I bought it.

The OTA is good and views are great but the GOTO and tracking is so hit and miss. Sometimes misses the target by 10-20 DEGREES!! but then other times once manually put on the right target it will track ok, sometimes not.

I think I'll start by tightening up the friction a bit in both axis.

Great info so far Richard and keep posting any updates

[squeaky]

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.

[Rossco72]

I generally use a GPS mouse (just coz I have 1) and it sets the date, time, location etc.

Alignment stars I like to hope not but maybe the odd clanger but for me given that I can do an alignment, slue to the first target and hit it dead on then slue to a second and be anywhere from close to miles out it seems more likely to me that its a clutch or some kind of slip on the encoder causing the problem.

[squeaky]

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.

[gary1968]

I have found that it is vital to get the base level! If it is not then the further off level it is then the further out the goto will be.

The sooner SW do a 3 star align routine the better these mounts will perform I think.

Gaz

[Rossco72]

I have found that it is vital to get the base level! If it is not then the further off level it is then the further out the goto will be.

The sooner SW do a 3 star align routine the better these mounts will perform I think.

Gaz

Thanks Gaz, I level the base with a bubble. If it needs to be more level than a bubble level then I'd say it is a design not fit for purpose...

Cheers

Stuart

[Rossco72]

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.

Unfortunately it may well be tricky, I'm off for the school mid-term holidays next week so hopefully I'll have some time to strip it all down, rebuild it and see where I go from there. Your guide will come in very useful.

Cheers

Stuart

[Rossco72]

OK, quick update.

I stripped everything down and refitted, tightening things up. Last night was the first semi clear night and I took the Dob out.

The alignment went ok, first alignment star Arcturus in the West, then chose Capella in the North-East. The Dob slued round and Capella was in the eyepiece if not central. A quick adjustment and Alignment success.

Then select a Goto M15, in the South West, again in the eyepiece (26mm) just off centre. I adjust a little on the handset to centre and it stays in the middle for a good 10 mins.

Then Select M31 and it completely misses! Re-Do alignment and then goto M31 and there it is in the eyepiece again. Holds ok and then do a GOTO Neptune and it misses.

Also on the Moon it just couldn't keep it steady at all for some reason and the moon kept drifting off the eyepiece even with lunar tracking enabled!

So to sum up it seems it can only do one goto and track per alignment....

:shocked:

Cheers

Stuart

[squeaky]

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.

[Rossco72]

My retailer was FLO, but mine was one of the first day issue 14" Flextube GOTOs so its over 2 years old now. Just has hardly been used!!

[squeaky]

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

[squeaky]

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!!

[Rossco72]

I wonder how much a replacement set of encoders would cost to see if it cured my problems :huh:

[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.

[tteedd]

Hi

Interesting topic.

I just got a 10" (used) from Astronomia. The discussion shows that my stiff AZ is not an easy problem to solve but that I can do it myself if forced to. But for now I'm leaving it to Astronomia.

Did the original machine get sorted Stuart?

Ted

[LisaC]

Hi.
If someone would like to disassemble azimuthal bearing from a newer version of SW GoTo dobson, you can see https://www.astro-forum.cz/viewtopic.php?f=35&t=25807.
Only in czech language, but lot of pictures.

Edited October 16, 2019 by LisaC

[Goliath Online]

I adjusted the friction nut accordingly on my Skywatcher 200p 8" and now I'm have trouble with the tracking. Seems like it starts to drift after 15-20 seconds and doesn't hold normally like it did from the factory. I was able to track the moon for almost 30 minutes if Im not mistaken...Is there any lbs per torque on the friction nut to get the right tension? Curious to know how the assembly technicians put this together. 

[Fraunhoffer]

Thanks for this post - still very useful and so glad I found it (again).
Just about to start tinkering.

 

[Fraunhoffer]

Re-awakening an old thread. .. Does anyone know what size the roller bearings are in the black thingy between the two metal sheets in the AZ bearing arrangement?

I noticed peering underneath that I have some play between the lower friction clutch plate housing (held by 4 blind bolts) below the gear wheel and the wooden baseplate.
I can only see it matters if the decoder spindle also has some play, but I cant see that with everything in place and thought I would get every thing ready and change the rollers whilst I'm at it.
I cant feel any play in the 4 'blind' bolts from underneath - but wish to at least check that out .

Thanks

 

[Fraunhoffer]

On 31/07/2012 at 12:07, squeaky said:

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

? How is the decoder shaft attached to the bottom clutch plate (last pic) please. ?

I think the holes in the bottom clutch plate on mine have worn as the plate moves slightly when the top is rotated ( so the decoder doesnt detect a movement).

[Fraunhoffer]

I took my Dob in hand in the end (300p Skyline) and made a new base for it.
Wearing my 'engineers hat' it seems the lower wood disk (original Chipboard) swells up after a time. In my case 2mm!. The result is the telescope tilts as it rotates (think fairground Wurlitzer ride).
Its only a bit, maybe 0.5 degree, but that's the width of the moon and the result is the alignment reference plane is then tilted and the stars are off using the go-to.
I chose some good quality marine play and finished it to be plat to within 0.125mm.
Result - my goto are within half the fov of a 10mm eyepiece. That's about 5 arc minutes, which is the stated spec of the Auxiliary decoder on the base unit.
Well pleased.
Conclusion - look after those wooden baseplate disks.

Ill get around to do the rest of the base in looovely wood next Summer I expect.

[Peter Batchelor]

Does anyone know if Squeaky is still a member of this forum? I wanted to get in touch with Richard, but his email address appears to no longer work, and the website isn't active either...