Polar Scope Alignment

[almcl]

 

In the quest for better tracking and more accurate gotos, I decided to check the alignment of my polar scope.  Having read Astrobaby’s web pages on the subject and watched Dion’s video, I was suffering from the delusion that I understood what was to be achieved and how to go about it.

Settled down in daylight using a bit of a neighbour’s roof as a target (no external tv aerials here, unfortunately), I spent half an hour adjusting the sub-miniature allen screws until a 180° degree rotation of the counter weight bar produced hardly any movement of the cross.  Great, I thought, polar alignment will be much quicker and possibly better.

Well it wasn’t.  Once outside in the dark, the movement of Polaris when rotating the RA axis was horrific -  worse than before.

So this weekend I settled down to try again.  This time I tried 360° rotations only to discover that  the cross almost never ends up back in the place it starts from.  It doesn’t describe a circle, either.  Sometimes it’s a rough spiral, others it’s more like a bent figure of eight.  Thinking I must be nudging the mount while turning the axis, I fired up Eqmod and used that to rotate the axis. No difference.  Perplexingly, too, sometimes, during movement around the axis, the cross suddenly starts to drift off by a large amount.

Clearly the chances of reliable polar alignment or accurate tracking are diminished by this, so does anyone have any advice on where to start looking for the problem?   I haven’t dared delve into the bearings on the EQ5 (yeah, I know, no one in their right mind would use one for AP), and am not sure if this is where the problem lies anyway (I have checked that they don’t suffer from the ‘thick grease’ issue - and they don’t - just a light lithium grease coating).  But where could the slack in the bearings be (if that’s what the issue is) ?  Or, if it’s not, where else should I look?  The reticule screws are tight before each rotation - so I don’t think it’s the reticule shifting, although it might be something else?

 

 

[wookie1965]

http://www.myastroscience.com/polarscopecalibration

Try this I found this to be easy to follow and had my polar scope set up in minutes. Outside it was really accurate.

[almcl]

Thanks for the link wookie.

I have tried that procedure but unfortunately the random nature of the error means that even four or five iterations don't reduce it.  Sometimes they even increase it in new and strange directions.  I think something is loose or mis-set in the RA bearings but information on how they are set up and adjusted is hard to come by.  

Does your EQ5 pro resemble the basic EQ5 (which is what mine was until I started messing around with motors and such) ?

[Starflyer]

Does your polar scope lens wobble? The one on my AZ EQ6 was terrible and I could rock it half of the field of view, this made calibrating it impossible.

I solved it by unscrewing the lens all the way and wrapping a few turns off plumbers PTFE tape around the threads. This tightens up the thread for nicely and allows you to calibrate it.

[almcl]

I don't -think- it does, but that's certainly one more thing to check.  The outer reticule ring is firmly held, as of today with three m3 thumbscrews, but not sure if the locking ring is fully home...

Thanks!

[SteveBz]

14 hours ago, Starflyer said:

Does your polar scope lens wobble? The one on my AZ EQ6 was terrible and I could rock it half of the field of view, this made calibrating it impossible.

I solved it by unscrewing the lens all the way and wrapping a few turns off plumbers PTFE tape around the threads. This tightens up the thread for nicely and allows you to calibrate it.

I agree my whole polar scope 'eyepiece', if I can call it that, wobbled. I put on a large plumber's washer so the lens sinks into it when I tighten.  The other thing is my polarscope tube is held in place by rubber pads inside the mount and alignment screws at the end. If the tube were to flop around without these pads it might create the effect you are seeing.  Good luck.

Steve

[Dr_Ju_ju]

Save your knees\back, get a Polemaster, much easier & I find accurate....

[SteveBz]

12 minutes ago, Dr_Ju_ju said:

Save your knees\back, get a Polemaster, much easier & I find accurate....

V nice but 279 quid? More than my entire setup.

[almcl]

5 minutes ago, Dr_Ju_ju said:

Save your knees\back, get a Polemaster, much easier & I find accurate....

I have certainly considered one, and read the enthusiastic reviews, but at nearly £300, I need to know that the fault doesn't lie with the mount, for if it does, even with Polemaster, an accurate polar alignment when the scope's axis can wobble by more than a degree, might not be achievable.

I must also confess that I sit on the grass on a cushion to look through the polar scope and don't find this too bad at all - maybe this is annoying the gods of PA and is the true cause of the issues?

[wookie1965]

This is my Eq5 sort of resembles yours as others have said the plumbers washer is good i stripped all mine down and put new lithium grease in then made sure everything was tight in the right places it's really quiet and smooth now. 

[almcl]

It's certainly got a lot of similar points, hasn't it?  

I'll give my O ring collection a once over this evening, but (fingers crossed, touching wood &c) tightening the polar scope (by using the newly acquired thumb screws like a capstan wheel) to a torque just short of thread stripping may have cured, or at least severely reduced the problem...  the circle stays on the apex of my neighbour's soffit boards all the way round now.

I'll need a clear night (wot's one of those then?) and a few trees cut down before I can really check with Alignmaster or PHD2's   drift alignment tool, but things look a bit better now than they did earlier in the week, so thanks to all for the thoughts and advice.

[Donkeiller]

I gave up and took the Polar scope out of my EQ5.

More trouble than it was worth.

Just sight Polaris through the hole and use "Polar Align" function.

seems to work OK.

[SteveBz]

On 09/15/2016 at 18:47, almcl said:

It's certainly got a lot of similar points, hasn't it?  

I'll give my O ring collection a once over this evening, but (fingers crossed, touching wood &c) tightening the polar scope (by using the newly acquired thumb screws like a capstan wheel) to a torque just short of thread stripping may have cured, or at least severely reduced the problem...  the circle stays on the apex of my neighbour's soffit boards all the way round now.

I'll need a clear night (wot's one of those then?) and a few trees cut down before I can really check with Alignmaster or PHD2's   drift alignment tool, but things look a bit better now than they did earlier in the week, so thanks to all for the thoughts and advice.

Did you ever get this sorted? I was out the other day aligning my scope and I had exactly the same problem. I fear bearings, but I thought i'd follow up here first.

Steve.

[jambouk]

I'd put my money on the polar scope just not being aligned rather than an issue worh bearings.

make sure you've not released the screws too much and the internal reticle no longer sits held by the ends of the three screws. 

It is a really tricky and ackward job. Remember to just make a correction of half the distance of the error when you've done a 180 degree rotation.

james

 

[almcl]

17 hours ago, SteveBz said:

Did you ever get this sorted? I was out the other day aligning my scope and I had exactly the same problem. I fear bearings, but I thought i'd follow up here first.

Steve.

Not entirely, Steve.  

Although improving the alignment of the polar scope certainly helped, the enduring problem of declination backlash plus a load of 13kg being on the limit of what a modified EQ5 can handle, even with Tom Carpenter's excellent Astroeq and belt drives, led me to acquire an Alt Az EQ6. Comes with belts from the factory and  seems much less prone to backlash, although it is certainly a much heavier beast.

[SteveBz]

5 hours ago, almcl said:

Not entirely, Steve.  

Although improving the alignment of the polar scope certainly helped, the enduring problem of declination backlash plus a load of 13kg being on the limit of what a modified EQ5 can handle, even with Tom Carpenter's excellent Astroeq and belt drives, led me to acquire an Alt Az EQ6. Comes with belts from the factory and  seems much less prone to backlash, although it is certainly a much heavier beast.

Well, it feels like I've got a strip down and check all the bearings and collars coming on.  I can see the problem very clearly as I'm looking through my polar scope against the TV aerial.  As I slowly rotate the mount, the image suddenly drops by a small amount as it passes some sort of tipping point, maybe a bearing, maybe a collar not tightened, maybe a washer missing, maybe a bolt not at the right tension: but it feels like the whole RA section of the mount moves.

I might also check the weight limits.  I have a Celestron CG5 with the c8n Newtonian - looks like yours.  But they are both manufactured by Celestron, I imagine they are specified to work together.

Good luck with the EQ6.

[jambouk]

SteveBz, can you feela clunk in the mount when it goes through this point? If not, I do wonder if the reticule in the polar scope is just loose rather than it being an issue with the RA axis. I'd hate for you to have to strip it all if not totally necessary.

James

 

[wookie1965]

I  had same thing and as jambouk says it was the polar scope reticule dropping use a rubber washer keeps it in place mine is much better now no movement at all. 

[SteveBz]

22 minutes ago, jambouk said:

SteveBz, can you feela clunk in the mount when it goes through this point? If not, I do wonder if the reticule in the polar scope is just loose rather than it being an issue with the RA axis. I'd hate for you to have to strip it all if not totally necessary.

James

 

Hmm..  I do have a washer on my other mount, but not on this one.  I suppose I could swap the polarscopes over and see if there is any difference.  At least then I'd be able to say definitively whether it was the mount or the polarscope.

Thanks for the idea.

[jambouk]

The other option is to unscrew the eye lens on the dodgy polar scope (its a long thread) but hold your hand underneath it as when it finally comes off, if the reticule is loose it will just drop out on the floor and smash...

[SteveBz]

Update.  So we've had a couple of rainy days yesterday with a clear, if cold, weekend predicted.  Yesterday I put in the plumber's washer and realigned. 

No diff.  Still get the drop.

So today, stripped down the RA axis to it's bearings.  Re-greased as a precaution, although it still looked good, and carefully and very slowly put it back together tightening and testing with each half turn of every screw.  Re-calibrated the motors and then realigned.

Success.  No random movement in the Polar alignment scope.  I now have movement much less than the smaller, inner-most Polaris circle.  A half-turn in RA probably gives rise to a movement equivalent to about a quarter of the diameter of the smallest circle.  So certainly there was something loose in my RA axis.

Anyhow, maybe a quick collimation check tomorrow to get ready for Friday and Sat, make sure batteries are charged and off we go!

It's lookin' goooood.

Steve

[almcl]

Hey, well done Steve.   

Do you reckon it was the polarscope that was moving, or something deeper within the RA axis?

I keep telling myself I should carry this procedure out on my Alt Az EQ6 but haven't quite got around to it...

[Owmuchonomy]

If you have got an AZ EQ 6 then you don't need a polar scope.  The cap has never been off mine.  Just use the handset to do your polar alignment.  No bending down required or awkward peering through potentially mis-aligned hardware. Simples.

[SteveBz]

27 minutes ago, almcl said:

Hey, well done Steve.   

Do you reckon it was the polarscope that was moving, or something deeper within the RA axis?

I keep telling myself I should carry this procedure out on my Alt Az EQ6 but haven't quite got around to it...

No, I think it was the main RA thingy um.. the sort of pipe thing that the polar scope screws into.  It is held in place by a collar that itself is held in place by three grub screws accessible only through the main RA locking screw-hole.  Without the OTA and counter-weights it rocks very very slightly if you lift it with the weight of 15 kgs or whatever it is, but with the weights mounted there must be a sort of tipping point where the whole axis tips by half a mm or less and this creates the wandering we've been seeing.

So in fact then I went out and did the collimation and that also was a bit out, so I feel I'm solid for tomorrow now.

[almcl]

Thanks for that.  Haven't completely retired my EQ5, so sorting that might be a task for the weekend.