Polar Alignment on static pier

[johnrt]

I commissioned my pier back in February, an Altair model screwed in to a concrete base (pic below).

 

For one reason and another it hasn't been used since the 20th of March, just over 2 months. The idea of the pier was to avoid having to regularly polar align, due to a very restricted sky I cannot drift align or see polaris for most of the year. The plan was to align when there are no leaves on the trees in winter and then I should be good to go until I can re-check in 12 months.

So after sitting for 2 months I was disappointed to find my PA was way out for imaging, specifically in RA. The mount was left parked as pictured and did have the weights left on and no OTA, could this be due to the weights pulling the axis of the mount down over time? The correction needed was in the opposite direction the the pull of the weights (if that makes sense?).

I'd be interested to hear how others keep their mount to preserve PA.....

 

John.

[bottletopburly]

Show deep is concrete , could be ground shift , rule of thumb keep digging until you hit the mantel????

[Kropster]

Perhaps the concrete base has settled slightly or moved as the ground dried out / got wet.

How long since it was put in place?

[johnrt]

The concrete base was 40cm x 40cm x 90cm

The concrete was poured in early Feb, so could still be drying out / settling a little bit I suppose.

[RayD]

I'd say it's more likely to be that top plate flexing (well not flexing as such but moving on its fixings).  Make sure the 4 bolts that secure the (bottom) top plate to the pier top are very tight and secure, and then reduce the height of the studs to as low as you possibly can.

[johnrt]

6 hours ago, RayD said:

I'd say it's more likely to be that top plate flexing (well not flexing as such but moving on its fixings).  Make sure the 4 bolts that secure the (bottom) top plate to the pier top are very tight and secure, and then reduce the height of the studs to as low as you possibly can.

HI Ray,

Everything is nice & tight, the threaded rods are 16mm solid steel, do you think they would flex with just the mount sitting on the pier?

[Davey-T]

Hi John, did you use a spirit level originally ? if so you can recheck it to see which direction it's moved, I'm in the settlement camp 

Just relevelling using the bolts may fix it.

Dave

[johnrt]

1 hour ago, Davey-T said:

Hi John, did you use a spirit level originally ? if so you can recheck it to see which direction it's moved, I'm in the settlement camp 

Just relevelling using the bolts may fix it.

Dave

Hi Dave,

I've re-checked with a level, and I can't really see that it has changed much since I set it up. I didn't level it perfectly, but as far as I can remember it is as it was.

[AngryDonkey]

Maybe a defoliant or chainsaw might help? 

[RayD]

6 hours ago, johnrt said:

HI Ray,

Everything is nice & tight, the threaded rods are 16mm solid steel, do you think they would flex with just the mount sitting on the pier?

No probably not John.  I just thought with the weights on one end the top plate (the one attached to the pier at the bottom of the studs) which appears to be a kind of cap held in place with 4 pinch fit bolts, could easily tilt.  If so this isn't an ideal scenario, but as others note, perhaps look at ground shifting.

[sophiecentaur]

12 hours ago, RayD said:

I'd say it's more likely to be that top plate flexing (well not flexing as such but moving on its fixings).  Make sure the 4 bolts that secure the (bottom) top plate to the pier top are very tight and secure, and then reduce the height of the studs to as low as you possibly can.

The arrangement of four parallel bolts does rely on the shear strength of the bolts and their holes through the plates. It looks relatively easy (?) to shorten them. The bolts appear to be threaded along their whole length - in particular as they go through the holes in the bottom plate. That arrangement may not be as strong as it looks and they could perhaps be rocking slightly and there is no inherent strength in a parallelogram. Some very fat rigid washers on either side of the holes could perhaps help or a thick block between the plates to reduce the angle change if there is horizontal movement. 

[michael8554]

Hi John

I see you've had the OTA off the mount.

I'd say it's far more likely that when you remounted the OTA it's not back in exactly the same position as before.

The precision required for good PA is tiny - I can make noticeable adjustments by loosening or tightening my Dec screw  a tiny amount with the Meade wedge plate clamps fully tightened.

That's because the Meade Wedge is made of cheese, but the OTA mounting arrangement on EQ mounts is not precision.

Michael

[johnrt]

I had a quick experiment last night, no time for imaging due to work even though it was clear all night.

I cannot generate any flex at all in the pier, even with my full body weight. However, if I transfer to the mount then I can easily generate movement. I think the weak point is the mechanism to transfer from EQ to AZ mode on the mount, this involves unscrewing the 2 large bolts on the RA axis and jacking up the altitude adjustment until horizontal. The bolts are tightened up as much as possible but this seems to be where the movement is - the axis is being dragged down over time by the unbalanced force of the counter weights. There is a big movement in polaris in the polar scope when adding and removing the weights with no OTA on the mount.

Hopefully I can minimise the shift by storing without weights on. But I'm not confident it's going to give me decent enough PA for the 9 months I need every year before the leaves drop in winter.

[RayD]

26 minutes ago, johnrt said:

I had a quick experiment last night, no time for imaging due to work even though it was clear all night.

I cannot generate any flex at all in the pier, even with my full body weight. However, if I transfer to the mount then I can easily generate movement. I think the weak point is the mechanism to transfer from EQ to AZ mode on the mount, this involves unscrewing the 2 large bolts on the RA axis and jacking up the altitude adjustment until horizontal. The bolts are tightened up as much as possible but this seems to be where the movement is - the axis is being dragged down over time by the unbalanced force of the counter weights. There is a big movement in polaris in the polar scope when adding and removing the weights with no OTA on the mount.

Hopefully I can minimise the shift by storing without weights on. But I'm not confident it's going to give me decent enough PA for the 9 months I need every year before the leaves drop in winter.

Good that you've found it John.  I would say that if you are definitely not wanting to polar align for 9 months, replace the plastic hand bolts with normal ones you can do up with a spanner.  I have the same mount and this is most definitely a weak point.

[johnrt]

1 hour ago, RayD said:

Good that you've found it John.  I would say that if you are definitely not wanting to polar align for 9 months, replace the plastic hand bolts with normal ones you can do up with a spanner.  I have the same mount and this is most definitely a weak point.

Good idea! Do you know what size bolts they are?

[RayD]

2 minutes ago, johnrt said:

Good idea! Do you know what size bolts they are?

I think they're M12 John but I'll have a look.  I think it's not the best design, and you'll probably find the same as me, which is you can easily move the Alt screw through them even when done up.

[RayD]

1 hour ago, johnrt said:

Good idea! Do you know what size bolts they are?

it looks like they are M12 John but my mount is in Spain so I can't check.  It's worth just having a quick look as it appears there may be an Allen headed socket inside the plastic bolt head, meaning you could use a (6mm?) Allen key to nip them up.  Not sure on that but that's what is looks like in some of the pictures I have.

Also when you do your polar alignment you have to make absolutely certain that your last adjustment on the altitude bolt is increasing altitude, meaning the weight is sat on the bolt and not just relying on the lock off bolts to hold it there.  

[johnrt]

1 hour ago, RayD said:

I think they're M12 John but I'll have a look.  I think it's not the best design, and you'll probably find the same as me, which is you can easily move the Alt screw through them even when done up.

Yes the altitude screw is easy to turn even with the 2 bolts firmly done up. 

[michael8554]

11 hours ago, michael8554 said:

Hi John

I see you've had the OTA off the mount.

I'd say it's far more likely that when you remounted the OTA it's not back in exactly the same position as before

Ummm, further thought overnight, I now realise that this suggestion was not "optimum".

Glad you found the problem.

Michael

[RayD]

14 minutes ago, johnrt said:

Yes the altitude screw is easy to turn even with the 2 bolts firmly done up. 

Yes I have the same with mine John.  It isn't the best design and whilst I love the mount I think this is a bit of a flaw and sort of lets it down a bit.  

Have a look to see if they are indeed allen headed bolts in there and if not you should be able to just swap them out with normal bolts.  The only reason I suspect they are hand operated is due to it being a portable mount, so should be usable without tools, but in your application you want it to stay static, so using bolts and a spanner will be better (albeit make sure you don't over tighten and break something of course).

As noted above I also make sure my last adjustment is in altitude as well (clockwise from memory), to make sure you are pushing the weights up.

[RayD]

14 minutes ago, michael8554 said:

Ummm, further thought overnight, I now realise that this suggestion was not "optimum".

Glad you found the problem.

Michael

You would actually be spot on Michael as putting the OTA back on would probably put the PA back to somewhere near, but of course this is just a result of the actual problem which is this pretty poor design to lock of the altitude once adjusted.

[johnrt]

7 hours ago, RayD said:

Yes I have the same with mine John.  It isn't the best design and whilst I love the mount I think this is a bit of a flaw and sort of lets it down a bit.  

Have a look to see if they are indeed allen headed bolts in there and if not you should be able to just swap them out with normal bolts.  The only reason I suspect they are hand operated is due to it being a portable mount, so should be usable without tools, but in your application you want it to stay static, so using bolts and a spanner will be better (albeit make sure you don't over tighten and break something of course).

As noted above I also make sure my last adjustment is in altitude as well (clockwise from memory), to make sure you are pushing the weights up.

I have had a quick look at the mechanism on the mount and it is basically an M12 threaded rod set in a larger piece that the body of the mount pivots on to set the altitude. The 2 hand turned nuts simply tighten up against plastic washer and clamp the threaded rod to stop it moving (which of course it doesn't). So to improve it all I can see working is replacing the hand turned bolts with normal ones that can be nipped up with a spanner & hope that will hold it tighter to stop any slop.

 

[johnrt]

On 25/02/2015 at 12:53, Starflyer said:

 

The thread on the altitude adjuster is far too course and the side handwheel nuts to lock the altitude axis do almost nothing to lock it in place.  I added leather washers underneath the handwheel nuts and they now hold the altitude axis firmly.  I can't do anything about the altitude adjuster without resorting to having one made and it's more an annoyance than a show stopper so I live with it now.  A finer thread would allow accurate PA to be achieved in less time while drift aligning, at the moment I find myself often overshooting the sweet spot, even with the finest of touches.

 

Cheers,

Ian

 

A quick google around brought me straight back to SGL with this post from @Starflyer seems like this could be the answer!

 

John.

 

[Jessun]

Perhaps not quite on topic but for a permanent pier, why use the four bolts to hold the mounting plate? The idea with a level mount is that is isolates RA from DEC as you adjust towards a good PA in the field or for any kind of mobile setup. It's just a wee bit quicker.

For a permanent rig it's easy to drift align or use another mean of PA, even if a RA adjustment will ever so slightly upset your last DEC adjustment and vice versa, but that's a one off. Four bolts that may come loose or expand/contract differently seem to do little to me on this type of rig.

The usual "only my 2C caveat" of course. The main thing is that you got it to work again!

For me, most of the drift - if I wait months - is due to the handset clock running a bit out of sync.

/Jesper

[Waldemar]

For another reason I wonder about those four bolts also: This construction is called a 'rat-cage'. The problem with it is that it enhances vibrations, introduced by the motors and even by wind gusts. The resonance frequency i.e. the number of vibrations per second that makes the whole construction vibrate depends on the diameter of the used bolts, the thickness of the plates and the in between plates distance, but the longer the bolts, the thinner the plates and the larger the distance, the lower that resonance frequency will be. That will introduce a dynamic with the motor frequency and the whole set up will vibrate on an unknown frequency which may very well interfere with imaging and prevent getting tight stars.
If visual the effect may be like bad seeing. This might even be noticeable by feeling or hearing, but not necessarily. 
If the construction is made stiffer, the resonance frequency gets higher and will not cause problems anymore.
I realise this sounds far fetched, but I had the same kind of thing going on and it took me quite a while to find the underlying problem:  the rat-cage construction!!

If you don't mind, I would like to suggest you shorten the bolts or even better still, just bolt the two plates together, to get an as rigid construction as possible.

Another 2 eurocents...