Stability of PA alignment

[alan4908]

Hopefully, this is a straightforward question. 

Assume that I align my scope with zero PA error. I come back at time X during which time I haven't touched anything on the scope/mount. In addition, assume my pier does not move or my mount/scope does not slip. Approximately, how long does it take before my PA is 1 arc minute out of alignment ?

The reason for my question is that I notice that my PA gradually degrades over time, I believe this is due to my pier/mount/scope moving slightly but I wanted to check that I haven't missed any alignment factors that would accumulate due to the motion of the Earth.

Since I don't alter the configuration of my imaging scope, I'm almost 100% convinced that I experience very slight Pier movements which change with the weather conditions- this is explained by considering that whilst my Pier sits on a substantial concrete block, the concrete block sits on clay soil. 

Alan

[coatesg]

Almost certainly due to the local movement of soil - this will depend on moisture content if you are on clay.

Polar motion (https://en.m.wikipedia.org/wiki/Polar_motion) is a real thing, however, but not sure you'd be able to detect this at all (needs a long baseline interferometer to do so).

 

[coatesg]

(Should add as well, even the pros see this type of thing - have a feeling that Pinpoint was used on the AAT and showed a slow drift in PA due to (possibly) the concrete curing in the observatory foundations)

[vlaiv]

Actually it can happen over the course of one year or so.

On Heq5 polar alignment scope there are engravings of exact position (I'll try to look up diagram online) that have year markers - correct pole star position changes in matter of 1-2 years - and shift is such that is more than usual visual PA error, or smallest movement that one can be certain there is good PA.

As you see, there is slight "wobble" in pole star position in relation to NCP.

[vlaiv]

Then there is this:

https://en.wikipedia.org/wiki/Polar_motion

Since we are floating on mantle - there is drift of crust with respect to earth rotation axis - that can also alter PA, but it's small in magnitude.

Better explanation for above is in precession and nutation.

[Skipper Billy]

I remove my mount from the observatory from April to August (no proper darkness this far North) so I cannot comment over a long period.

At the start of the imaging season in late August I drift align and usually that's it until early April.

The exception is if/when we have small earth tremors which is usually once or twice a year ( we are in a glacial valley on a fault line) and even though my mount is secured to 1.4 cubic meters of concrete is usually needs a wee tweak after a tremor - so it doesnt take much !

[coatesg]

4 hours ago, vlaiv said:

Actually it can happen over the course of one year or so.

Yes - but Polaris shifting in the polar scope isn't the same as polar alignment shifting - precession affects where the pole points, not where the axis of rotation is.

But then, for accurate polar alignment I wouldn't use the polar scope - esp in a permanent setup.

[R26 oldtimer]

Do you take the scope of the mount? If you do, then you apply changes to the cone error which you took into account if you drift-polar aligned in the first place.

[coatesg]

7 minutes ago, R26 oldtimer said:

Do you take the scope of the mount? If you do, then you apply changes to the cone error which you took into account if you drift-polar aligned in the first place.

Come error doesn't affect drift alignment. The scope will still follow circles of RA/Dec (but it will affect pointing models on goto mounts)

[vlaiv]

1 hour ago, coatesg said:

Yes - but Polaris shifting in the polar scope isn't the same as polar alignment shifting - precession affects where the pole points, not where the axis of rotation is.

But then, for accurate polar alignment I wouldn't use the polar scope - esp in a permanent setup.

Yes, but Polaris does not shift due to its proper motion, at least on level of engraving in reticle. That engraving is displaying Polaris relation to NCP because of change in NCP position over time (Earth precession and nutation) and not due to Polaris proper motion - at least I think so.

Wiki states Polaris proper motion as being order of 44 and 11 Mas/Year (RA and DEC) - that is far less than single year division in polar scope.

 

 

[Stub Mandrel]

When I image planets with an x3 barlow for 3,600mm focal length the impact of standing near a tripod leg is very easy to see.

I now try to be away from the legs and lean over to adjust the mount when doing an accurate PA.

[alan4908]

19 hours ago, coatesg said:

Almost certainly due to the local movement of soil - this will depend on moisture content if you are on clay.

Polar motion (https://en.m.wikipedia.org/wiki/Polar_motion) is a real thing, however, but not sure you'd be able to detect this at all (needs a long baseline interferometer to do so).

 

 

19 hours ago, coatesg said:

(Should add as well, even the pros see this type of thing - have a feeling that Pinpoint was used on the AAT and showed a slow drift in PA due to (possibly) the concrete curing in the observatory foundations)

Thanks for the replies -  It is good to know I'm in the company of professional observatories . I will assume that the vast majority of the effect is from movement of my Pier which arises due to the changing moisture clay content. 

 

19 hours ago, vlaiv said:

Actually it can happen over the course of one year or so.

Thanks vlaiv - however, what I didn't understand from the (very interesting) Wikipedia article is how you calculate the time period X  to give 1 arc minute misalignment.  As you indicate, I presume this is will come out to be many years, so compared to my Pier movement it will be insignificant. 

16 hours ago, Skipper Billy said:

The exception is if/when we have small earth tremors which is usually once or twice a year ( we are in a glacial valley on a fault line) and even though my mount is secured to 1.4 cubic meters of concrete is usually needs a wee tweak after a tremor - so it doesnt take much !

I normally try to operate at a PA of 1 arc minute or less - as you say, it doesn't take much to knock this out - thankfully Earth tremors in East Sussex are somewhat rare ! 

14 hours ago, R26 oldtimer said:

Do you take the scope of the mount? If you do, then you apply changes to the cone error which you took into account if you drift-polar aligned in the first place.

 No, I don't take the scope off the mount, I also don't drift align to obtain my PA. 

 To initially polar align, I use a piece of software that instructs my mount take 10 images which are then plate solved over various positions of the sky. The plate solve results are then synched to the mount which builds up an sky model error map of RA and DEC. To perform the PA, I select from a list of polar alignment stars which the mount provides and the scope slews to the selected star and then its simply a case of adjusting the alt and azimuth controls until the star is centered in the CCD.  It's quite a simply process and it appears to give an PA accuracy of a few arc seconds.  After I've done the initial alignment and because I image unguided, I get the software to repeat the procedure for 100 images which builds a very comprehensive sky model for the entire sky. I've found that to image 30min subs at 0.7 arc seconds/s unguided I need a PA better that 1.5 arc minutes, in addition my Pier also needs to be very horizontal (unlikely a normal equatorial mount where this doesn't matter). 

13 hours ago, Stub Mandrel said:

When I image planets with an x3 barlow for 3,600mm focal length the impact of standing near a tripod leg is very easy to see.

I now try to be away from the legs and lean over to adjust the mount when doing an accurate PA.

Thanks. I haven't noticed any similar effects but I'm only at 0.7 arc seconds/pixel, its also probably helps that my observatory floor is semi- isolated from the Pier.  

[coatesg]

13 hours ago, vlaiv said:

Yes, but Polaris does not shift due to its proper motion, at least on level of engraving in reticle. That engraving is displaying Polaris relation to NCP because of change in NCP position over time (Earth precession and nutation) and not due to Polaris proper motion - at least I think so.

Yes - I'm not saying it does - just that precession of the pole/nutation doesn't require realignment of a polar aligned scope. The scope's polar axis will still be aligned with the axis of the earth's rotation - it's just that the NCP isn't in the same place with respect to background stars.

[Stub Mandrel]

55 minutes ago, alan4908 said:

Thanks. I haven't noticed any similar effects but I'm only at 0.7 arc seconds/pixel, its also probably helps that my observatory floor is semi- isolated from the Pier.  

This is at 0.25" per pixel ... but on grass over a fairly sandy soil.

[vlaiv]

1 hour ago, coatesg said:

Yes - I'm not saying it does - just that precession of the pole/nutation doesn't require realignment of a polar aligned scope. The scope's polar axis will still be aligned with the axis of the earth's rotation - it's just that the NCP isn't in the same place with respect to background stars.

Ah, quite right!

Once aligned mount precesses / nutates together with earth - did not think of that.